9th Class (CBSE) Science: Force and Laws of Motion

Question: When you kick a football it flies away but when you kick a stone you get huh why?

Answer: This is because stone is heavier than football and heavier objects offer larger inertia. When we kick a football its mass is less and inertia is also less so force applied by our kick acts on it and hence it shows larger displacement but in case of stone, it has larger mass and offers larger inertia. When we kick (action) the stone it exerts an equal and opposite force (reaction) and hence it hurts the foot.

Question: If a person jumps from a height on a concrete surface he gets hurt. Explain.

Answer: When a person jumps from a height he is in state of inertia of motion. When he suddenly touches the ground he comes to rest in a very short time and hence the force exerted by the hard concrete surface on his body is very high, and the person gets hurt.

Question: What is the relation between Newton’s three laws of motion?

Answer: Newton’s first law explains about the unbalanced force required to bring change in the position of the body.

Second law states / explains about the amount of force required to produce a given acceleration.

And Newton’s third law explains how these forces acting on a body are interrelated.

Question: Give any three examples in daily life which are based on Newton’s third law of motion.

Answer: Three examples based on Newton’s third law are :

1. Swimming: We push the water backward to move forward.
   action – water is pushed behind
   reaction – water pushes the swimmer ahead
2. Firing gun: A bullet fired from a gun and the gun recoils.
   action – gun exerts force on the bullet
   reaction – bullet exerts an equal and opposite force on the gun
3. Launching of rocket:
   action – hot gases from the rocket are released
   reaction – the gases exert upward push to the rocket

Question: A bullet of m.ass 20 g is horizontally fired with a velocity 150 m/s from a pistol of mass 2 kg. What is the recoil velocity of the pistol?

Answer:

Negative sign indicates that the direction in which the pistol would recoil is opposite to that of bullet.