# Wood Friction Question

1. Mar 6, 2008

### vanquish

1. The problem statement, all variables and given/known data
You place a piece of wood onto a 227cm long piece of metal on a 25 angle. The $$\mu$$=.34 How long does it take for the piece of wood to reach the bottom of the metal?

2. Relevant equations
$$\Delta$$X=227
Vi=0
a=?
t=?

$$\Delta$$X=ViT+.5at^2
227=.5at^2

a=Fnet/m
$$\mu$$=Ff/Fn

3. The attempt at a solution
I have 2 pages of work for this problem and it's obvious to me that I'm missing something crucial. I tried this problem a few times, the first time I calculated the wrong $$\mu$$ and the second time I have no idea what happened

I can't reproduce my free body diagram for you but I can tell you what I broke the components down into:
Fg=y'/sin25
Fg=x'/cos25

2. Mar 6, 2008

### Staff: Mentor

You need to find the acceleration. To do that, you'll need the net force and Newton's 2nd law.

Try to give symbolic answers (not numbers) to these questions:

What are the components of the wood's weight parallel and perpendicular to the incline?

What's the normal force between the surfaces?

What's the friction force acting on the wood?

What's the net force acting on the wood?

3. Mar 6, 2008

### vanquish

Thats what I ended up doing but my answer came out to be 9.7 seconds, which is extremely wrong.

If I leave a as Fnet/m I end up with T=sqrt(454m/Fnet) So then I need to solve for Fnet in a way in which m cancels out (thats what my teacher said atleast). So Fnet=Fn-Ff
Fg=Ff/sin25
Fg=Fn/cos25

Fn=Fgcos25
Ff=Fgsin25

EDIT: Just worked it out that way and got the same answer 9.6 seconds... which is horribly wrong... any ideas?

Last edited: Mar 6, 2008
4. Mar 6, 2008

### Staff: Mentor

Why don't you try answering my questions?