Homework Help: Force on two touching blocks

1. Jan 4, 2012

Heather_

1. The problem statement, all variables and given/known data
For two touching blocks, a constant horizontal force Fa is applied to block A to the right, which pushes against block B with a 26.0 N force directed horizontally to the right. In figure (b), the same force Fa is applied to block B to the right (the blocks switched places); now block A pushes on block B with a 13.0 N force directed horizontally to the left. The blocks have a combined mass of 6.00 kg (A is smaller than B).

I need to know the system's total acceleration and the force being applied.

2. Relevant equations
Newton's laws; friction is being ignored

3. The attempt at a solution
is it safe to assume that the mass of A is half the mass of B? (a pushes on b with half the force that b forces on a). That would make the acceleration 52, which isn't correct. Have i missed something?

Last edited: Jan 4, 2012
2. Jan 4, 2012

PhanthomJay

Hi Heather, welcome to Physics Forums!
Yes, this is a very good observation. Now since A is half of B, and together their mass is 6 kg, then A = ???? kg and B = ??? kg.
Please show how you arrived at this number. Draw free body diagrams of each block for each case, and a free body diagram of the system of 2 blocks.

3. Jan 5, 2012

Heather_

Well obviously that would make the mass of A 2 kg and the mass of B 4. My thinking was that the block that was being pushed by the other one would need to exert an opposite force, so if A is pushing on B with a 26 N force and weighs 2 kg, the acceleration would be 13, which doesn't work (if A pushes on B with 26N force, though, why isn't that the force being applied to the system). As for the 52...I understand now why that was wrong.

4. Jan 5, 2012

PhanthomJay

If , drawing a free body diagram of B, A pushes on B with 26 N, and B has a mass of 4 kg, then per Newton 2, a = ?
The force applied to the system is not the same as the force between blocks. The force applied to the system (the applied force acting on A in your first case) must move BOTH blocks.