Solving a Problem Using Conservation of Energy

[Nivoh]

I apologize in advance for my lack of knowledge of "physics english".

First off, this is high school level. :)

I'm terribly sorry if this is confusing due to my lack of understand of the subject, the metric system, and my lack of grasp for the English language as well as terminology. What is μ?

Homework Statement

An object is moves up a flat board, the board forms an angle of 30° with the x-axis. During 0.76s, the object accelerates from 5m/s to 0. The only forces affecting the board is gravity and friction.

Homework Equations

ƩF=ma, R=μ*N, E=0.5mv^2+mgh, W=F*s.

The Attempt at a Solution

I initially solved this quite easily using ƩF=ma=mg*sin30°+μ*cos30°, where a=Δt/Δv.

Out of curiousity, I figured I could solve this using conservation of mechanical energy, as R is a constant force.

So I went ahead and used R=μ*N, R=E/s, where E is the mechanical energy lost due to friction. N=mg/cos30°, E=0.5mv^2-mgh, therefore μ=R/N=(0.5mv^2-mgh)*cos30°/mgs. s=1.9m, h=0.95m, g=9.81m/s^2, v=5m/s.

I seem however, to have failed, I wonder why? :)

 

[Nivoh]

I am sorry for being scum of the Earth and bumping my own post, but I really cannot rest without knowing what I've done wrong. Please let me know if my question is unclear, my English not sufficient to describe the problem, or if I'm not following guidelines.

 

[Rokas_P]

Is it only me or is there no actual mention of what we have to calculate in this problem?

 

[Nivoh]

Is it only me or is there no actual mention of what we have to calculate in this problem?

Duh, how silly of me, my apologies. Thank you for making me aware, was looking for μ, the friction constant.

 

[Rokas_P]

The general outline of how I would approach this problem is this:

1. calculate acceleration
2. write down Newton's Second Law for this problem (here all the sines and cosines come in)
3. calculate μ (since μ is in R=μN)

Edit: I see that you're asking not about how to solve it but why you can't get the right answer when you try to solve it using another method. Hopefully someone can help you out with that :)