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# M2 work, energy + power? watch

1. Hi, can anyone help with this question?

A box of mass 10kg is being pulled up a line of greatest slope of a rough plane that is inclned at an angle of 15 degrees to the horizontal. The coefficient of friction between the planes and the box is 0.25. At the instant when the box is moving at 10ms the rope pulling the box breaks. Find the further distance moved by the box before coming to rest.

I first tried resolving but without the pulling frce this was useless.

I have got the kinetic energy of the box when it is at 10ms to be 500J. Im thinking that kinetic enregy lost = gain in potential + friction.

So 500 = mgh + Fr x (distance) Is this right? The problem I have is that h is measured vertically and friction is down the plane at 15 degrees.

The answer is 10.2 which is 2(500/10g)
2. (Original post by Womble548)

I have got the kinetic energy of the box when it is at 10ms to be 500J. Im thinking that kinetic enregy lost = gain in potential + friction.

So 500 = mgh + Fr x (distance) Is this right? The problem I have is that h is measured vertically and friction is down the plane at 15 degrees.

The answer is 10.2 which is 2(500/10g)
h = sin15 S (h up, S along slope)

500 = (10g)*0.25* cos15 * S + 10g * sin15 * S

gives 23.6645 S + 25.36 S =500

S = 10.2

Aitch
3. (Original post by Womble548)
Hi, can anyone help with this question?

A box of mass 10kg is being pulled up a line of greatest slope of a rough plane that is inclned at an angle of 15 degrees to the horizontal. The coefficient of friction between the planes and the box is 0.25. At the instant when the box is moving at 10ms the rope pulling the box breaks. Find the further distance moved by the box before coming to rest.
KE loss = (0.5)(10)(10^2) = (5)(100) = 500J
PE gain = (10)(g)(xsin15), let x=the further distance travelled.
F = UR = 0.25[10gcos15] = 2.5gcos15
Work against Fr = 2.5gxcos15

Using conservation of momentum:
PE gain + Work against Fr = KE loss
10gxsin15 + 2.5gxcos15 = 500
x[10gsin15 + 2.5gcos15] = 500
x = 500/[10gsin15 + 2.5gcos15]
x = 10.2m
4. Thanks!
5. hi are all the three of you doing the M2 paper tomoz... cause you guys are definately on the ball, and look very ready for it!!
6. Me? I am not ready for it, what gave you that idea?

I can do the simple questions and can apply the concepts (dont understand half of them though) but when it gets complex I am in trouble
7. lol i hate mechanics and i dnt get any of it!!!!!
8. Good luck to u all for M2 exam tomorrow
9. (Original post by BCHL85)
Good luck to u all for M2 exam tomorrow
thnx! are you doing it too?
10. I'm doing it tomorrow. Good luck everyone.
I just need to ensure i don't rush or lose my composure. After taking M3 last week M2 shouldn't be so bad.
11. Yes I am. I got 87% last summer cuz of stupid mistakes and want to make it higher

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