Mechanics Work done Question

I had to calculate the work done BY air resistance in a particular question. I gave the answer as a negative number (as the air resistance does work against the particle’s motion). However, the mark scheme doesn’t include the minus sign and my teacher cut a mark for it. Why is it inappropriate to include the negative sign? Or is it not?

Any help is greatly appreciated.

Reply 1

mqb2766

21

Original post by Richie112233

I had to calculate the work done BY air resistance in a particular question. I gave the answer as a negative number (as the air resistance does work against the particle’s motion). However, the mark scheme doesn’t include the minus sign and my teacher cut a mark for it. Why is it inappropriate to include the negative sign? Or is it not?

Any help is greatly appreciated.

Can you upload the question/solution? The by/against wording is probably important.

OP

ThAnks. Which exam is this - will check properly when I wake up.

Reply 4

Richie112233

OP

2

Original post by mqb2766

ThAnks. Which exam is this - will check properly when I wake up.

https://www.theallpapers.com/papers/CIE/AS_and_ALevel/Mathematics%20(9709)/9709_s08_qp_4.pdf

From CAIE A levels Mathematics.

Reply 5

mqb2766

21

Tbh, I've tried to justify a positive answer for the work done on the object, but the simplest explanation is that work done is the change in KE+PE (which is what they've done) but it's
final - initial
They've done initial-final. I can't see any subtlety in the wording, so I suspect a typo in the m/s?

Mechanics Work done Question

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