# M2 Help Work-energy

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#1
I was taught that "loss in K.E=Gain in G.P.E" but then i did an m2 paper and it turns out i was supposed to use "loss in K.E=Gain in G.P.E+WORK DONE AGAINST FRICTION" so which one is right??

0
3 years ago
#2
Always use WD against friction = losses - gains
e.g say a particle goes up a plane: KE is lost and gpe is gained
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3 years ago
#3
Energy loss = work done against resistance/friction etc.

So if a particle travelled up on a rough slope then it gains GPE but it loses KE.
Therefore, (KE loss) - (GPE gain) = (work done against friction) [because I mentioned the slope was rough and resistance is negligible in this example]. So when you rearrange, (KE loss) = (GPE gain) + (work done against friction).

Remember, the total energy loss is [loss - gain] i.e. (KE loss) - (GPE gain) = (WD against....) or (PE loss) - (KE gain) = WD against....

However, if you're given a situation where a skier for example losses velocity and height so you have both KE and GPE loss then (KE loss + GPE gain) = WD against....

When you assume, KE loss = GPE gain then you're assuming there are no external forces acting on the particle such as friction or resistance.
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3 years ago
#4
(Original post by NigelK98)
I was taught that "loss in K.E=Gain in G.P.E" but then i did an m2 paper and it turns out i was supposed to use "loss in K.E=Gain in G.P.E+WORK DONE AGAINST FRICTION" so which one is right??
Kinetic energy only changes if a force or forces act on the body. GPE is just a way of calculating the work done against / by gravity, but it's no different from any other force - you could say the work done against forces, for the general case.

The right equation is therefore: Loss in KE = work done against all forces (with the correct signs for the conventions)

When it's gravity, you may think of it as GPE, but it's still just force (mg) times distance in the direction of the force (h).
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