11. Action and reaction forces

Assume a person is at rest for some time and intends to walk on a road. According to the second law, the person must apply force to produce acceleration to perform this action.

 

But can you guess the type of force applied to the ground, whether muscular or something else?

 

 

While walking on the road, the man pushes the road below backwards with his feet. The road then exerts an equal but opposite force on the person's feet to help him move forward.

 

The important point is that the action and reaction forces may not produce accelerations of equal magnitudes even though these forces are in equal magnitude. This happens when each force acts on different objects with varying mass.

1. When a gun is shot, the bullet is pushed forward. The gun is subjected to an equal and opposite force by the bullet. As a result, the gun recoils. The gun has a comparatively larger mass than the bullet, so the gun's acceleration is much lower than the bullet's acceleration.

 

This activity requires a glass test tube, water, strings or wires and a stop cork.

 

 

The water inside the test tube vaporises after some time on heating, and the cork blows out. The test tube recoils in the direction opposite to the direction of the cork when it bows out.

 

2. When a boy leaps from a rowing boat, the third law of motion is explained. When the boy leaps forward, the boat is pushed backwards by applying force.

 

 

Imagine two children standing on a separate rolling cart with wheels. The wheels are noted during any movement of the cart. The children have a bag filled with sand or a heavy object to play catch and throw game. Now, as the game starts, note the movement of the cartwheels.

 

 

While the boy throws the bag to the girl, the cartwheels get pushed backward as the force exerted by the boy is in the forward direction. Similarly, as the girl throws the bag, the cartwheels get pushed backward due to the forward force exerted by the girl.

 

Here both the children receive an instantaneous reaction while throwing the bag. This activity thus explains Newton’s third law of motion.