Edexcel M2/M3 June 6th/10th 2013

yes but a better way to think about it is work done by the resistance on you

Why 1/root2 ?

First find the magnitude of the direction $\sqrt {1^2 + (-1)^2} = \sqrt {2}$

Then do the inverse of the magnitude x the Impulse x the parallel vector

$\frac{1}{\sqrt {2}} \times 9\sqrt {2} \times (i - j)$

$= 18i - 18j$

Couldn't you just do the first time to get root 2, and then realise to get 9 root 2 it would have to be 9(i-j)?

Because impulse is a vector which has magnitude and direction, so you have to do the inverse of the direction x the direction x the magnitude (sorry not the best way of explaining it)

This is just getting more confusing. work against resistance, work by cyclist, and ke and pe loss are all different things!
I just want to know why we're saying energy loss + wd by cyclist = wd against resistance

Thanks !!

This is just getting more confusing. work against resistance, work by cyclist, and ke and pe loss are all different things!
I just want to know why we're saying energy loss + wd by cyclist = wd against resistance

Because the question said there's an energy loss therefore whatever force is causing that loss must be greater than what's causing the gain

No no, because resistance causes energy loss but wd causes energy gain so the difference between Wd by R and energy loss is WD by cyclist

I think you are overcomplicating it. All energy loss in a system must be due to an external force opposing the motion. This force must be the resistance as there is no other force opposing the motion. This question was quite confusing as the cyclist has put a force into the system as well so the work done by the cyclist is adding to the input whereas the work resistance is taking energy out, just read this and attempt the question yourself, you should understand

I guess ill have a go at explaining. Lets take it back to the start. The work energy principle states the change in energy is equal the work done for any situation. So think of it as a change in energy not a loss. Say we lose pe and ke for a situation over a know distance. The change in energy is the sum of the two losses and hence this equals work done.

Hopefully that helps?

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This made things clearer! thanks!

No problem, I had to get my teacher explain this to me yesterday

Ah yes, that's the basic concept, I understood that. This question is confusing since there's extra work input by the cyclist. Wasted around 2 hours on this! -.-

Review exercise is killing me :|

So basically, energy loss in a system = wd against resistance. Tell me I'm right? :P
Haha. Good luck to you!

Just looked at that question and its the perfect example of a loss in both pe and ke. The difference between the work done due to resistance and the sum of the energy losses is the work done by the cyclist

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