# Homework Help: 2 blocks on top of each other, applied force, friction, and tension

1. Oct 12, 2011

### TFOrange

1. The problem statement, all variables and given/known data
A 4.32 kg block is placed on top of a 9.16 kg
block. A horizontal force of F = 66.6 N is
applied to the 9.16 kg block, and the 4.32 kg
block is tied to the wall. The coefficient of
kinetic friction between all moving surfaces
is 0.169. There is friction both between the
masses and between the 9.16 kg block and the
ground.
The acceleration of gravity is 9.8 m/s^2.
Determine Tension of string in N

(see image)
2. Relevant equations
ƩF = ma
fs = μk*FN

3. The attempt at a solution
ƩF = ma
m = 4.32 kg
atop block = 0
ƩF = Fapplied - fk - T
0 = (44.274) - (7.155) - T
T = 44.274 - 7.155
T = 37.119

Apparently I did something wrong somewhere, but I'm not sure where.

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Last edited: Oct 12, 2011
2. Oct 12, 2011

### Staff: Mentor

What forces act on the upper block? (I don't understand your FBDs.)

3. Oct 12, 2011

### TFOrange

Well maybe it isn't supposed to be there, but I was thinking that since the bottom block has a force in that direction that is not completely canceled out by friction, it would cause the top block to have a force as well, which is equal to Fapplied (to the bottom block) - fk

4. Oct 12, 2011

### Staff: Mentor

That applied force acts on the bottom block, not on the top block. So it doesn't belong in your analysis of the top block. The only influence that the bottom block exerts on the top block is through the normal force and friction.

5. Oct 12, 2011

### TFOrange

Oh! so T = f
and so it's T = 7.155
Thanks!

6. Oct 12, 2011

Yep!