# Homework Help: Action-reaction pair

1. Oct 22, 2006

### OVB

A concept I don't understand is the force exerted by one block of mass m1 on a block of mass m2 and vice versa when a force F is applied to m1.

What I mean is, say you have to blocks next to each other and a force F is applied to m1. My physics teacher says m1 will accelerate, but m2 will accelerate in the opposite direction. How can the forces be equal if m1 and m2 aren't? My teacher says F - (m2)(a) = (m1)(a). How is this possible? It makes absolutely no sense to me how this can be possible if (m2)(a) and (m1)(a) form an action-reaction pair.

2. Oct 22, 2006

### semc

hmm...lets say u exert a force on m1 and they both move to the right.draw the FBD of both mass and u will notice that other than the force exerted and frictional force,theres a force on m1 by m2 in the left direction and a force on m2 by m1 to the right.this forms the action reaction pair??

3. Oct 25, 2006

### OVB

yeah, but why? Why can you use the acceration derived from the formulae I used for the mass m2 if it is not being accelerated in that direction?

4. Oct 25, 2006

### Staff: Mentor

Are you sure that's what he said? If you have two blocks next to each other and you push against the left one with a force acting to the right, both blocks will accelerate together--in the same direction. Perhaps he said that the two blocks will exert equal and opposite forces on each other, which is certainly true.
What's their mass have to do with it? If you push on an elephant with a force F, it will push back on you with a force F. That's Newton's 3rd law.

Your teacher is correct, but (m2)(a) and (m1)(a) do not form an action-reaction pair. (They aren't even equal!) (m1)(a) will equal the net force on m1; (m2)(a) will equal the net force on m2.

If you consider the two blocks as a single system, the only force acting on them is the applied force F. Apply Newton's 2nd law: F = (m1 + m2)a. That's equivalent to what your teacher said, but it says nothing about the forces between m1 and m2.

To find those forces, consider each block as a separate system. What forces act on m1? What forces act on m2? (Note that two forces act on m1, but only one force acts on m2.)