NCERT Solutions Class 9 Science Chapter 8 - Force and Laws of Motion

Force and Laws of Motion – Important Questions with Answers

1. What is force?
Answer: Force is a push or pull that can change the state of motion or shape of an object.

2. What is the SI unit of force?
Answer: The SI unit of force is the newton (N).

3. Define balanced and unbalanced forces.
Answer:

• Balanced forces do not change the state of motion of an object.

• Unbalanced forces change the state of motion or cause acceleration.

4. What happens when two equal and opposite forces act on an object?
Answer: The object remains in its state of rest or uniform motion (balanced force).

5. State Newton’s First Law of Motion.
Answer: An object remains at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force.

6. What is inertia?
Answer: Inertia is the tendency of an object to resist any change in its state of motion or rest.

7. On what factor does inertia depend?
Answer: Inertia depends on the mass of the object. Greater the mass, greater the inertia.

8. State Newton’s Second Law of Motion.
Answer: The force acting on an object is equal to the product of its mass and acceleration.
F = ma

9. Define 1 newton of force.
Answer: 1 newton is the force required to accelerate a mass of 1 kg by 1 m/s².

10. A force of 10 N is applied on a mass of 2 kg. Find the acceleration.
Answer:
F = ma → a = F/m = 10 / 2 = 5 m/s²

11. State Newton’s Third Law of Motion.
Answer: For every action, there is an equal and opposite reaction.

12. Why does a gun recoil when fired?
Answer: Due to Newton’s Third Law – the bullet moves forward, and the gun experiences an equal and opposite reaction backward.

13. What is momentum?
Answer: Momentum is the product of mass and velocity of an object.
Momentum (p) = mass × velocity

14. What is the SI unit of momentum?
Answer: The SI unit is kg·m/s.

15. State the law of conservation of momentum.
Answer: The total momentum of a system remains constant if no external force acts on it.

16. A ball of mass 0.5 kg is moving with a velocity of 10 m/s. Find its momentum.
Answer:
p = mv = 0.5 × 10 = 5 kg·m/s

17. Why do passengers fall forward when a moving bus stops suddenly?
Answer: Due to inertia of motion, the body tends to remain in motion while the bus stops.

18. Why do passengers fall backward when a stationary bus suddenly starts?
Answer: Due to inertia of rest, the body tends to remain at rest while the bus moves forward.

19. How does a karate player break a slab of ice with a single blow?
Answer: By applying a large force in a very short time, producing a large momentum change.

20. A 2 kg mass is moving at 4 m/s. A force stops it in 2 seconds. What is the force applied?
Answer:
Initial momentum = 2 × 4 = 8 kg·m/s
Final momentum = 0
Change in momentum = 8
Force = Change in momentum / Time = 8 / 2 = 4 N

21. What does the slope of a momentum-time graph represent?
Answer: Force

22. What causes the motion of a rocket?
Answer: The expulsion of gases downward (action) produces an upward thrust (reaction) – Newton’s Third Law.

23. Name the factor affecting the rate of change of momentum.
Answer: Force and time duration.

24. A body of mass 3 kg accelerates at 2 m/s². Find the force acting on it.
Answer:
F = ma = 3 × 2 = 6 N

25. Why do athletes run a few steps before taking a long jump?
Answer: To gain momentum which helps in jumping further due to greater initial velocity.