The question in question is this (q2 pg 161 of the edexcel M2 textbook)
A box of mass 2kg is projected down a rough inclined plane with speed 4ms-1. The plane is inclined to the horizontal at an angle b, where tanb = 4/3, and the coefficient of friction between the box and the plane is 0.5. The box is modelled as a particle and air resistance is negligible. Using the work-energy principle, find the speed of the box after it has moved a distance of 5m down the plane.
So this is what I did first.
Resolving perpendicular to the plane, R = 2g cosb = 2g * 3/5 = 6g/5
Resolving parallel to the plane, downwards positive, 2g * sinb - 0.5R = 2g * 4/5 - 3g/5 = g
So the net force acting down the plane is gN.
Ek before = 1/2 * 2 * 4^2 = 16J
Ek after = 1/2 * 2 * v^2 = v^2
So delta Ek = v^2 - 16
Ep before = 0
Ep after = 4 * 2g = 8g
So delta Ep = -8g
Using the work - energy principle,
Wd * s = delta Ek + delta Ep
g * 5 = v^2 - 16 -8g
13g = v^2 - 16 => v = 12.0 ms-1 (3sf)
This is the incorrect answer
I retried it without including the Ep
Wd * s = delta Ek
5g = v^2 - 16 => v = root 65 = 8.06 ms-1 (3sf)
8.06 ms-1 is the correct answer - so why is the potential energy not included in this question?
M2 Work Energy Principle Question
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