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# Little confused with this question about energy Watch

1. http://filestore.aqa.org.uk/subjects...W-QP-JUN10.PDF

Question 4b(i)

So I thought that GPE lost = KE gained so surely as it comes back down 3.2m then it would just be 3.2 * 9.81 * 55 = 1726.56 J?

The mark scheme says this however

(Ek = 3.2/4.2 x 2264 or uses suitable kinematics equation)1= 1700(J) ! (= 1724.8 = 1720)

I don't understand why they divided 3.2/4.2 at all. Can anyone explain?
Thanks
2. Any ideas?
3. (Original post by CrazyFool229)
http://filestore.aqa.org.uk/subjects...W-QP-JUN10.PDF

Question 4b(i)

So I thought that GPE lost = KE gained so surely as it comes back down 3.2m then it would just be 3.2 * 9.81 * 55 = 1726.56 J?

The mark scheme says this however

(Ek = 3.2/4.2 x 2264 or uses suitable kinematics equation)1= 1700(J) ! (= 1724.8 = 1720)

I don't understand why they divided 3.2/4.2 at all. Can anyone explain?
Thanks
mgh = 55 x 9.81 x 4.2 = 2266 Joules

GPE lost = 55 x 9.81 x 3.2 = 1726.56 Joules

In other words, the mark scheme simply cancels out the 4.2m to avoid writing the whole expression again.

i.e.

GPE lost =

So it seems like you did the same equation as me but for some reason the answer is still different to the mark scheme? Sorry if this is stupid as I know it is only one mark!

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5. I just noticed that the mark scheme used a different energy rather than the one calculated which will affect the answer. Is there a reason for that also?

Posted from TSR Mobile
6. (Original post by CrazyFool229)
I just noticed that the mark scheme used a different energy rather than the one calculated which will affect the answer. Is there a reason for that also?

Posted from TSR Mobile
It's the same, they just rounded it to 1700J which tells other markers that rounding is acceptable in this instance since it's the method they are after.
7. Oh I see now! Mark schemes can be a little confusing for me sometimes

Thank you so much for your help

Posted from TSR Mobile
8. (Original post by CrazyFool229)
Oh I see now! Mark schemes can be a little confusing for me sometimes

Thank you so much for your help

Posted from TSR Mobile
No worries.

Mark schemes are not produced for students. They are there for the army of markers who will all have a suitable qualification in Physics and Maths and therefore it's an aid for them to get consistency between each other. The fact that students can get access to them is helpful to an extent but also causes confusion as you have discovered!

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