Force & Springs Homework: Find Magnitude of F & Compression of Spring

[minitorpedo]

Homework Statement

A 30.0 kg block is resting on a flat horizontal table. On top of this block is resting a 15.0 kg block, to which a horizontal spring is attached. The spring constant of the spring is 315 N/m. The coefficient of kinetic friction between the lower block and the table is 0.555, while the coefficient of static friction between the two blocks is 0.925. A horizontal force F is applied to the lower block as shown. This force is increasing in such a way as to keep the blocks moving at a constant speed.
At the point where the upper block begins to slip on the lower block determine the following. (a) the amount by which the spring is compressed. (b) the magnitude of the force F.

Homework Equations

F = -kx

The Attempt at a Solution

For part a) the magnitude by which the spring is compressed is 0.432m. (that is a right answer)
For part b) i think there are many factors: the friction of the big block with the table, the friction of the big block with the small block, the force of the spring and how it varies as the force pushes in more. I don't know how to combine all of those into an equation.

 

[radou]

For part b) i think there are many factors: the friction of the big block with the table, the friction of the big block with the small block, the force of the spring and how it varies as the force pushes in more. I don't know how to combine all of those into an equation.

Hint: constant velocity implies equilibrium.

 

[prettyinpink]

Hey I am confused on a question like this too! I had no clue how to even start?...

 

[prettyinpink]

How did u even get part a? F=? K=315

 

[radou]

How did u even get part a? F=? K=315

Probably the most standard advice - draw a free body diagram. Do that for every box separately.