Extra 30 short questions and answers from the chapter-9 Force and Laws of Motion chapter of Class 9 CBSE Science:-
1. Define force. What are its effects?
Answer:
Force is a push or pull acting on an object that changes its state of motion or shape.
Effects of Force:
It can change the speed of an object.
It can change the direction of motion.
It can change the shape of an object.
It can make a stationary object move.
2. State Newton’s First Law of Motion with an example.
Answer: Newton’s First Law states that an object remains at rest or in uniform motion unless acted upon by an external force.
Example: A book remains on a table until someone pushes it.
3. What is inertia? Explain its types with examples.
Answer: Inertia is the tendency of an object to resist a change in its state of motion.
Types:
Inertia of Rest: A passenger leans backward when a bus starts suddenly.
Inertia of Motion: A passenger leans forward when a bus stops suddenly.
Inertia of Direction: A car turning sharply causes passengers to move sideways.
4. What is the relation between mass and inertia?
Answer:
Inertia is directly proportional to mass. Heavier objects have greater inertia.
5. State and explain Newton’s Second Law of Motion.
Answer: Newton’s Second Law states that the force acting on an object is equal to the rate of change of its momentum.
Formula: F=maF = maF=ma
Example: A heavier object requires more force to accelerate than a lighter one.
6. Define momentum and give its SI unit.
Answer: Momentum is the product of mass and velocity.
Formula: p=mvp = mvp=mv
SI Unit: kg m/s
8. Define impulse and give its formula.
Answer: Impulse is the product of force and time during which the force acts.
Formula: Impulse = Force × Time = Change in momentum
9. Explain Newton’s Third Law of Motion with an example.
Answer:
Newton’s Third Law states that for every action, there is an equal and opposite reaction.
Example:
A rocket moves upward because gases are expelled downward.
10. Why does a gun recoil when fired?
Answer:
When a bullet is fired, it moves forward with momentum.
The gun moves backward (recoils) with equal and opposite momentum due to Newton’s Third Law.
11. Why do athletes run before taking a long jump?
Answer:
Running increases their velocity, which increases momentum, helping them jump farther.
12. Why is it easier to stop a moving cycle than a moving truck?
Answer:
A truck has more mass and momentum, requiring more force to stop than a cycle.
13. Why are airbags used in cars?
Answer:
Airbags increase the time of impact during an accident, reducing the force acting on passengers.
14. How does a cricketer lower his hands while catching a ball?
Answer:
Lowering hands increases the time of impact, reducing the force exerted on the hands.
15. Why do passengers fall forward when a bus stops suddenly?
Answer:
Due to
inertia of motion
, the upper body continues moving forward when the bus stops.
16. Explain why a rocket moves upward.
Answer:
The rocket expels gases downward with force, and the reaction force pushes it upward (Newton’s Third Law).
17. Why is it difficult to walk on a slippery surface?
Answer:
Friction provides the necessary reaction force for walking. On a slippery surface, friction is low, making it hard to walk.
18. What happens to momentum when two objects collide?
Answer:
Law of Conservation of Momentum:
The total momentum before and after collision remains constant if no external force acts.
19. State the law of conservation of momentum.
Answer:
The total momentum of a system remains constant if no external force acts on it.
20. Why does a rocket have multiple stages?
Answer:
To reduce mass gradually and increase acceleration according to Newton’s Second Law.
21. Why does a moving bus come to rest when brakes are applied?
Answer:
The force of friction between the tires and the road slows it down.
22. Why do wet clothes dry faster when they are wrung?
Answer:
Wringing removes water by applying centrifugal force, reducing inertia of rest.
23. How do seat belts help in preventing injuries?
Answer:
They stop passengers from moving forward due to inertia when a car stops suddenly.
24. Explain why a freely falling body has increasing velocity.
Answer:
Gravity acts as an unbalanced force, continuously increasing velocity.
25. Why does an inflated balloon fly forward when air is released?
Answer:
The air moves backward (action), and the balloon moves forward (reaction).
27. Why does a person feel a jerk when a car starts moving suddenly?
Answer:
Due to
inertia of rest
, the body resists motion initially.
28. If action and reaction forces are equal and opposite, why don’t they cancel each other?
Answer:
Because they act on different objects.
29. A ball is thrown vertically upward. What force acts on it?
Answer:
The force of gravity acts downward, reducing velocity until the ball stops and falls.
30. Why do astronauts experience weightlessness in space?
Answer:
In space, gravitational force is weak.
Astronauts and the spaceship fall together, creating a sensation of weightlessness.
