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    Hi,

    When does work done against resistances = change in energy?

    I thought always, but, when asked to find out the work done by a skier:

    work done against resistance = change in energy + work done by skier.

    So when does one have to take into account the work done by the skier?

    Here's the question: http://img690.imageshack.us/img690/655/imagexq.jpg
    The solution: http://img691.imageshack.us/img691/1333/imagewh.jpg

    If we're not asked to find the work done by the skier, when should i take it into consideration?

    Thanks

    JT80
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    (Original post by JTeighty)
    Hi,

    When does work done against resistances = change in energy?

    I thought always, but, when asked to find out the work done by a skier:

    work done against resistance = change in energy + work done by skier.

    So when does one have to take into account the work done by the skier?

    Here's the question: http://img690.imageshack.us/img690/655/imagexq.jpg
    The solution: http://img691.imageshack.us/img691/1333/imagewh.jpg

    If we're not asked to find the work done by the skier, when should i take it into consideration?

    Thanks

    JT80
    My understanding of your question is this:

    If there is no force being put into the system (by say an engine, a cyclist pedaling etc...), then any change in energy is equal to the work done against resistance.

    In the question you've used, the cyclist would not be able to get up to 12m with 5ms-1 of speed with those intial conditions, without the cyclist doing some Work. He would need a speed of 10.9ms-1 at the start to arrive at point B with a speed of 5ms-1, so he must do some work in the intervening distance to acheive such a speed.

    I would hope that the exam board, at M2 level, would tell you to find the Work Done by the cyclist if any work is done by them.

    Hope that helps.
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    (Original post by insane2642)
    My understanding of your question is this:

    If there is no force being put into the system (by say an engine, a cyclist pedaling etc...), then any change in energy is equal to the work done against resistance.

    In the question you've used, the cyclist would not be able to get up to 12m with 5ms-1 of speed with those intial conditions, without the cyclist doing some Work. He would need a speed of 10.9ms-1 at the start to arrive at point B with a speed of 5ms-1, so he must do some work in the intervening distance to acheive such a speed.

    I would hope that the exam board, at M2 level, would tell you to find the Work Done by the cyclist if any work is done by them.

    Hope that helps.
    Thanks for explaining it.Are u also saying that unless asked find out wd by cyclist or skier don't take any of this into consideration?
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    (Original post by JTeighty)
    Thanks for explaining it.Are u also saying that unless asked find out wd by cyclist or skier don't take any of this into consideration?
    I don't know that for a fact, but I feel that at M2 level it would be pretty unfair to expect you to work it out unless there is some explicit mention of it in the question.

    I would ask your teacher though, just to be sure, as I don't deal with this stuff everyday.
 
 
 
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