How Can I Solve This Newton's Forces Problem for My Homework?

[rzn972]

Homework Statement

Homework Equations

The Attempt at a Solution

A)
I am not sure how to go about solving this. I would say no you can't find Us. Can you just say that you do not know when the object will move?
B) First I found acceleration x= 1/2 at^2, 18= .5a(9), a= 4.
Then I used the free body diagram of m1. Fk pointing to the left, Tension 2 pointing to the right.
T2- Fk= ma
10g - 5g Uk = (5)(4), but my teacher uses the mass of both the blocks, 5+ 10. I am not sure why...

 

[mrnike992]

I don't understand how there can be static AND kinetic friction between M1 and the surface..

 

[dauto]

I don't understand how there can be static AND kinetic friction between M1 and the surface..

Not at the same time, obviously...

 

[Chestermiller]

Your mistake is that T2 is not 10g. Just do a force balance on M2 and you will see what it really is, and you will also find out why your teacher used (10 + 5).

Chet