Fnet=ma (kind of confusing question )

[laddoo12]

Homework Statement

In the diagram below are two blocks. Block A sitting on top of block B has a mass mA = 10.0 kg and is attached to the wall by massless string. Block B, mass mB = 35.0 kg, is pulled by a force F = 75.0 N. The surface under block B is frictionless. The coefficients of friction for block A and B are μs = 0.34 and μk = 0.17.
(a) Determine the acceleration of block B.
(b) Determine the tension in the string.

Homework Equations

Fnet=ma
Ffriction-mg*coefficient friction

The Attempt at a Solution

For the first part i went
fnet=ma
Ft=(m1+m2)a+M*g*μk
75=45a+35*9.8*.17
a=41.68/45
a=.37 m/s^2

Not sure if i am doing this correct, the coefficient of static friction is confusing me, do i need it in this equation?

Part B
Fnet=ma
Ft=Ffriction+Ffstatic
Ft=.37*35*.17+35*.34*9.8
ft=118.82N (seems right not sure)

I am not fully confident in my work an answers can someone tell me if idid this correct, i have no answers unfortunately given to me. Please tell me if i went wrong somewhere or not thank you!

 

[azizlwl]

Mass A will stay on top of mass B as long the acceleration less than the static friction + the tension of the spring.
Nothing is said about the spring.

 

[PhanthomJay]

The top block cannot accelerate in the direction of the applied force, with respect to the ground, because it is attached to the wall. So your approach is incorrect. You should use free body diagrams for each block and identify the forces acting on each , and apply all of Newton's laws.