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# M2 watch

1. A box of mass is resting on a sloping platform which is inclined at 20 degrees to the horizontal. The coefficient of friction between the box and the platform is 0.2. The box is released from rest and slides down the platform. Calculate:

a, the speed of the box after it has been moving for 3 seconds.
b, the potential energy lost by the box during this time.

I can't seem to get the right answer for part a, even though I know how to find each individual component
2. (Original post by DOJO)
A box of mass m is resting on a sloping platform which is inclined at 20 degrees to the horizontal. The coefficient of friction between the box and the platform is 0.2. The box is released from rest and slides down the platform. Calculate:

a, the speed of the box after it has been moving for 3 seconds.
b, the potential energy lost by the box during this time.
Resultant F = mgsin20 - Friction = mgsin20 - 0.2R = mgsin20 - 0.2(mgcos20)
= mg[sin20 - 0.2cos20]

-> ma = mg[sin20 - 0.2cos20]
-> a = g[sin20 - 0.2cos20]

u = 0, a = 9.8[sin20 - 0.2cos20], t = 3.
-> v = u + at = 29.4[sin20 - 0.2cos20] = 4.53 ms^-1

b.) s = ut + at^2/2 = 9.8[sin20 - 0.2cos20]*(9/2) = 6.795 m

Change In Height During Motion = (sin20) * (6.795) = 2.324 m

Lost PE = mg*(6.795) = 66.6m J (Where m is mass of box)
3. As usual you will get a rep for the help you have given me. But no, not today you won't because it is not letting me.

anyway, back to collisions (M2 is so fun!)....

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