equal and opposite to the force on the body B by A. . The terms action and reaction in the third law may give a wrong impression that action comes before reaction i.e action is the cause and reaction the effect. There is no cause- effect relation implied in the third law.
The force on A by B and the force on B by A act at the same instant. By the same reasoning, any one of them may be called action and the other reaction. . Action and reaction forces act on different bodies, not on the same body.
Consider a pair of bodies A and B . According to the third law, F AB = – F BA ( . ) (force on A by B ) = – (force on B by A ) Thus if we are considering the motion of any one body ( A or B ), only one of the two forces is relevant. It is an error to add up the two forces and claim that the net force is zero.
However, if you are considering the system of two bodies as a whole, F AB and F BA are internal forces of the system ( A + B ). They add up to give a null force. Internal forces in a body or a system of particles thus cancel away in pairs. This is an important fact that enables the second law to be applicable to a body or a system of particles (See Chapter ).
Example . Two identical billiard balls strike a rigid wall with the same speed but at different angles, and get reflected without any change in speed, as shown in Fig. . .
What is (i) the direction of the force on the wall due to each ball? (ii) the ratio of the magnitudes of impulses imparted to the balls by the wall ? Fig. .
Answer An instinctive answer to (i) might be that the force on the wall in case (a) is normal to the wall, while that in case (b) is inclined at ° to