Class 9 Science: Chapter 9 Force and Laws of Motion

  Chapter 9 Force and Laws of Motion

Question 1: 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: We know  that Inertia is the measure of the mass of the body. The greater is the mass of the body; the greater is its inertia and vice-versa.

(a) Mass of a stone is more than the mass of a rubber ball for the same size. Hence, inertia of the stone is greater than that of a rubber ball.

(b) Mass of a train is more than the mass of a bicycle. Hence, inertia of the train is greater than that of the bicycle.

(c) Mass of a five-rupee coin is more than that of a one-rupee coin. Hence, inertia of the five-rupee coin is greater than that of the one-rupee coin.

Question 2: Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.

Answer:  Initially, the branch of tree and its leaves  are in state of rest. But when force is applied on the branch of a tree, the tree comes in motion but due to inertia of rest, the leaves  remain in state of rest hence, falls down.

Question 3: 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:

1. When a moving bus brakes to a stop, the lower portion of our body also comes to rest but the upper part of our body remains in motion due to inertia of motion. Hence, we fall in the forward direction.
2. When a bus accelerates from rest, the lower portion of our body also comes in motion with the bus but the upper part of our body remains at rest due to inertia of rest. Hence we fall backwards.

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