In the figure above, a box of mass 2.5kg is initially at rest on a rough inclined plane. Assuming the
mass is released and acquires a velocity of 4m/s at point P, the base of the incline. Find
A) The Work done against friction. (6 marks)
B) The Friction force.

Explanation:

By work - energy theorem on the box

Wmg + wn + wf = kf - ki

=> mgh + 0 + wf = 1/2 mv^2f - 1/2mv^2i

=> wf = 1/2mv^2f - 1/2mv^2i - mgh

:. Wf = 1/2×2.5×4^2 - 0- 2.5×9.8×1.5

= 20 - 36.75

= -16.75 J

Work done comes out negative here which is expected as the friction always act opposite to the velocity of an object. Friction is a non conservative force and it causes loss of mechanical energy.

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