Newtons law, interacting objects question?

[Xinio64]

Newtons law, interacting objects question??

A block of mass 5.00 Kg sits on top of a second block of mass 15.0 Kg, which in
turn sits on a horizontal table. The coefficients of friction between the two blocks are
: μs = 0.300 and μk = 0.100. The coefficients of friction between the lower block and
the table surface are: μs = 0.500 and μk = 0.400. You apply a constant horizontal
force to the lower block so that this force is just large enough to make this block start
sliding out from between the upper block and the table.

(a) Draw the free body diagram for each block, naming the forces on each of them.
(b) Determine the magnitude of each force on each block at the instant you have
started pushing but motion has not started.
(c) Determine the acceleration you measure for each block.

 

[tiny-tim]

Welcome to PF!

Hi Xinio64! Welcome to PF!

You must tell us how far you've got, so that we know what's troubling you, and how to help.

What have you tried? have you drawn the diagram?

What equations have you got?

 

[Xinio64]

Hi Xinio64! Welcome to PF!

You must tell us how far you've got, so that we know what's troubling you, and how to help.

What have you tried? have you drawn the diagram?

What equations have you got?

Well I drawn two free body diagrams, one representing each block. The upper block has a weight and normal force of of 49N and a static frictional force pointing to the right. The lower block has a weight and normal force of 196N and a static frictional force of 78.4N pointing to the left and a constant force F pointing to the right. I don't know if in the free body diagrams I use kinetic friction or static friction forces. Equations I guess is F=ma. That's all I have.

 

[tiny-tim]

I don't know if in the free body diagrams I use kinetic friction or static friction forces. Equations I guess is F=ma. That's all I have.

Hi Xinio64!

To get the starting force, you must use the static coefficient.

To get the movement, you use the kinetic coefficient (but of course you use the force you got from the static coefficient).

So … what value force do you get, and how do you carry on from there?