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does power equate to power/time

 
 
Reply Mon 10 Dec, 2012 04:26 pm
Can I say (and be correct):

Work done = energy used (work - energy theorem)

Power = Work done/time or P = f*d/t

and so

Work done/time = energy used/time and = Power

Therefore

ΔKE/t-(f*d)/t=0 => ΔKE/t - fv = 0 => P-fv = 0

so does Power equate to energy/time

Cheers Dave
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raprap
 
  1  
Reply Mon 10 Dec, 2012 07:39 pm
@David Smith,
power is energy divided by time.

Example

Horse Power is 550 ft-lbf/sec+33000 ft-lbf/min

1 horse Power = 756 Watt = 0.756 Kilowatt

Rap

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maxdancona
 
  1  
Reply Tue 11 Dec, 2012 12:02 pm
@David Smith,
You are basically on track, but you have to be careful with the terminology. You can "use" energy without doing any work. Depending on the problem you are solving there can be waste energy which also doesn't count.
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David Smith
 
  1  
Reply Tue 11 Dec, 2012 12:18 pm
Thanks
Thought it was right but I was cautious because it seemed to easy too solve the problem I was thinking of, which was calculating the energy used when footwear sole is compressed by body weight Vs ground reaction force. I can measure the discreet forces and the interface deformation magnitude and time, so I should be able to find the energy used.

Regards Dave
maxdancona
 
  2  
Reply Tue 11 Dec, 2012 01:36 pm
@David Smith,
I am not really sure where KE (which I assume means Kinetic energy) comes in to the calculation.

Assuming the sole is approximately elastic, there is an easy relationship between work and amount of deformation. But there will be multiple forces doing this work including forces from and varying share of weight, and actions of muscles.

It will be difficult to deduce much about any velocity involved from these measurements.

This calculation will only get you the small proportion of work done (and energy used) to deform the soles.
David Smith
 
  1  
Reply Wed 12 Dec, 2012 04:49 pm
@maxdancona,
"This calculation will only get you the small proportion of work done (and energy used) to deform the soles."

Yes that's the idea, we're just considering the effects of various types of insole, outersole and foot orthotic included in or added to a shoe in terms of energy storage and return.

Dave
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