Physics Friction Question?

L

Lee R

New member
At the bottom of a hill, a snowboarder with a velocity of 16m/s (FWD) skids to a stop after moving a distance of 9.4m. The mass of the board and rider is 71kg, and the magnitude of the force of kinetic friction during the skid is 950N. Determine the work done by the force of friction on the snowboarder during skidding and determine the coefficient of friction during skidding. How do I do this? Thanks
 
Since friction is a non-conservative force, the net work done by friction is the change in energy between the time the snowboarder was moving at 16m/s and the time when he came to rest. Potential energy may be ignored as the event takes place with no vertical displacement:

W(friction) = ΔKE
= 0 - 0.5(71kg)(16m/s)²
= -9.1kJ (9100J backward)

You need to find the acceleration of the snowboarder to come to rest:

v² = v₀² + 2aΔx
a = (v² - v₀²) / 2Δx
a = [0 - (16m/s)²] / (2 x 9.4m)
= -14m/s²

Because friction equals the friction coefficient multiplied by the normal force (mg on a horizontal surface), the friction coeeficient is:

f = μmg
μ = f / mg
= 950N / (71kg x 9.8m/s²)
= 1.4

I don't like that number, but it is possible to have a friction coefficient greater than one. Hope this helps.
 
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