# Homework Help: Box is placed on a plank, one end is raised, find friction...

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1. Jun 12, 2015

### leafjerky

1. The problem statement, all variables and given/known data
A box is placed on a plank. One end of the plank is gradually raised. When the angle of inclination with the horizontal reaches 30°, the box starts to slip, and it then slides 2.5m down the plank in 4.5 seconds. Find (a) the coefficient of static friction, and (b) coefficient of kinetic friction between the box and the plank.

2. Relevant equations
∑Fx = max
∑Fy = may
ΔX = (1/2)at2
ƒk = μk⋅FN
ƒsmax = μs⋅FN

3. The attempt at a solution

After using the third equation from the top, I get my a = .313 m/s2

For part a, I solved μs = tanΘ = .577

I'm having difficulty trying to get part b.

I drew my diagram and for my x part, I get FN = mgcos(30) and for my y I get mgsin(30) - μk(mgcos(30)) = ma, but I am almost positive that's not right, because I try entering the values on my calculator and I'm just getting some weird small numbers. Can someone shove me in the right direction and explain what I'm doing wrong/right? I'm new to physics and I have to take it in the summer they are just shoving my head with all kinds of stuff. Sorry if anything I wrote wasn't in proper syntax.

2. Jun 12, 2015

### leafjerky

Nevermind I figured out my problem. Can a mod please delete this? Thanks