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# Edexcel M2 - Some Energy help please watch

1. In the image attached, why is the change in total energy KE - GPE, and not GPE - KE, and in what case would it be GPE - KE.

If someone can please explain this to me it would be great, thanks.

2. It doesn't matter which way round you write it you still get the same answer.

gain in KE- loss in GPE = 0 so v^2 - 196 = 0 so v^ = 196 so v = 14 m/s

OR

loss in GPE - gain in KE = 0 so 196 - v^2 = 0 so 196 = v^2 so v = 14 m/s
3. (Original post by Teenie2)
It doesn't matter which way round you write it you still get the same answer.

gain in KE- loss in GPE = 0 so v^2 - 196 = 0 so v^ = 196 so v = 14 m/s

OR

loss in GPE - gain in KE = 0 so 196 - v^2 = 0 so 196 = v^2 so v = 14 m/s
Yes, but in this question then here, why is it GPE - KE. It won't be the same answer in this question.
4. (Original post by Reda2)
In the image attached, why is the change in total energy KE - GPE, and not GPE - KE, and in what case would it be GPE - KE.

If someone can please explain this to me it would be great, thanks.

Can you post the whole question and the full working?
5. (Original post by notnek)
Can you post the whole question and the full working?
This for example, attached. You can see it is, Loss in KE - gain in GPE = work done against friction.

Why is it that, and why is it not, gain in GPE - KE = work done against friction?
Is it because it is going from KE to GPE?
Thanks
6. (Original post by Reda2)
This for example, attached. You can see it is, Loss in KE - gain in GPE = work done against friction.

Why is it that, and why is it not, gain in GPE - KE = work done against friction?
Is it because it is going from KE to GPE?
Thanks
Referring to the image from your first post:

That's the solution but you haven't posted the question. Is the working on the left related to the working on the right?

It's confusing since they talk about "work done against friction" but it seems to be equal to 0. I'd need to see the question before I can help you.
7. (Original post by notnek)
Referring to the image from your first post:

That's the solution but you haven't posted the question. Is the working on the left related to the working on the right?

It's confusing since they talk about "work done against friction" but it seems to be equal to 0. I'd need to see the question before I can help you.
Sorry I forgot to attach them. The question in this reply.
Attachment 551477551479
Attached Images

8. (Original post by Reda2)
Sorry I forgot to attach them. The question in this reply.
Attachment 551477551479
I thought your issue was with the working that you posted in your original post.

This is a different question.

Can you please start again, and post the question and working that confused you and tell us again why.
9. (Original post by notnek)
Referring to the image from your first post:

That's the solution but you haven't posted the question. Is the working on the left related to the working on the right?

It's confusing since they talk about "work done against friction" but it seems to be equal to 0. I'd need to see the question before I can help you.
Always use this formula..

WORK DONE AGAINST RESISTANCE = LOSS IN ENERGY (KE OR GPE) - GAIN IN ENERGY (KE OR GPE)

Always do whatever energy is lost - whatever energy is gained
10. In both questions (GPE at start) + (KE at start) = (GPE at finish) + (KE at finish) + (work done)

In question 1 there is no loss in energy (e.g. no air resistance) this means that

(GPE at start) + (KE at start) = (GPE at finish) + (KE at finish) + (work done) so (mgh) + (0) =(0) + (0.5mv^2) + (0) so 2 * 9.8 * 10 = 0.5 * 2 * v^2 so v^2 = 196 so v = 14 m/s

In question 2 there is work done to overcome friction so this means that

(GPE at start) + (KE at start) = (GPE at finish) + (KE at finish) + (work done) so

(GPE at start) - (GPE at finish) + (KE at start) - (KE at finish) = (work done against friction) so

(GPE at start - GPE at finish) + (KE at start) - (KE at finish) = (work done against friction) so

(mgh) + (0.5 * m * initial velocity ^2) - (0.5 * m * final velocity ^2) = (work done against friction) so

(6 * 9.98 * 2) + (0.5 * 6 * 4) - (0.5 * 6 * v^2) = (0.25 * FN * 4)

There's a youtube video at https://www.youtube.com/watch?v=dj8Y1Pof9XU that explains this using an example of a block starting from rest sliding down a slope that might help you.
11. (Original post by fpmaniac)
Always use this formula..

WORK DONE AGAINST RESISTANCE = LOSS IN ENERGY (KE OR GPE) - GAIN IN ENERGY (KE OR GPE)

Always do whatever energy is lost - whatever energy is gained
That's the answer I was looking for, thank you very much.

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