Year 12s: what will be the first way you research your future options? >>

M2 work done by a variable force

I'm trying question 9.

I managed part a) OK the extension being 0.395 (book answer 0.4). Part b) I've done this:

As it's a string not a spring then the string goes slack when the extension runs out. I've found the EPE with extension and I've got 15.6025 not 16 as I used 0.395 rather than 0.4. Then I've done Work Done against friction just for the extension (using 0.395) and subtracted that from EPE and then equated it to KE.

My answer was close but not close enough to round to the book answer so I decided to use the book answer for part a) rather than my own. I got 3.425 and the book answer is 3.46.

Is my logic incorrect and if so where?

Also, in subsequent parts to a question, should I be using the rounded value from a previous part of the less rounded value?