Willis thinks over the discussion of the second part of the first experiment described on pages 30-31 . He is interested in the equality of the force he exerts on the clone and the one the clone exerts on him

Question

Later he tells his girlfriend Sheena "It may be right in that case, because both people had the same mass. It's wrong if the masses are different. If I met the Hulk and he pushed me, it would be with a greater force than I could use to push him back." Discuss this reasoning.

Answer 1

This is thought to be the case by many people, not just Willis. It seems to make
sense that the greater the mass, the greater the force (which is correct, if we think of objects
having the same accelerations). In this case, Willis goes further—he says that they exert the
same forces only because they have the same masses; if the masses were different, the
forces would be different. Newton's Third Law says, however, that the forces should be
equal to one another and act in opposing directions.
So Willis would say that if a car hit a big truck, the truck would exert a greater force than
the car. This seems to agree with experience. If a truck and a car going the same speed hit
head-on, we'd expect to see the truck continue in its same direction (with reduced speed).
The car would be moving backwards. Willis would say that this must mean that the forces
couldn't have been equal.
This is motion Willis is considering, but is it actually the force he's thinking about? We
need to think further.
Newton would say that the forces were the same size. Equal magnitudes of force on two
different masses should lead to two different results (Newton's Second Law). The smaller
mass would have the greater acceleration, the larger mass the smaller acceleration. From
forces being equal, we would predict differing outcomes for the car and truck The car
would slow down more than the truck. But that's just the common-sense evidence Willis
used above to assert that the forces couldn't be equal!
Willis's reasoning is incorrect. The two objects having the same force acting will behave
just as he thinks they should. Willis is thinking of something physical with his reasoning,
but it is not force. It is called momentum. If two objects of differing masses move at the
same speed, the larger-mass object has the larger momentum. Force actually is the rate of
change in momentum. This is the same for both objects in a collision.