Newton's Third Law of Motion

18. Another example: A football player kicks a
football. Therefore according to the 3rd Law,
the football kicks the player back.
19. The kick is an impulsive force, lasting for a
very short time, say, 1 of a second. Taking
10
the mass of the ball to be ¼ kg, we have
calculated the kick force to be 250 N.
20. The 3rd Law tells us that the ball also kicks
the player back in the opposite direction.
However, since the mass of the player is 100
kg, i.e., 400 times the mass of the ball, his
acceleration is 400 times less and lasts exactly
for 1
10 second.
21. Here we see the complete picture. The
mutual kick forces are equal and opposite. But
they do not cancel each other, because they act
on different bodies.
218 Physics Education • July − September 2009