M3 Elastic strings on inclined planes

Anyone?

To calculate the change in GPE & EPE I'm doing (Final EPE/GPE - Start EPE/GPE) - should it be the other way round?

I think of it as:

Starting energy - work done by system = final energy.

Which rearranges to "Work done = starting energy - final energy".

I note that you have the GPE as positive, although P is below your zero level.

I was going to put GPE as -ve but then I thought that energy is a scalar so it doesn't matter?

I was going to put GPE as -ve but then I thought that energy is a scalar so it doesn't matter?

It matters.

In the case of GPE, it arises from work done against a force, in this case gravity. As the amount of work done increases - i.e. you raise the object higher - so its energy increases.

Ah - That gives me the +ve answer. Thanks.

Can you see any error in the second part? The way I've interpreted the question, is that the system is still released at AP=0.6, but I need to work out the speed after P has moved to AP=0.8?

Also, I have a similar problem with the second part:

$AP=0.8,[br]EPE = \dfrac{20 \times 0.2^2}{1.2}, GPE = 2g(0.8)\sin(45), KE = \dfrac{1}{2}(2)v^2= v^2$

Work done,W, against friction in moving from AP=0.6 to 0.8:

$W = \dfrac{\sqrt{2}}{25}g$

After rearranging I get v=1.63ms^-1 when the correct answer is 1.25.

For the GPE, the distance moved down the plane is 0.2, not 0.8

But A is 0 GPE, and P is below A, so when P has moved down the plane by 0.2, isn't it then 0.8m from A?

True; my mistake. I was assuming that you had the change in GPE there, since you'd only posted one, rather than the initial and final GPEs.

Again, this would be negative relative to A.

Edit: Never mind, I was being stupid - forgot the g. Thanks for your help.