# Laws of Motion - Short Question Answers (Solution) | Physics Grade XI

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#### Kinematics

Introduction to Kinematics

# Laws of Motion - Short Question Answers

1. Can a body be at rest and in motion at the same time?

Yes, a body can be said to be in rest and in motion at the same time. Nothing is in absolute rest or in motion. A body is said to be in a state of rest or motion with respect to its surrounding. A passenger on a bus is in motion for a person outside the bus whereas he is in rest for another passenger on the same bus.

2. A woman let go of her briefcase but it does not fall to the floor. How is the elevator moving?

If the briefcase doesn’t fall to the floor, then the elevator is moving with the acceleration equal to the acceleration due to gravity i.e. 9.8 m/s because the briefcase is experiencing free fall.

3. It hurts our foot more to kick a big rock than a small pebble. Why?

A big rock has more mass than that of a small pebble. This means big rock has more change in momentum than the small pebble. So, when we kick a big rock, the force exerted by it is more than the force exerted by the smaller pebble. So, it hurts our foot more to kick a big rock than a small pebble.

4. In rain, a scooter may slip in the turning of a road. Why?

In a rain, the coefficient of friction between the tires of the scooter and the road is very less due to small water droplets making the road more smooth and slippery lowering the frictional force between the scooter and the road. Due to less frictional force, the scooter lacks adequate centripetal force and falls down.

5. Is friction a necessary evil? Explain.

Despite having a lot of disadvantages, friction is a necessary evil. Friction reduces the efficiency of a machine, produces more and more heat however it is necessary because of the following reasons:

• It is friction due to which we can walk, sit, and work,
• It is friction that stops the vehicles after applying brakes.
• If friction would not be there, it would not be possible to fasten nails on walls, hold the screws, and shaping metals.

6. It is easier to run on the road than in sand. Why?

When we run on the road, as we push the road gives a necessary reaction to us to run. But when we push sand, it gets pushed away and cannot give a necessary amount of reaction and we get small reaction due to which we feel difficult to run in the sand.

7. When a balloon filled with air and its mouth downwards is released, it moves upward why?

According to Newton’s third law of motion, "Every action has an equal and opposite reaction". In the case of the balloon, when it is released, the air inside it escapes with higher velocity (action) and as a result, it moves upward quickly (reaction).

8. What do you mean by cold welding?

Cold welding is a process of joining two flat surfaces of solids without fusion than by any liquid or other process.

Practice yourself following questions;

1. A cricketer moves his hand while catching balls?
2. When we shake trees, fruits fall. Why?
3. A man drops his briefcase in an elevator but it does not fall to the floor. What can be concluded about the situation?
4. When a large heavy truck and a car collide, it is more likely to hurt the occupants of the car.
5. Why a man getting out of a moving bus must run in the same direction for a certain distance?
6. Explain hoe newton’s first law follows the second law of motion.
7. State newton’s second law of motion.

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