Advertisement
9-10th Class Competitive Exam

FORCE AND LAWS OF MOTION IN EASY WORDS

Afifa Asad 0

FORCE AND LAWS OF MOTION

1️⃣ Introduction

Force and motion are closely related concepts in physics. In daily life, we observe objects moving, stopping, speeding up, slowing down, or changing direction. All these changes occur due to force. The scientific study of force and motion was revolutionized by the great scientist Isaac Newton, who gave the three fundamental laws of motion.

This chapter explains:

  • What is force?
  • Effects of force
  • Newton’s three laws of motion
  • Momentum
  • Conservation of momentum
  • Applications in daily life

2️⃣ What is Force?

A force is a push or pull acting on an object that can change its:

  • Shape
  • Speed
  • Direction
  • State of motion

Examples:

  • Pushing a door
  • Pulling a rope
  • Kicking a football
  • Applying brakes to a bicycle

Effects of Force

  1. Can move a stationary object
  2. Can stop a moving object
  3. Can change speed
  4. Can change direction
  5. Can change shape

3️⃣ Balanced and Unbalanced Forces

🔹 Balanced Forces

When two forces acting on an object are equal and opposite, they cancel each other.

Result: No change in motion.

Example:
A book resting on a table.

🔹 Unbalanced Forces

When forces are not equal or not opposite.

Result: Motion changes.

Example:
Pushing a stationary cart.

4️⃣ Newton’s First Law of Motion (Law of Inertia)

newton first law

Statement:
An object remains at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force.

This law is also called the Law of Inertia.

What is Inertia?

Inertia is the tendency of an object to resist change in its state of motion.

Types of Inertia:

  1. Inertia of Rest
  2. Inertia of Motion
  3. Inertia of Direction

Examples of First Law:

  • Passengers fall backward when a bus starts suddenly.
  • Dust comes out when we beat a carpet.
  • Fruits fall when tree is shaken.

5️⃣ Momentum

Momentum measures the quantity of motion in a body.

momentum

Where:
p = momentum
m = mass
v = velocity

Momentum is a vector quantity (has direction).

SI Unit:

kg m/s

Example:

A truck and a bicycle moving at same speed:

  • Truck has greater mass
  • Therefore, truck has greater momentum

6️⃣ Newton’s Second Law of Motion

newton second law

Statement:
The rate of change of momentum of a body is directly proportional to the applied force and occurs in the direction of the force.

Mathematically:
Force = Mass × Acceleration

Explanation:

If a force acts on a body:

  • It produces acceleration.
  • Greater force → greater acceleration.
  • Greater mass → smaller acceleration (for same force).

Derivation:

Initial velocity = u
Final velocity = v
Time = t

Change in momentum = m(v − u)

Acceleration = (v − u)/t

Therefore:
F = ma

Unit of Force

SI Unit: Newton (N)

1 Newton = Force required to produce acceleration of 1 m/s² in 1 kg mass.

Applications:

  • A cricket player moves hands backward while catching a ball.
  • Airbags in cars increase time of collision.
  • Helmets reduce impact force.

7️⃣ Newton’s Third Law of Motion

newton third law

Statement:
For every action, there is an equal and opposite reaction.

Important Points:

  • Action and reaction act on different bodies.
  • They are equal in magnitude.
  • They are opposite in direction.

Examples:

  1. Recoil of a gun
  2. Walking on ground
  3. Rocket propulsion
  4. Swimming

8️⃣ Law of Conservation of Momentum

momentum

Statement:
The total momentum of a system remains constant if no external force acts on it.

Explanation:

Before collision:
Total momentum = m₁u₁ + m₂u₂

After collision:
Total momentum = m₁v₁ + m₂v₂

Therefore:

m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂

Applications:

  • Collision of billiard balls
  • Firing of a bullet
  • Rocket launching

9️⃣ Thrust and Pressure

🔹 Thrust

Force acting perpendicular to a surface.

🔹 Pressure

Force per unit area.

Pressure = Force / Area

SI Unit: Pascal (Pa)

Applications:

  • Sharp knife cuts easily.
  • Camels have broad feet.
  • Porters use turbans to carry loads.

🔟 Mass and Weight

Mass:

  • Amount of matter
  • Constant everywhere
  • Unit: kg

Weight:

  • Force of gravity on object
  • Changes with gravity
  • Unit: Newton

Weight = Mass × g

1️⃣1️⃣ Friction (Brief Introduction)

Friction is a force that opposes motion.

Examples:

  • Walking possible due to friction
  • Brakes work due to friction

1️⃣2️⃣ Applications of Newton’s Laws in Daily Life

🚗 Vehicles

  • Seat belts prevent injury (First Law)
  • Airbags increase time of impact (Second Law)

🚀 Rocket Launch

Based on Third Law.

⚽ Sports

  • Kicking ball (Second Law)
  • Catching ball safely (Impulse concept)

1️⃣3️⃣ Impulse

Impulse = Force × Time

It changes momentum.

Example:

  • Boxing gloves increase time of impact.
  • Crash helmets reduce injury.

1️⃣4️⃣ Numerical Examples

Example 1:

A body of 5 kg accelerates at 2 m/s².
Force = ma = 5 × 2 = 10 N

Example 2:

A 1000 kg car moving at 20 m/s stops in 5 s.
a = (0 − 20)/5 = −4 m/s²
Force = 1000 × (−4) = −4000 N

1️⃣5️⃣ Common Mistakes Students Make

  • Confusing mass and weight
  • Forgetting direction in momentum
  • Writing incomplete units
  • Ignoring sign in acceleration

1️⃣6️⃣ Important Differences

Force vs Momentum

ForceMomentum
Push or pullQuantity of motion
Unit: NewtonUnit: kg m/s

Mass vs Weight

MassWeight
ConstantChanges
ScalarVector

1️⃣7️⃣ Summary of Important Formulas

  1. F = ma
  2. p = mv
  3. Impulse = F × t
  4. Pressure = F/A
  5. Momentum conservation equation

1️⃣8️⃣ Real-Life Case Study

Car Collision

When two cars collide:

  • Momentum before collision equals momentum after.
  • Airbags reduce force by increasing time.
  • Seat belts prevent inertia effect.

1️⃣9️⃣ Higher Order Thinking Points

  • Why does a heavy truck take longer to stop?
  • Why do astronauts float in space?
  • Why do rockets work without air?

2️⃣0️⃣ Conclusion

Force and Laws of Motion form the foundation of mechanics. The three laws given by Isaac Newton explain nearly all types of motion in our daily life—from walking and driving to launching rockets.

Understanding:

  • Inertia
  • Momentum
  • Force
  • Action-reaction
  • Conservation of momentum

helps us analyze real-world situations scientifically.

This chapter builds the base for advanced physics topics like work, energy, gravitation, and rotational motion.

✅ SECTION A – MULTIPLE CHOICE QUESTIONS (1–100)

1. The SI unit of force is:

a) Joule
b) Watt
c) Newton
d) Pascal
Answer: c) Newton

2. Force is a:

a) Scalar
b) Vector
c) Speed
d) Energy
Answer: b) Vector

3. Newton’s First Law is also called:

a) Law of Acceleration
b) Law of Momentum
c) Law of Inertia
d) Law of Gravity
Answer: c) Law of Inertia

4. Inertia depends on:

a) Speed
b) Mass
c) Force
d) Velocity
Answer: b) Mass

5. Momentum is equal to:

a) m/a
b) m × v
c) v/m
d) F/t
Answer: b) m × v

6. The unit of momentum is:

a) kg m/s
b) N
c) m/s
d) kg
Answer: a) kg m/s

7. If force is zero, acceleration is:

a) 1
b) Zero
c) Infinite
d) Negative
Answer: b) Zero

8. According to Newton’s second law:

a) F = mv
b) F = ma
c) F = m/a
d) F = a/m
Answer: b) F = ma

9. Action and reaction forces act on:

a) Same body
b) Different bodies
c) Same direction
d) Same magnitude only
Answer: b) Different bodies

10. Balanced forces result in:

a) Acceleration
b) Motion
c) No change in motion
d) Rotation
Answer: c) No change in motion

11. A moving object stops due to:

a) Gravity
b) Friction
c) Mass
d) Pressure
Answer: b) Friction

12. The rate of change of momentum gives:

a) Speed
b) Acceleration
c) Force
d) Pressure
Answer: c) Force

13. 1 Newton equals:

a) 1 kg m/s
b) 1 kg m/s²
c) 1 m/s
d) 1 kg
Answer: b) 1 kg m/s²

14. The tendency of a body to resist change is called:

a) Pressure
b) Momentum
c) Inertia
d) Speed
Answer: c) Inertia

15. A rocket works on:

a) First law
b) Second law
c) Third law
d) Law of gravity
Answer: c) Third law

16. Impulse equals:

a) F/t
b) F × t
c) m/a
d) v/t
Answer: b) F × t

17. The SI unit of pressure is:

a) Newton
b) Pascal
c) Joule
d) Watt
Answer: b) Pascal

18. Pressure increases when area:

a) Increases
b) Decreases
c) Doubles
d) Remains same
Answer: b) Decreases

19. Momentum is conserved when:

a) Force acts
b) No external force acts
c) Acceleration occurs
d) Mass changes
Answer: b) No external force acts

20. Weight is equal to:

a) m/g
b) m × g
c) g/m
d) m + g
Answer: b) m × g

21. Mass is measured in:

a) Newton
b) Pascal
c) Kilogram
d) Joule
Answer: c) Kilogram

  1. Weight is a:
    a) Scalar
    b) Vector
    c) Speed
    d) Distance
    Answer: b) Vector
  2. Friction is a:
    a) Push
    b) Pull
    c) Opposing force
    d) Energy
    Answer: c) Opposing force
  3. A cricket player lowers his hands to:
    a) Increase force
    b) Decrease time
    c) Increase time
    d) Increase speed
    Answer: c) Increase time
  4. Momentum depends on:
    a) Mass only
    b) Velocity only
    c) Both mass and velocity
    d) Force only
    Answer: c) Both mass and velocity

26. Acceleration is change in: Velocity

Answer: b

  1. If mass doubles and force same, acceleration: Halves
    Answer: b
  2. A heavy truck has more inertia because: More mass
    Answer: c
  3. Unit of impulse: Newton-second
    Answer: a
  4. Law of conservation of momentum applies in: Collisions
    Answer: d

31–40

  1. Inertia of rest example: Passenger falls backward
    Answer: b
  2. Inertia of motion example: Person falls forward
    Answer: c
  3. Inertia of direction example: Stone tied in string
    Answer: a
  4. Pressure = Force/Area
    Answer: c
  5. Sharp knife cuts due to: High pressure
    Answer: b
  6. Camels have broad feet to: Reduce pressure
    Answer: a
  7. Recoil of gun shows: Third law
    Answer: d
  8. A body at rest stays at rest due to: First law
    Answer: a
  9. Momentum of stationary body: Zero
    Answer: b
  10. Force causes change in: Motion
    Answer: d

41–50

  1. If velocity increases, momentum: Increases
    Answer: a
  2. Balanced forces have net force: Zero
    Answer: c
  3. Unit of mass: kg
    Answer: b
  4. SI unit of thrust: Newton
    Answer: d
  5. Third law forces are: Equal and opposite
    Answer: a
  6. F = ma given by: Newton
    Answer: b
  7. Airbags reduce: Force of impact
    Answer: c
  8. Seat belts prevent injury due to: Inertia
    Answer: d
  9. Force needed to stop body depends on: Momentum
    Answer: a
  10. Greater mass → greater: Inertia
    Answer: c

51–60

  1. Change in momentum per second gives: Force
    Answer: b
  2. Unit of acceleration: m/s²
    Answer: a
  3. Force required to move object depends on: Mass
    Answer: d
  4. Third law acts in: Pairs
    Answer: b
  5. Impulse changes: Momentum
    Answer: c
  6. Pressure depends on: Area
    Answer: a
  7. Walking possible due to: Friction
    Answer: d
  8. Firing bullet shows: Momentum conservation
    Answer: b
  9. If mass zero, momentum: Zero
    Answer: c
  10. Greater stopping time → smaller: Force
    Answer: a

61–70

  1. Collision obeys: Momentum conservation
    Answer: d
  2. Gravity is a: Force
    Answer: a
  3. Newton studied: Motion
    Answer: c
  4. Force can change: Shape
    Answer: b
  5. SI unit of impulse: Ns
    Answer: a
  6. Push or pull is: Force
    Answer: d
  7. More mass → more resistance to change
    Answer: b
  8. If no force acts, velocity: Constant
    Answer: a
  9. Law of inertia explains: Sudden jerk
    Answer: c
  10. Pressure unit Pascal equals: N/m²
    Answer: d

71–80

  1. Thrust acts: Perpendicular
    Answer: a
  2. Rocket propulsion uses: Action-reaction
    Answer: c
  3. Balanced forces produce: No acceleration
    Answer: b
  4. If acceleration zero, force: Zero
    Answer: d
  5. Momentum is vector because: Has direction
    Answer: a
  6. Weight varies with: Gravity
    Answer: b
  7. Mass remains: Constant
    Answer: c
  8. Heavy object harder to move due to: Inertia
    Answer: a
  9. Impulse formula: F × t
    Answer: d
  10. Force causes acceleration in: Same direction
    Answer: b

81–90

  1. Third law always acts simultaneously
    Answer: a
  2. SI unit of pressure: Pa
    Answer: c
  3. Momentum before = after in: Isolated system
    Answer: b
  4. Net force zero → acceleration zero
    Answer: d
  5. Inertia proportional to mass
    Answer: a
  6. A body moving with constant velocity has: Zero net force
    Answer: c
  7. Acceleration produced by force depends on: Mass
    Answer: b
  8. Larger area → less pressure
    Answer: a
  9. Friction opposes: Motion
    Answer: d
  10. Newton’s laws apply in: Everyday life
    Answer: c

91–100

  1. F = ma shows force proportional to acceleration
    Answer: a
  2. Bullet recoil shows: Third law
    Answer: b
  3. Conservation law valid when: No external force
    Answer: d
  4. SI unit of force named after: Newton
    Answer: c
  5. Action and reaction are: Equal
    Answer: a
  6. Momentum increases with: Velocity
    Answer: b
  7. Net force causes: Change in motion
    Answer: c
  8. Weight is gravitational force
    Answer: d
  9. Object resists change due to: Inertia
    Answer: a
  10. Laws of motion were given by: Newton
    Answer: b

✅ 100 Very Short Answer Questions (With Answers)

1. What is force?

Force is a push or pull acting on a body.

2. Write the SI unit of force.

Newton (N).

3. What is inertia?

Inertia is the tendency of a body to resist change in its state of motion.

4. On what factor does inertia depend?

Mass.

5. State Newton’s First Law.

A body remains at rest or in uniform motion unless acted upon by an unbalanced force.

6. What is momentum?

Momentum is the product of mass and velocity.

7. Write formula of momentum.

p = mv.

8. Write SI unit of momentum.

kg m/s.

9. What is acceleration?

Rate of change of velocity.

10. Write formula of force.

F = ma.

11. Who gave the laws of motion?

Isaac Newton.

12. What is impulse?

Impulse is force multiplied by time.

13. Write formula of impulse.

Impulse = F × t.

14. What is balanced force?

Equal and opposite forces acting on a body.

15. What is unbalanced force?

Forces that cause change in motion.

16. What is weight?

Gravitational force acting on a body.

17. Write formula of weight.

W = mg.

18. What is mass?

Amount of matter in a body.

19. SI unit of mass?

Kilogram (kg).

20. What is pressure?

Force per unit area.

21. Write formula of pressure.

Pressure = Force/Area.

22. SI unit of pressure?

Pascal (Pa).

23. What is thrust?

Force acting perpendicular to a surface.

24. What is friction?

Force that opposes motion.

25. What is net force?

Sum of all forces acting on a body.

26. What happens if net force is zero?

Acceleration is zero.

27. What is Newton’s Second Law?

Force equals mass times acceleration.

28. What is Newton’s Third Law?

For every action, there is equal and opposite reaction.

29. Is momentum scalar or vector?

Vector.

30. Is force scalar or vector?

Vector.

31. What is inertia of rest?

Tendency to remain at rest.

32. What is inertia of motion?

Tendency to remain in motion.

33. What is inertia of direction?

Tendency to resist change in direction.

34. What is SI unit of impulse?

Newton-second (Ns).

35. What is law of conservation of momentum?

Total momentum remains constant if no external force acts.

36. When is momentum conserved?

In absence of external force.

37. What is recoil of gun?

Backward motion of gun after firing.

38. Which law explains rocket propulsion?

Newton’s Third Law.

39. What happens to pressure when area decreases?

Pressure increases.

40. What happens to acceleration if mass increases (force constant)?

Acceleration decreases.

41. What is effect of force on shape?

Force can change shape.

42. Define 1 Newton.

Force that produces 1 m/s² acceleration in 1 kg mass.

43. What causes a body to accelerate?

Unbalanced force.

44. What is direction of acceleration?

Same as applied force.

45. What is momentum of stationary body?

Zero.

46. Can balanced force change shape?

Yes.

47. What is unit of acceleration?

m/s².

48. What happens when force acts for longer time?

Greater change in momentum.

49. What is stopping time?

Time taken to bring body to rest.

50. What is isolated system?

System with no external force.

51. What happens to momentum if velocity doubles?

Momentum doubles.

52. What happens to momentum if mass doubles?

Momentum doubles.

53. What is relationship between force and acceleration?

Directly proportional.

54. What is relationship between acceleration and mass?

Inversely proportional.

55. What is effect of friction?

Opposes motion.

56. Why do camels have broad feet?

To reduce pressure.

57. Why are knives sharp?

To increase pressure.

58. What law explains jerks in bus?

First Law.

59. Why do we fall forward when bus stops suddenly?

Due to inertia of motion.

60. What is SI unit of weight?

Newton.

61. Does mass change on Moon?

No.

62. Does weight change on Moon?

Yes.

63. What is magnitude of action and reaction?

Equal.

64. Do action and reaction act on same body?

No.

65. What type of quantity is mass?

Scalar.

66. What type of quantity is weight?

Vector.

67. What is main cause of injuries in accidents?

Large force in short time.

68. How do airbags reduce injury?

Increase time of impact.

69. What is change in momentum called?

Impulse.

70. What is rate of change of momentum?

Force.

71. What is gravitational acceleration on Earth?

9.8 m/s².

72. Can a body have zero velocity but non-zero acceleration?

Yes.

73. What is uniform acceleration?

Constant acceleration.

74. What is direction of friction?

Opposite to motion.

75. What is effect of heavy mass on inertia?

Increases inertia.

76. Can force change direction of motion?

Yes.

77. What is collision?

Impact between two bodies.

78. Which law applies in collision?

Conservation of momentum.

79. What is net force in balanced condition?

Zero.

80. What is force required to change momentum?

Unbalanced force.

81. What is contact force?

Force acting through contact.

82. What is non-contact force?

Force acting without contact.

83. Example of non-contact force?

Gravitational force.

84. What is stopping distance affected by?

Speed and mass.

85. What is effect of greater stopping time?

Smaller force.

86. What is impulse equal to in terms of momentum?

Change in momentum.

87. Is momentum conserved in explosion?

Yes.

88. Is force needed to maintain uniform motion?

No.

89. What is force required to change rest to motion?

Unbalanced force.

90. What is tendency of object to resist acceleration?

Inertia.

91. Which law gives F = ma?

Second law.

92. Which law explains walking?

Third law.

93. Which law explains stationary object remains at rest?

First law.

94. What is mass measure of?

Inertia.

95. What is greater in truck compared to car?

Momentum (if speed same).

96. What is force needed to increase velocity?

Unbalanced force.

97. Can balanced forces change motion?

No.

98. What is opposite of acceleration?

Retardation.

99. What is SI unit of thrust?

Newton.

100. What is foundation of mechanics?

Newton’s Laws of Motion.

📘 Class 9 Science – Force and Laws of Motion

✅ Short Answer Questions

1. What is force? Explain with examples.

Force is a push or pull acting on a body that can change its speed, direction, or shape. For example, pushing a cart makes it move, and pulling a rope draws an object closer.

2. Define inertia and state its types.

Inertia is the property of a body to resist change in its state of rest or motion. It is of three types: inertia of rest, inertia of motion, and inertia of direction.

3. Explain inertia of rest with example.

Inertia of rest is the tendency of a body to remain at rest. For example, passengers fall backward when a bus starts suddenly because their body tends to remain at rest.

4. Explain inertia of motion with example.

Inertia of motion is the tendency of a body to remain in motion. Passengers fall forward when a moving bus stops suddenly due to inertia of motion.

5. Explain inertia of direction.

Inertia of direction is the tendency of a body to resist change in direction. A stone tied to a string flies off tangentially when the string breaks due to inertia of direction.

6. State Newton’s First Law of Motion.

Newton’s First Law states that a body remains at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force.

7. Why is Newton’s First Law called Law of Inertia?

It is called the Law of Inertia because it explains the property of bodies to resist change in their state of motion or rest.

8. Define momentum and write its formula.

Momentum is the quantity of motion possessed by a body. It is the product of mass and velocity. The formula is p = mv.

9. State SI unit of momentum and explain.

The SI unit of momentum is kg m/s. It represents mass in kilograms multiplied by velocity in meters per second.

10. State Newton’s Second Law of Motion.

Newton’s Second Law states that the rate of change of momentum of a body is directly proportional to the applied force and occurs in the direction of force.

11. Derive the formula F = ma.

According to the second law, force equals rate of change of momentum. Since momentum is mv, change in momentum over time equals ma. Therefore, F = ma.

12. Define 1 Newton.

One Newton is the force required to produce an acceleration of 1 m/s² in a body of mass 1 kg.

13. State Newton’s Third Law of Motion.

For every action, there is an equal and opposite reaction. The forces act on different bodies and are equal in magnitude but opposite in direction.

14. Give examples of Newton’s Third Law.

Examples include recoil of a gun, walking on ground, swimming, and rocket propulsion.

15. What is impulse?

Impulse is the product of force and time for which it acts. It is equal to the change in momentum of the body.

16. Write the relation between impulse and momentum.

Impulse equals change in momentum. A large force acting for a short time can produce the same change as a small force acting for a longer time.

17. State Law of Conservation of Momentum.

The total momentum of a system remains constant provided no external force acts on it.

18. Explain conservation of momentum with example.

When a gun fires a bullet, the bullet moves forward and the gun recoils backward. The total momentum before and after firing remains constant.

19. Differentiate between mass and weight.

Mass is the amount of matter in a body and remains constant. Weight is the force of gravity acting on the body and varies with gravitational field.

20. Define balanced forces.

Balanced forces are equal in magnitude and opposite in direction. They cancel each other and do not change the state of motion.

21. Define unbalanced forces.

Unbalanced forces are forces that do not cancel each other. They result in acceleration or change in motion of the object.

22. What is friction?

Friction is the force that opposes the relative motion between two surfaces in contact.

23. What is thrust?

Thrust is the force acting perpendicular to a surface.

24. Define pressure and write its formula.

Pressure is force acting per unit area. Pressure = Force / Area.

25. Why do camels have broad feet?

Camels have broad feet to increase surface area and reduce pressure on sand, preventing them from sinking.

26. Why are knives sharp?

Sharp knives have smaller area of contact, increasing pressure and making cutting easier.

27. Explain why passengers jerk in bus.

Passengers jerk due to inertia. When bus starts or stops suddenly, their body resists change in motion.

28. Why do we use seat belts?

Seat belts prevent passengers from moving forward during sudden stops due to inertia of motion.

29. Why do airbags reduce injury?

Airbags increase the time of impact, reducing the force experienced during collision.

30. Explain rocket propulsion.

Rockets move upward by expelling gases downward. According to Newton’s Third Law, action of gases downward produces upward reaction force.

31. What is net force?

Net force is the vector sum of all forces acting on a body.

32. What happens if net force is zero?

The body continues in its state of rest or uniform motion.

33. What is acceleration?

Acceleration is the rate of change of velocity per unit time.

34. State relationship between force and acceleration.

Acceleration is directly proportional to force.

35. State relationship between mass and acceleration.

Acceleration is inversely proportional to mass.

36. Explain recoil of gun.

When a bullet is fired, it moves forward with high momentum. According to Newton’s Third Law, the gun experiences an equal and opposite reaction, causing it to move backward. This backward movement is called recoil of the gun.

37. Define isolated system.

An isolated system is a system on which no external force acts. In such a system, the total momentum remains constant according to the law of conservation of momentum.

38. Why does a heavy truck have more inertia?

Inertia depends on mass. Since a truck has greater mass compared to a car or bicycle, it has greater resistance to change in motion. Therefore, it possesses more inertia.

39. What is stopping time?

Stopping time is the time taken by a moving object to come to rest after applying force such as brakes. Increasing stopping time reduces the force of impact.

40. Explain collision briefly.

Collision occurs when two bodies strike each other for a short time. During collision, momentum is transferred, but total momentum remains conserved if no external force acts.

41. Why is momentum a vector quantity?

Momentum depends on both mass and velocity. Since velocity has direction, momentum also has direction, making it a vector quantity.

42. What happens to momentum if velocity doubles?

Since momentum equals mass × velocity, if velocity doubles and mass remains constant, momentum also doubles.

43. What is effect of increasing mass on momentum?

Momentum is directly proportional to mass. If mass increases while velocity remains constant, momentum increases proportionally.

44. Explain why walking is possible.

While walking, we push the ground backward with our feet. According to Newton’s Third Law, the ground exerts an equal and opposite reaction force that pushes us forward.

45. Define gravitational force.

Gravitational force is the attractive force between two masses. On Earth, it pulls objects toward the center of the planet, giving them weight.

46. Explain role of friction in daily life.

Friction helps us walk, write, and hold objects. It prevents slipping and allows vehicles to move. However, excessive friction causes wear and tear.

47. What is effect of balanced forces on shape?

Balanced forces may not change the motion of a body but can change its shape. For example, squeezing a rubber ball changes its shape without moving it.

48. Why do porters use turbans?

Porters use turbans to increase the area of contact between the load and head. Increased area reduces pressure and makes carrying heavy loads easier.

49. Explain relation between impulse and safety.

Impulse equals change in momentum. Increasing the time of impact reduces the force experienced. Safety devices like airbags and helmets increase impact time to reduce injury.

50. What is significance of Newton’s laws?

Newton’s laws explain motion of objects under force. They form the foundation of mechanics and help understand everyday phenomena like walking, driving, and flying rockets.

51. Explain momentum in collision.

During collision, bodies exchange momentum. Although individual momentum may change, total momentum of the system remains conserved if no external force acts.

52. What happens in head-on collision?

In head-on collision, two bodies moving in opposite directions collide. Momentum is redistributed between them, but total momentum remains conserved.

53. Define rate of change of momentum.

Rate of change of momentum is the change in momentum divided by time taken. According to Newton’s Second Law, it equals the applied force.

54. Explain force in terms of momentum.

Force is defined as the rate of change of momentum. A larger force produces a greater change in momentum in a shorter time.

55. Why does heavy object need more force?

Heavier objects have greater mass and inertia. According to F = ma, more force is needed to produce the same acceleration in a heavier object.

56. Explain stopping distance.

Stopping distance is the distance covered by a vehicle after brakes are applied. It depends on speed, mass, and friction between tires and road.

57. Define contact force.

Contact force is a force that acts only when two objects are in physical contact, such as friction and muscular force.

58. Define non-contact force.

Non-contact force acts without physical contact between objects. Examples include gravitational, magnetic, and electrostatic forces.

59. Example of non-contact force.

Gravitational force is a non-contact force that pulls objects toward Earth without physical contact.

60. Why is mass measure of inertia?

Inertia depends on mass. Greater the mass of an object, greater its resistance to change in motion. Hence, mass is considered a measure of inertia.

61. Explain sudden jerk in lift.

When lift starts or stops suddenly, passengers feel a jerk due to inertia. Their body resists change in motion, causing temporary imbalance.

62. Why do athletes bend while landing?

Athletes bend their knees while landing to increase stopping time, reducing impact force and preventing injury.

63. What is direction of friction?

Friction always acts opposite to the direction of motion or attempted motion.

64. Explain effect of force on shape.

Force can deform objects. For example, stretching a rubber band or compressing a sponge changes their shape.

65. Define constant velocity.

Constant velocity means an object moves with uniform speed in a fixed direction without any change.

66. What happens when external force is absent?

If no external force acts, a body remains at rest or continues in uniform motion according to Newton’s First Law.

67. Explain conservation law in explosion.

In explosion, fragments fly apart in different directions. Though individual momenta differ, total momentum before and after explosion remains constant.

68. Why does bullet move fast?

A large force acts on the bullet for a short time, giving it high acceleration and large momentum.

69. Define action force.

Action force is the force exerted by one body on another.

70. Define reaction force.

Reaction force is the equal and opposite force exerted by the second body in response to action.

71. Why is action-reaction pair equal?

According to Newton’s Third Law, action and reaction are always equal in magnitude and opposite in direction.

72. What is magnitude of reaction?

The magnitude of reaction force is always equal to the magnitude of action force.

73. Explain motion in terms of force.

Motion changes only when unbalanced force acts. Force produces acceleration, changing speed or direction.

74. Why are heavier vehicles harder to stop?

Heavier vehicles have greater momentum and inertia, requiring larger force and longer distance to stop.

75. Define stopping force.

Stopping force is the force applied to bring a moving object to rest.

76. Why does pressure decrease with area?

Pressure equals force divided by area. Increasing area reduces pressure if force remains constant.

77. Explain impulse in cricket.

While catching a ball, a player moves hands backward to increase time of impact, reducing force and preventing injury.

78. Define vector quantity.

A vector quantity has both magnitude and direction, such as force and momentum.

79. Define scalar quantity.

A scalar quantity has only magnitude and no direction, such as mass and time.

80. Give examples of vector quantities.

Force, velocity, acceleration, and momentum are examples of vector quantities.

81. What law applies to space motion?

Newton’s laws of motion apply to motion of planets, satellites, and rockets in space.

82. Why do astronauts float?

Astronauts float due to microgravity conditions where gravitational pull is very weak.

83. Define gravitational acceleration.

Gravitational acceleration is acceleration due to gravity, approximately 9.8 m/s² on Earth.

84. Why does weight vary on Moon?

Weight depends on gravitational force. Since Moon’s gravity is weaker, weight decreases there.

85. Explain friction in braking.

Brakes apply frictional force between brake pads and wheels, reducing speed and stopping vehicle.

86. Define resisting force.

Resisting force opposes motion of an object, such as friction or air resistance.

87. What force is needed for uniform motion?

No net force is required to maintain uniform motion in absence of friction.

88. Why do rockets not need air?

Rockets work on action-reaction principle and do not require air; they carry their own fuel and oxidizer.

89. Explain Newton’s contribution.

Newton formulated three laws of motion that explain relationship between force and motion, forming the foundation of classical mechanics.

90. What is fundamental principle of motion?

Motion changes only when unbalanced force acts, as stated in Newton’s laws.

91. Explain inertia in daily life.

Inertia explains jerks in vehicles, dust removal by beating carpets, and falling forward during sudden stops.

92. What is law behind swimming?

Swimming works on Newton’s Third Law. Water pushed backward exerts reaction force pushing swimmer forward.

93. Define uniform acceleration.

Uniform acceleration means constant change in velocity per unit time.

94. Explain pressure formula briefly.

Pressure equals force divided by area. Smaller area increases pressure for same force.

95. Why do trucks cause more damage?

Trucks have larger mass and momentum, producing greater force during collision.

96. Define mechanical system.

A mechanical system is a group of bodies interacting under forces.

97. Why do we fall forward?

We fall forward when bus stops due to inertia of motion.

98. Define change in momentum.

Change in momentum equals final momentum minus initial momentum.

99. What is effect of time on force?

Increasing time of impact decreases force for same change in momentum.

100. Summarize laws of motion briefly.

First law explains inertia, second law gives F = ma, and third law states action-reaction principle.

LONG ANSWER QUESTIONS

1. State and explain Newton’s First Law of Motion with examples.

Newton’s First Law of Motion states that a body remains at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force. This law is also called the Law of Inertia because it describes the tendency of objects to resist changes in their state of motion. For example, passengers fall backward when a bus starts suddenly because their body tends to remain at rest. Similarly, when a moving bus stops suddenly, passengers fall forward due to inertia of motion. This law explains why seat belts are necessary in vehicles. It also forms the foundation of classical mechanics by establishing that force is required only to change motion, not to maintain it.

2. Define inertia and describe its types with examples.

Inertia is the property of matter by which it resists any change in its state of rest, motion, or direction. It depends on mass; greater the mass, greater the inertia. There are three types of inertia. Inertia of rest is the tendency to remain at rest, such as a book lying on a table. Inertia of motion is the tendency to remain in motion, like passengers falling forward when a bus stops suddenly. Inertia of direction is the tendency to resist change in direction, such as a stone flying off tangentially when a string breaks during circular motion. Inertia plays a crucial role in understanding motion and safety measures in vehicles.

3. State and explain Newton’s Second Law of Motion. Derive F = ma.

Newton’s Second Law states that the rate of change of momentum of a body is directly proportional to the applied force and occurs in the direction of force. Let a body of mass m change its velocity from u to v in time t. Initial momentum = mu and final momentum = mv. Change in momentum = m(v − u). Rate of change of momentum = m(v − u)/t. Since acceleration a = (v − u)/t, the rate of change of momentum becomes ma. Therefore, Force F = ma. This law explains why heavier objects require greater force to achieve the same acceleration and forms the mathematical foundation of dynamics.

4. Define momentum. Explain its importance in collisions.

Momentum is the product of mass and velocity of a body and is represented as p = mv. It is a vector quantity because it has direction. Momentum plays an important role in collisions. During a collision, objects exchange momentum, but the total momentum of the system remains constant if no external force acts. This principle is known as the law of conservation of momentum. For example, when a gun fires a bullet, the bullet moves forward and the gun recoils backward. Though their velocities differ, the total momentum before and after firing remains the same. Momentum helps explain accidents, explosions, and vehicle crashes scientifically.

5. State and explain Newton’s Third Law of Motion with examples.

Newton’s Third Law states that for every action, there is an equal and opposite reaction. These forces act on different bodies but are equal in magnitude and opposite in direction. For example, while walking, we push the ground backward, and the ground pushes us forward. In rocket propulsion, gases expelled downward produce an upward reaction force that lifts the rocket. Recoil of a gun is another example where the bullet moves forward and the gun moves backward. This law explains motion in swimming, flying birds, and jumping. It shows that forces always occur in pairs and are fundamental in understanding interactions between objects.

6. State and prove the Law of Conservation of Momentum.

The Law of Conservation of Momentum states that the total momentum of an isolated system remains constant if no external force acts on it. Consider two bodies of masses m₁ and m₂ moving with velocities u₁ and u₂. After collision, their velocities become v₁ and v₂. According to Newton’s Third Law, forces during collision are equal and opposite. Therefore, change in momentum of one body equals and opposes that of the other. Hence, m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂. This proves total momentum remains conserved. This law explains gun recoil, rocket motion, and collisions in daily life.

7. What is impulse? Explain its relation with safety devices.

Impulse is the product of force and the time for which it acts. It is equal to the change in momentum of a body. Mathematically, Impulse = F × t. For the same change in momentum, increasing the time of impact decreases the force applied. Safety devices like airbags, helmets, and seat belts increase the time over which force acts, reducing injury. For example, cricketers pull their hands backward while catching a ball to increase impact time. Thus, impulse plays a significant role in designing protective equipment and reducing accident injuries.

8. Differentiate between mass and weight.

Mass is the amount of matter in a body and is measured in kilograms. It remains constant everywhere. Weight is the gravitational force acting on a body and is calculated as W = mg. Weight is measured in Newtons and changes depending on gravitational acceleration. On the Moon, weight decreases due to lower gravity, but mass remains unchanged. Mass is a scalar quantity, whereas weight is a vector quantity because it has direction toward Earth’s center. Understanding the difference helps explain why astronauts feel lighter in space but still retain their mass.

9. Explain balanced and unbalanced forces with examples.

Balanced forces are equal in magnitude and opposite in direction. They cancel each other and do not change the state of motion. For example, a book resting on a table experiences gravitational force downward and normal reaction upward. Unbalanced forces are unequal forces that result in acceleration. For instance, pushing a stationary cart with force causes it to move because forces are unbalanced. Balanced forces can change shape but not motion, while unbalanced forces change speed, direction, or both. This concept is essential to understand motion and Newton’s laws.

10. Explain friction and its advantages and disadvantages.

Friction is a force that opposes relative motion between two surfaces in contact. It is necessary for walking, writing, and driving. Without friction, vehicles cannot move, and we cannot grip objects. However, friction also has disadvantages. It causes wear and tear of machine parts and produces heat, leading to energy loss. Lubricants are used to reduce friction in machines. Thus, friction is both helpful and harmful, depending on the situation

Related Story

Leave a Reply

Your email address will not be published. Required fields are marked *