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# work energy power Watch

1. I did this twice and got the same answer both times, but it's different to the book?
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2. (Original post by pippabethan)
I did this twice and got the same answer both times, but it's different to the book?
Your picture is rather poor quality, and I can't make out what you're doing clearly.

It does look however that you've misinterpreted the question. When the cyclist is freewheeling they are providing no power, i.e. driving force = 0.
3. (Original post by ghostwalker)
Your picture is rather poor quality, and I can't make out what you're doing clearly.

It does look however that you've misinterpreted the question. When the cyclist is freewheeling they are providing no power, i.e. driving force = 0.
Oh okay, I had done it as if they were providing a driving force.

I tried it again and got the answer in the book, but with the opposite sign, so please could you take a look to see if my explanation sentence is right (otherwise I've messed up somewhere else)

Hopefully this picture is clearer
Attached Images

4. (Original post by pippabethan)
I tried it again and got the answer in the book, but with the opposite sign, so please could you take a look to see if my explanation sentence is right (otherwise I've messed up somewhere else)

Hopefully this picture is clearer
Yes, I can see that one clearly.

Your v should be positive. And your explanation is really just an attempt to explain away why it's negative.

The error lies in your initial equation for going uphill. Both the resistance and the component of the weight along the plane are acting in the same direction, and so would have the same sign; but you have one positive and one negative.
5. (Original post by ghostwalker)
Yes, I can see that one clearly.

Your v should be positive. And your explanation is really just an attempt to explain away why it's negative.

The error lies in your initial equation for going uphill. Both the resistance and the component of the weight along the plane are acting in the same direction, and so would have the same sign; but you have one positive and one negative.

Aha I see, thank you for being so helpful
6. (Original post by pippabethan)
Aha I see, thank you for being so helpful
You're welcome.

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