Calculate the force required to pull the truck

[oMovements]

Homework Statement

You are dragging a 110 kg trunk across a floor at a constant velocity with a horizontal force of 380N.
a) Calculate the coefficient of kinetic friction
b)A friend decides to help by pulling on the truck with a force of 150N [up]. Will this help? Calculate the force required to pull the truck at a constant velocity to help you decide.
c) Instead of pulling on the trunk, your 55 kg friend just sits on it. What force is required to keep the trunk moving at a constant velocity?

Homework Equations

μ=F_f/F_n

The Attempt at a Solution

a) μ_k= F_k/F_n
= 380/(110x9.8)
μ_k = 0.35

b)

c) F_k = μ_kF_n
= (0.35)(165x9.8)
F_k = 566N

Did I solve a) and c) correctly? Also how would I go about doing b).

 

[Simon Bridge]

(a) sort of - you should start from a free-body diagram, which makes the equations look different.
(b) use a free-body diagram
(c) again - sort of - same reason. Where did you get that relationship from? (i.e. to add the masses.)