# Friction + Force Between Two Blocks

1. Oct 20, 2008

### Davetozz

2. Relevant equations
ΣF=m*a
Ff=μ*Fn
Should be it..

I gave it a shot for about the 4th time this week, my teacher said he made this up (couldn't just Google it) and that none of the other physics teachers in the school could get it. Since in my previous attempts I confused myself towards the end with numbers, I didn't substitute any numbers yet. I can't really understand the force on the large block, or the whole system for that matter so I need less of actual math help and more of just conceptual help.
As far as my try goes, I usually just calculate the forces in each direction and things usually fall together but not here, I hit a point in the friction equation where I couldn't move forward, then realized that I'm not taking into account the frictionless big block.

2. Oct 21, 2008

### alphysicist

Hi Davetozz,

It looks to me like you have most of it. The main problem in your work that I see is that you are thinking that the small block has an acceleration of zero, but that is not true.

(And in your work you have already found an expression relating this acceleration to the applied force.) Does this help?

3. Oct 21, 2008

### Davetozz

Hey alphysicist,

I think I understand what you're saying. The small block does not move on the big block, but with the big block?

I just realized that the small block puts a force (weight) on the big block and I think that the force applied to the small block transfers that to the big block. So the FBD of the big block I would think is Fn up, both weights and Fsin@ down and Fcos@ to the right. I'm gonna try and work that out with the new info with acceleration this time.

One last question, does the frictionless surface under the big block do anything in this problem?

I just threw some numbers in assuming that the big block was affected by Fcos@ to the right but came up with cos(50)=5/5.5 so I take it that its affected by F to the right..

Does the big blocks normal force get included in the small blocks FBD?

Last edited: Oct 21, 2008