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M2 Work, Energy and Power

I'm trying question 7.

I've managed part a) but having tried many thoughts with part b) I have now run out of ideas.

Below is the question and my workings. As the question asks to find the distance AC and then says "Hence" I have to use 8.93 somewhere but I can't think of where/how other than what I've already done on my second attempt.

Thanks.
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Original post by maggiehodgson
I'm trying question 7.

I've managed part a) but having tried many thoughts with part b) I have now run out of ideas.

Below is the question and my workings. As the question asks to find the distance AC and then says "Hence" I have to use 8.93 somewhere but I can't think of where/how other than what I've already done on my second attempt.

Thanks.


"The air resistance acts on the child as she moves from A to C" not only from B to C.And its mentioned find the distance AC hence,so have to consider about the full distance AC.Let me try,and let me know if you get the answer correct!
Original post by maggiehodgson
I'm trying question 7.

I've managed part a) but having tried many thoughts with part b) I have now run out of ideas.

Below is the question and my workings. As the question asks to find the distance AC and then says "Hence" I have to use 8.93 somewhere but I can't think of where/how other than what I've already done on my second attempt.

Thanks.

The actual answer for v is 6.41 or 6.51 that you have written on the paper??:confused:
b)

I'd work in terms of energy, and not use suvat for this.

From a) you know the child's energy at C assuming no resistances.

So just subtract the work done against air resistance to get the new KE at C. And convert to a velocity. 6.41 (3sig.fig) is correct.

c)

Again use energy considerations.

Energy at B assuming air resistance from A to B is....

This becomes the energy available to overcome all resistances from B onwards.

And then W=Fd.
Sorry,calculation error.Use the length AB not the height to find the work done against resistance.:wink:
Original post by Hyper Beast
The actual answer for v is 6.41 or 6.51 that you have written on the paper??:confused:



The actual answer, in the book, is 6.41.
Original post by ghostwalker
b)

I'd work in terms of energy, and not use suvat for this.

From a) you know the child's energy at C assuming no resistances.

So just subtract the work done against air resistance to get the new KE at C. And convert to a velocity. 6.41 (3sig.fig) is correct.

c)

Again use energy considerations.

Energy at B assuming air resistance from A to B is....

This becomes the energy available to overcome all resistances from B onwards.

And then W=Fd.


Oh Blimey. That's what I did in my second attempt at part b0 but I must have pressed the wrong buttons on my calculator and got 4.53!!

Thanks for the part c too. Hadn't got to that but it makes sense to me.

You will be happy to know I've finished the chapter. Very enjoyable. I'm now going to be starting on Elasticity.
Original post by Hyper Beast
"The air resistance acts on the child as she moves from A to C" not only from B to C.And its mentioned find the distance AC hence,so have to consider about the full distance AC.Let me try,and let me know if you get the answer correct!



Thanks Hyper Beast. It turns out that I had taken that into consideration but despite using the correct numbers and method, I got the wrong answer!!! With help from you and GhostWalker, I did eventually get the correct answer. I'm finding M2 a challenge but you seem to have a good handle on it. Thanks for chipping in.
I can see it from your workings that you have tried everything including a negative value for the energy!!just kidding,no offence

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