Calculating Forces in a Pulling System: Fp, Wp, Wm

[Thea Woods]

Homework Statement

A physics student pulls a block of mass m = 21 kg up an incline at a slow constant velocity for a distance of d = 4.5 m. The incline makes an angle Δ = 31° with the horizontal. The coefficient of kinetic friction between the block and the inclined plane is µk = 0.2. What is the work Wm done by the student?
Wm =

Relevant Equations

W = F*d
K = 1/2 mv^2
U = mgy
E = U + K

Fp = Fg +Ff
Fp = (21*9.81*sin(31)) + (0.2*21*9.81)
Fp = 147.30
Fp*d = 147.3 *4.5
Wp = Wm = 662.87 = 663 J

I am not sure what I am doing wrong. Should I subtract the force of friction from the force of gravity? Have I assumed that the pulling force is equal to the sum of gravity and friction when I shouldn't have? Should I use cos instead of sin?

 

[Delta2]

The friction force $F_f$ is equal to $0.2N$ where $N$ is the normal force from the incline to the block. What is the correct value of $N$?? It is not 21*9.81 , because that's the full weight of the block...
Hint: Decompose weight of the block, into a component parallel to the incline and a component perpendicular to the incline. How does the normal force relate to the component perpendicular to the incline?