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# Work=ForcexDistance?

Registered User
edited October 2009
To start, I'm not looking for answers. I just want to know if I'm on the right track. I'm helping my wife with her homework, and the powerpoint notes aren't great, so here we go.

The scenario is a 150lb subject walking up 4m worth of stairs in 12 seconds. First, we are supposed to convert the subject's weight into Newtons, which gives us 675 (Actually, I'm not so sure about this. Now I'm getting 667?). Then, the work in Joules. Work=forcexdistance, so W=675x4m, which would be W=2700? Then, we are supposed to convert it into Power. Power=Work/time, so P=675/12s?

Basically, are we on the right track? Thanks for any help.

Guffrey on
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## Posts

• Registered User regular
edited October 2009
You've got the right methodology. I haven't checked your numbers.

A simpler way to conceptualize this is to calculate the potential energy change, though.

MWO User Name: Gorn Arming
StarCraft II User Name: DeadMenRise
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• Registered User
edited October 2009
You've got the right idea. Might want to double check that last step though...
Guffrey wrote:
Power=Work/time

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• Registered User regular
edited October 2009
You've got the right idea. Might want to double check that last step though...
Guffrey wrote:
Power=Work/time
Er... what's the problem here?

MWO User Name: Gorn Arming
StarCraft II User Name: DeadMenRise
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• Registered User regular
edited October 2009
You've got the right idea. Might want to double check that last step though...
Guffrey wrote:
Power=Work/time
Er... what's the problem here?

He wrote work/time, but reused the weight. It needs to be 2700N*M/12s

... but you're fucking Metalbourne. You rock a blue hawk like it's not even a thing. Nothing is too much for you.
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• Registered User
edited October 2009
Ok, double checked the Newtons. I did the formula, and checked with google conversions, and it should be 667. Thanks for all the help so far, I thought we were on the right track, but I needed some confirmation. One more question. The problem also gives the time the subject walked up the stairs (12 seconds), and the time it took to run up the stairs (4). Now, using the formulas, the Work should be the same for both times, but the Power would be different? So, the subject did the same amount of work (covered the same distance) regardless of speed, but generated more Power the second time because of increased speed?

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• Registered User
edited October 2009
Google indicates 150 lbf is around 667 N.

Everything else looks right, so long as that 4 meters is vertical displacement. The distance has to be in the same direction as the force (which is up--opposite gravity).

EDIT: Metalbourne caught your numeric error and you're correct, the work is the same but it takes more power to do it more quickly.

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• Registered User
edited October 2009
You've got the right idea. Might want to double check that last step though...
Guffrey wrote:
Power=Work/time
Er... what's the problem here?

He wrote work/time, but reused the weight. It needs to be 2700N*M/12s

Yeah, my bad. In my mind I had already converted to Newtons, but failed to do so on paper. Thanks for catching it.

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• Registered User
edited October 2009
Google indicates 150 lbf is around 667 N.

Everything else looks right, so long as that 4 meters is vertical displacement. The distance has to be in the same direction as the force (which is up--opposite gravity).

I didn't even think of that, but yes, its vertical

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• Registered User regular
edited October 2009
You've got the right idea. Might want to double check that last step though...
Guffrey wrote:
Power=Work/time
Er... what's the problem here?

He wrote work/time, but reused the weight. It needs to be 2700N*M/12s
Oh, I thought he was pointing out an error in the formula. Yeah, I didn't look at the numbers.

MWO User Name: Gorn Arming
StarCraft II User Name: DeadMenRise
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• Registered User
edited October 2009
Alright guys, thanks for all the quick responses. It all makes sense, and we've got it figured out, so thanks for all the help. Lock please.

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