NCERT Solutions for Class 9 Science Chapter 9
1. Suppose you are travelling in a car, why do you fall forwards when the car comes to stop? And why do you tend to fall towards your back when the car starts to move from rest?
Answer: When the car is moving, the traveller in it is also in the state of motion. When the brakes are applied, the car suddenly comes to the state of rest. At this time, the lower part of the traveller’s body, in contact with the floor of the car, comes to rest, but the upper part of the traveller’s body is in the inertia of motion. Hence the traveller falls forward.
When the car begins to move from the state of rest, the lower part of the traveller’s body, in contact with the floor of the car, comes to the state of motion. But the upper part of the traveller’s body is still in the inertia of rest. Hence, the traveller seems to fall backward, when the car begins to move.
2. Why do the fruits fall off the branches, when the fruit-bearing branches of a tree are shaken?
Answer: When the fruit-bearing branches of a tree are shaken, the force exerted on the branches, set them in a to and fro motion. Meanwhile, the fruits are still in the inertia of rest. This causes the fruits to oppose the direction of the motion of the branches, thereby, detaching them from the branches. When the fruits are detached from the branches, the force of gravity pulls them downwards and the fruits fall off the branches onto the ground.
3. What happens when you are sitting in a moving train and you flip a coin when the train is:
Begins to move
Moving at a constant speed
Answer: a) Accelerating: When you flip a coin inside the train, which just begins to move, the coin will fall backward from your position. This happens because when the coin was flipped the train was in the state of rest, and when the train begins to move, the coin is still in the inertia of rest and falls behind you as if the train were stationary.
b) When the train is moving at a constant speed: When you flip a coin inside a train moving at a constant speed, it will fall right back in your hands when you are sitting in a train moving at a constant speed, because, the coin is moving in the same direction as the train at the same speed.
c) Slowing Down: When you flip a coin inside a train when it is slowing down, the coin will fall ahead of your position on the train. It is because the coin is in the inertia of motion and it is still moving with the previous speed when the train slows down.
4. Why do we see dust particles coming out of a carpet when it is beaten with a rod, in the sunlight?
Answer: When the carpet is beaten with a rod, it is set to motion by the force exerted on it by the rod. Meanwhile, the dust particles on the carpet are still in the inertia of rest. This causes the dust particles to oppose the motion of the carpet and get comes out of it. The dust particles become visible when the carpet is beaten with the rod in sunlight.
There are several other questions on reasoning, numerical sums, and definitions in the NCERT Solutions for Class 9 Science Chapter 9, available on Vedantu. You can download the PDF of these NCERT Solutions for free and refer to them for a better understanding of the concepts.