Case Study Questions for Class 9 Science Chapter 6 How Forces Affect Motion (Exploration Book) 2026-27

By pulling the hands back, the fielder increases the time it takes for the ball’s velocity to reach zero. According to Newton’s second law, increasing the time decreases the acceleration (deceleration), which in turn significantly reduces the force exerted on the fielder’s hands, preventing injury.

Airbags act as a soft cushion that compresses as the passenger hits it. This compression extends the duration of the impact. Because the change in velocity happens over a longer time, the deceleration is smaller, resulting in a much lower force being exerted on the passenger’s body.

Using the mathematical expression for Newton’s second law of motion (F = ma), we multiply the mass of the football (0.4 kg) by its acceleration (50 m/s²). This gives a net force of 20 N.

The magnitude of acceleration is directly proportional to the magnitude of the net force applied (a = F/m). Therefore, if the force is doubled and the mass is kept constant, the acceleration will also double.

While walking, a person pushes the ground backwards. In response, the ground applies an equal and opposite reaction force in the forward direction. This forward force, which actually propels the person, is provided by the force of friction between the ground and the feet.

A rocket does not need air or the ground to push against. It functions on the principle of action and reaction. The engine produces exhaust gases and expels them rapidly downwards (action). The gases, in turn, exert an equal and opposite force upwards on the rocket (reaction), causing it to lift off.

According to Newton’s third law of motion, for every action, there is an equal and opposite reaction. If the paddle exerts a force of 150 N on the water, the water simultaneously exerts an exact equal force of 150 N back onto the paddle.

Forces can only cancel each other out if they are acting on the same object (like two teams pulling the same rope in a tug of war). In Newton’s third law, the action and reaction forces act on two completely different objects (e.g., the foot and the ground), so they do not cancel each other out.