CBSE Class 9 Science Chapter 9: Force and Laws of Motion NCERT Solutions

Solution: Inertia is the resistance of an object to changes in its state of motion or rest. It is directly proportional to the mass of the object. Therefore, the object with a greater mass will have greater inertia.

1. A stone of the same size as a rubber ball generally has a greater mass. Thus, the stone has more inertia than the rubber ball.
2. A train has a significantly larger mass compared to a bicycle. Therefore, the train has much more inertia than the bicycle.
3. A five-rupees coin typically has more mass than a one-rupee coin. Consequently, the five-rupees coin possesses greater inertia than the one-rupee coin.

Solution: The velocity of the football changes four times in this scenario. Velocity is a vector quantity, meaning it has both magnitude (speed) and direction. A change in either speed or direction constitutes a change in velocity.

1. First change: When the first player kicks the football. The ball starts from rest (zero velocity) and gains a certain speed and direction. The player is the agent supplying the force.
2. Second change: When the second player kicks the football towards the goal. The direction of the ball's motion is changed, which alters its velocity. The second player is the agent supplying the force.
3. Third change: When the goalkeeper collects the football. The ball, which was in motion, comes to rest (or its speed significantly reduces). The goalkeeper applies a force to stop the ball.
4. Fourth change: When the goalkeeper kicks the football towards a player of his own team. The ball starts from rest (or near rest) and gains a new speed and direction. The goalkeeper is the agent supplying the force.

Therefore, the velocity of the ball changes four times, with a player or goalkeeper acting as the agent supplying the force in each instance.

Solution: When the branch of a tree is shaken vigorously, it is forced to move back and forth. However, the leaves attached to the branch tend to resist this change in their state of motion due to their inertia. Inertia is the tendency of an object to remain in its state of rest or uniform motion. Because the leaves tend to remain at rest (or in their original position) while the branch moves, the connection between the leaves and the branch is stressed. If the shaking is vigorous enough, this stress overcomes the strength of the attachment, causing the leaves to detach and fall.

Solution: This phenomenon is explained by the concept of inertia, which is the tendency of an object to resist changes in its state of motion.

When a moving bus brakes to a stop: While you are inside the moving bus, your body is also in motion with the same velocity as the bus. When the driver applies the brakes, the bus decelerates and comes to a stop. However, your body, due to inertia, tends to continue its state of motion. This tendency to keep moving forward causes you to lurch or fall in the forward direction relative to the decelerating bus.

When a bus accelerates from rest: When the bus starts moving from rest, it accelerates forward. Your body, again due to inertia, tends to remain in its state of rest. As the bus moves forward beneath you, your body lags behind its motion. This makes it appear as if you are falling or being pushed backward relative to the accelerating bus.