Physics HELP!!!!!!!!!?

J

jack

New member
Can anyone give me detailed description of how to this and what formula to use?
Thank you so much for the help!

A block is at rest on a plank whose angle can be varied. The angle is gradually increased from 0 deg. At 34.1°, the block starts to slide down the incline, traveling 2.90 m down the incline in 2.00 s. Calculate the coefficient of static friction between the block and the plank.

Calculate the kinetic coefficent of friction between the block and the plank.
 
The static friction is found when the frictional force equals the gravity force when the block starts moving.

Frictional force = µs*m*g*cosθ
Gravity force = m*g*sinθ

Equate these to get µs*m*g*cosθ = m*g*sinθ

or µs = tanθ

µs = 0.68

The motion of the block down the plank is given by

s = 0.5*a*t²

The acceleration down the plank is then a = 2*s/t²

That acceleration is produced by the difference in gravity force and frictional force, Fd

Fd = m*g*sinθ - µk*m*g*cosθ

a = Fd/m = g*sinθ - µk*g*cosθ

g*sinθ - µk*g*cosθ = 2*s/t²

µk*g*cosθ = g*sinθ - 2*s/t²

µk = tanθ - 2*s/t² / g*cosθ

θ = 34.1º
s = 2.90 m
t = 2.0 s

µk = 0.50
 
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