Chapter 9. Force and Laws of Motion
(3) The Third law of motion:
The third law of motion states that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first.
Some important facts about this law-
- These two forces are always equal in magnitude but opposite in direction.
- These forces act on different objects and never on the same object.
- The two opposing forces are also known as action and reaction forces.
How to show the action and reaction forces are equal and opposite:
Let us consider two spring balances connected together as shown in Figure. The fixed end of balance B is attached with a rigid support, like a wall. When a force is applied through the free end of spring balance A, it is observed that both the spring balances show the same readings on their scales. It means that the force exerted by spring balance A on balance B is equal but opposite in direction to the force exerted by the balance B on balance A.
Action force : The force which is applied on an object A and object A exerts that force on object B is called action force.
Reaction force : When object B exerts a force on object A as resulting force applied by A is called reaction force.
Remember:-
- The action and reaction always act on two different objects.
- Every action there is an equal and opposite reaction.
Why action and reaction force always equal in magnitude but may not produce acceleration of equal magnitudes:
The action and reaction forces are always equal in magnitude, these forces may not produce accelerations of equal magnitudes. This is because each force acts on a different object that may have a different mass.
An Example of gun and bullet:
When a gun is fired, it exerts a forward force on the bullet. The bullet exerts an equal
and opposite reaction force on the gun. This results in the recoil of the gun. Since the gun has a much greater mass than the bullet, the acceleration of the gun is much less than the acceleration of the bullet.
The Law of Conservation of Momentum:
The sum of momenta of the two objects before collision is equal to the sum of momenta after the collision provided there is no external unbalanced force acting on them. This is known as the law of conservation of momentum.
This law gives the following mathematical formula,
mAuA + mBuB = mAvA + mBvB