Inclined plane question

  • #1
A 3kg block starts at the top of a 30 degree incline and slides 2m down the incline in 1.5 seconds.
a) Find acceleration of the block.
b) Find the coefficient of kinetic friction between the blcok and the incline.

For a) x = vot + 1/2at^2 --> a = 1.778m/s^2
For b) *here's where i'm having some problems*
I tried doing it two different ways and i get two different answers.

Way 1:
Ff = uFn
Ff/Fn = u
mgsinx/mgcosx=u
tanx=u
u = .577

Way 2:
ΣF=ma
mgsinx - umgcosx = ma
gsinx - ugcosx =a
a-gsinx/-gcosx = u
(1.778m/s^2 - 5m/s^2)/-(10*cos30) = u
u = .372

which way is wrong?
 

Answers and Replies

  • #2
So after reviewing what i typed in, i think that you can only use u = tanx when ΣF=0, is my assumption correct?
 
  • #3
radou
Homework Helper
3,120
7
Way 1 is wrong, since the force of friction does not equal mgsinx.
 

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