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

Solution: Inertia is the property of an object that resists changes in its state of motion or rest. It is directly proportional to the mass of the object. This means that an object with a greater mass will have greater inertia, and an object with a smaller mass will have less inertia. We need to compare the masses of the objects given in each part:

1. A stone of the same size as a rubber ball generally has a greater mass than the rubber ball. Therefore, the stone has more inertia.
2. A train is significantly more massive than a bicycle. Thus, the train has much greater inertia.
3. A five-rupees coin is typically made of different material or has a larger size/thickness than a one-rupee coin, giving it a greater mass. Hence, the five-rupees coin has more inertia.

Solution: The velocity of the football changes four times during this sequence of events. 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 supplies the force.
2. Second change: When the second player kicks the football towards the goal. The direction of the ball's motion changes, even if its speed remains similar. A change in direction means a change in velocity. The second player supplies the force.
3. Third change: When the goalkeeper collects the football. The ball, moving with a certain velocity, is stopped, bringing its speed to zero. This is a significant change in velocity. The goalkeeper supplies an opposing force.
4. Fourth change: When the goalkeeper kicks the football towards a player of his own team. The ball starts from rest (zero velocity) and gains a new speed and direction. The goalkeeper supplies the force.

Solution: When the branch of a tree is shaken vigorously, it undergoes rapid to-and-fro motion. However, the leaves attached to the branch tend to remain in their state of rest or uniform motion due to their inertia. Inertia is the resistance of any physical object to any change in its state of motion; this includes changes to its speed, direction, or state of rest. Because the leaves resist this sudden change in motion imposed by shaking the branch, they may become detached from the branch. The force required to move the leaves along with the branch is greater than the force holding them to the branch, causing them to fall off.

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 the bus brakes to a stop: While you are inside the moving bus, your body is also moving at the same speed as the bus. When the driver applies the brakes, the bus suddenly slows down or stops. However, due to inertia, your body tends to continue moving forward at the original speed. This causes you to be thrown forward relative to the decelerating bus.

When the bus accelerates from rest: When the bus starts moving from rest and accelerates forward, your body, due to inertia, tends to remain in its state of rest. As the bus moves forward, your body lags behind momentarily. This makes it appear as if you are falling backward relative to the accelerating bus.