Two block problem

1. Mar 26, 2013

ZanyCat

I'm sure you guys have never answered a physics question involving two blocks before :P

My question is regarding parts (b) and, by extension, (c).

For part (a), I called the force between the two blocks 'z' and solved $$\frac{F - z}{m1} = \frac{z}{m2}$$ which gave me z = 1.1N, which was correct.

Conceptually for part (b), I imagined the two blocks as one system. Two forces are acting on the system in opposite directions and they have the same magnitude, so the system is in equilibirum and not accelerating. Now considering just the large mass, this must be in equilibrium also, as it must not be accelerating either. Thus to oppose the F force pushing right, the z force 'pushing' left on it from the smaller block must have the same magnitude as F, 3.2N.
When I solved a pair of simultaneous equations again, I also ended up with z = 3.2N.

So where is my thinking wrong? Where does the 2.1N come from?

Thanks :)

2. Mar 26, 2013

ap123

In part (b), I think they mean that the force is removed from m1 and applied to m2, so there is only 1 force acting.

3. Mar 26, 2013

Staff: Mentor

Right, otherwise the problem does not make sense.
You can calculate the force as in (a).