9th Class (CBSE) Science: Force and Laws of Motion

Question: What is inertia? Explain different types of inertia. Give 3 examples in daily life which shows inertia.

Answer:

Inertia: The natural tendency of an object to resist change in their state of rest or of motion is called inertia.
The mass of an object is a measure of its inertia. Its S.I. unit is kg.

Types of inertia:

Inertia of rest: The object at rest will continue to remain at rest unless acted upon by an external unbalanced force.
Inertia of motion: The object in the state of uniform motion will continue to remain in motion with same speed and direction unless it is acted upon by an external unbalanced force.

Three examples of inertia in daily life are:

1. When we are travelling in a vehicle and sudden brakes are .applied we tend to fall forward.
2. When we shake the branch of a tree vigorously, leaves fall down.
3. If we want to remove the dust from carpet we beat the carpet so that dust fall down.

Question: Which of the following has more inertia:
(a) a rubber ball and a stone of the same size?
(b) a bicycle and a train?
(c) a five-rupees coin and a one-rupee coin?

Answer:
(a) A stone of the same size
(b) a train
(c) a five-rupees coin

As the mass of an object is a measure of its inertia, objects with more mass have more inertia.

Question: In the following example, try to identify the number of times the velocity of the ball changes.
“A football player kicks a football to another player of his team who kicks the football towards the goal The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team”.
Also identify the agent supplying the force in each case.

Answer:

The velocity of football changed four times.

Question: Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.

Answer: When the tree’s branch is shaken vigorously the branch attain motion but the leaves stay at rest. Due to the inertia of rest, the leaves tend to remain in its position and hence detaches from the tree to fall down.

Question: Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?

Answer: When a moving bus brakes-to a stop: When the bus is moving, our body is also in motion, but due to sudden brakes, the lower part of our body comes to rest as soon as the bus stops. But the upper part of our body continues to be in motion and hence we fall in forward direction due to inertia of motion.

When the bus accelerates from rest we fall backwards: When the bus’ is stationary our body is at rest but when the bus accelerates, the lower part of our body being in contact with the floor of the bus comes in motion, but the upper part of our body remains at rest due to inertia of rest. Hence we fall in backward direction.