Force Needed to Slide Trunk Down Inclined Plane with Constant Velocity

[imatreyu]

Homework Statement

"You push a 325-N trunk up a 20.0 degree inclined plane at a constant velocity by exerting a 211-N force parallel to the plane's surface."

What force must be exerted on said trunk so that it would slide down the plane with a constant velocity? In which direction should the force be exerted?

Homework Equations

W=mg
Fnet= ma

I'm not really sure about this section.

The Attempt at a Solution

I drew a free body diagram with the known forces that are acting on the object (the Normal--perpendicular to the inclined plane, and the weight straight down). I also drew in the supposed pushing force and the opposing frictional force. I aligned the diagram on a coordinate plane so that all forces lie on an axis EXCEPT the weight.

I resolved the weight:

Wx= 325cos70
Wy= 325sin70

And found the coeff. of friction:

sum of all forces y-axis:
Fnet= N + Wy
Fnet= N-Wy
Fnet= 0

sum of all forces x-axis:
Fnet= Pushing force + Ff + Wx
Fnet= Fp + Wx - Ff
ma= Fp + Wx -Ff
(velocity is constant--> there is no acceleration):
0= Fp + Wx - Ff
0= 211 - (325cos70) - Ff
Ff= approximately 100 N, the coeff of friction is thus approximately .3278

I am totally stuck now, though on how to find the force needed to cause the trunk to slide down the plane with constant velocity. . . .

And I might have done something wrong above. :( Please help!

 

[paul232]

sum of all forces x-axis:
Fnet= Pushing force + Ff + Wx
Fnet= Fp + Wx - Ff
ma= Fp + Wx -Ff

1) Why is Fnet = Fp + Wx - Ff? Both Wx and Ff oppose to the Fp.

2) Also about the new force. Imagine that you want to get a box down of a hill. In order it comes down sweet how you need to act? Also the Constant velocity is the key point here. Also think that the velocity must be Opposite of the pushing up part. That means here you have Wx opposing to the Frictional force? Which of those is greater? If you consider that Fnet x=0 you understand the where this force acts and you can find its magnitude.

Hope I helped..

 

[imatreyu]

Hi, thank you so much for replying!

I actually figured it out. . .hahaha. . .shouldn't have been so quick to post.
Your post helped me make sure I did it correctly. Thank you!