A weight lifter struggles but manages to keep a heavy barbell above his head. Infrequently he slips and the barbell starts to fall downward, however he always recovers. Contrast the force exerted by the weight lifter on the barbell to that exerted by the barbell on the weight lifter. Is one force larger than the other or are they equal in magnitude? Does this change through the times when the barbell slips downward?
Same with a bowling ball rolls down the alley and hits a pin and tennis player hits the ball by her racket. Does the relationship of the magnitudes of the two forces change at any time through the collision?