Calculating Forces on an Inclined Plane

[BuBbLeS01]

Homework Statement

A block weighing 73.7 N rests on a plane inclined at 24.3° to the horizontal. The coefficient of the static and kinetic frictions are 0.28 and 0.11 respectively.

A.) What is the minimum magnitude of the force F, parallel to the plane, that will prevent the block from slipping?

B.) What is the minimum magnitude of F that will start the block moving up the plane?

C.) What is the magnitude of F is required to move the block up the plane at constant velocity?

Homework Equations

w = 73.7
theta = 24.3
Mus = 0.28 (Don't know how to make that little symbol lol)
Muk = 0.11

The Attempt at a Solution

A.) F = w sin theta - Mus*wcos theta = 11.52 N

B.) F = fs + wx
Fmin = Mus*wcos theta + wsin theta = 49.14 N

C.) F = Wx + fk
fk = Muk*N = 7.3889
Wx = fs + F = 11.775 + 11.52
fs = Mus*wcos theta


Part C I don't think is right but I don't know what I am doing wrong?

 

[Mindscrape]

What does it mean for something to be moving at a constant velocity?

 

[BuBbLeS01]

What does it mean for something to be moving at a constant velocity?

It means that it is not accelerating...Newtons first law I think

 

[asd1249jf]

It means that it is not accelerating...Newtons first law I think

No, that has nothing to do with Newton's first law.

http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/newtlaws/u2l1a.html


Constant velocity means there is no acceleration, meaning from Newton's second law

F = ma

there is no force since there is no acceleration.

 

[BuBbLeS01]

Oh ok Newtons second law.
So why isn't it...
F = Wx + fk

 

[Mindscrape]

What do you mean by F? So, you understand that the forces must sum to zero because there is no acceleration? Also, since velocity is kinetic, you need to use kinetic friction.

-Wx - fk + F = 0

I think you were using static friction. Did you actually get the question wrong, or do you just think you are wrong? It looks like you are right.

 

[BuBbLeS01]

the force. Yea I understand that it must = to 0.
so
Wx = fs + F
fk = Muk x W cos theta
?

 

[Mindscrape]

Looks fine to me. Watch your signs though.

F = fk + Wx

Actually I am sort of confused now that I look that over once more. Wx = mgsinØ, why do you think it is fs + F? What did you do there?

 

[BuBbLeS01]

Nevermind I was looking at the wrong thing I think for Wx, but I did try to do that...
F= (Muk x W cos theta) + (mg sin theta) = 49.14 N
but it says its wrong

 

[Mindscrape]

I get a different answer, make sure you are in degrees and all that.

 

[BuBbLeS01]

F = (0.28 x 73.7 cos 24.3) + (73.7 sin 24.3) = 49.14
its in degrees

 

[Mindscrape]

Kinetic friction coefficient is .11 :)

 

[BuBbLeS01]

omg lol i make the dumbest mistakes!