How Does Friction Affect a Penguin Sled on an Inclined Plane?

[mmiller9913]

Homework Statement

A loaded penguin sled weighing 66 N rests on a plane inclined at angle θ = 18° to the horizontal (Fig. 6-23). Between the sled and the plane, the coefficient of static friction is 0.27, and the coefficient of kinetic friction is 0.18. (a) What is the minimum magnitude of the force , parallel to the plane, that will prevent the sled from slipping down the plane? (b) What is the minimum magnitude F that will start the sled moving up the plane? (c) What value of F is required to move the sled up the plane at constant velocity?

Homework Equations

Fk=uFn
Fsmax=uFn

The Attempt at a Solution

a) I know the acceleration of the sled would be negative but if I set it up like
Fa-Fs=-ma... it can't be solved

b) Fsmax=uFn
=.27(6.73)(9.8)
=17.81

c)Fa-Fk=ma
Fa-Fk=0
Fa=Fk
Fa=uFn
=11.87N

 

[Doc Al]

a) I know the acceleration of the sled would be negative but if I set it up like
Fa-Fs=-ma... it can't be solved

What forces act on the sled parallel to the plane? There are three, one of them being the applied force that you need to solve for. What are the other two? What must they add to for the sled to not move?

 

[mmiller9913]

I don't see the three forces that act on it...the applied force, the static friction force, and?

 

[Doc Al]

I don't see the three forces that act on it...the applied force, the static friction force, and?

What force is always there, acting on any mass? (What force acts to make it slide down the hill?)

 

[mmiller9913]

okay so ...
Fa-Fs-Fgcostheta=-ma

I don't see how this can be solved though

 

[fizzynoob]

you need to sum the forces in the Y as well, so you can find the normal force

 

[Doc Al]

okay so ...
Fa-Fs-Fgcostheta=-ma

I don't see how this can be solved though

What direction does friction act? What does the friction force equal? What's the 'cos theta' business? What's the acceleration? (Remember--you are preventing it from sliding.)