What is the magnitude of the normal force in this block stacking scenario?

[lacar213]

Homework Statement

Three blocks are arranged in a stack on a frictionless horizontal surface. The bottom block has a mass of 37.0 kg. A block of mass 18.0 kg sits on top of it and a 20.0 kg block sits on top of the middle block. A downward vertical force of 170 N is applied to the top block. What is the magnitude of the normal force exerted by the bottom block on the middle block?

Homework Equations

Fn + T + (-mg) = 0

The Attempt at a Solution

20 + 18 = 38*9.8 = 372.4 N
Fn + 170 - 372.4 = 0
Fn = 202.4

 

[Doc Al]

The Attempt at a Solution

20 + 18 = 38*9.8 = 372.4 N

Good.

Fn + 170 - 372.4 = 0

Careful: That 170 N force acts downward.

 

[lacar213]

So the 170 needs to be negative .. ?
Fn + (-170) - 372.4 = 0
Fn = 542.4

 

[Doc Al]

Looks good!