Corey S.

asked • 12/15/15

Work & force

I am a weight lifter and want to calculate how much "work" I do when performing the deadlift. I did the math and want to ensure my concepts and calculations are correct, as it has been a while since I took physics. Here is what I did:

I am lifting 225lb (1000.85 newton) a distance of 0.613m. So the work (in joules) = 1000.85n X 0.613m = 613 joules. I did 32 repetitions of this in total, so the total work would be 19,632.64 joules. So, if I convert that to watt hours, it is about 5.45 watt hours. Does this mean that to do that same amount of work, 5 watts for 1 hour would be required? Or 20 watts for 15 minutes? Or 300 watts for 1 minute?

Did I use the right calculations?

Ultimately I want to compare several of my lifts to see how much total work I performed (for example 10 sets of 5 reps on Bench Press at 150lbs VS 6 sets of 7 reps on Deadlift at 225 lbs)...

Thank you for any feedback and input!

Raphael D.

Regarding your question:
 
Does this mean that to do that same amount of work, 5 watts for 1 hour would be required? Or 20 watts for 15 minutes? Or 300 watts for 1 minute?
 
Now: what is work in terms of power and time?  It's simply Power*time.
So,
5W*1hr = 20*1/4(W*hr)=300*1/60 (W*hr)
 
Same work can be performed with less power for a long time, or, with a more power for (surely) shorter time (i.e. quicker).
 
While comparing your performances, always keep an eye on both power and time!
 
Otherwise, all calculations look good. (I assume, 225 lb was exactly the WEIGHT ... that is FORCE .. Just be careful to mass/weight convention ...)
 
Good luck!
 
 
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12/15/15

Corey S.

Thank you! 
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12/15/15

1 Expert Answer

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Bryan P. answered • 12/15/15

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Corey,
There is nothing wrong with your concept. The amount of work for lifting a particular weight will always be the same, regardless of speed or time held. The energy that you expend, however, can vary significantly. I think you'd have to be in a lab hooked up to machines while lifting to measure that.
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Corey S.

Thank you for your answer! 
 
I am just trying to get the "theoretical" number so I can compare one workout to another (I. E. So I know if I did more total work in week 2 vs week 1). I really just need a relative number with which I can compare my total work week to week. 
 
I think you're right, if I wanted to measure ACTUAL calories or something, they'd have to measure oxygen in and oxygen out to determine that, since our bodies are not 100 percent efficient machines. 
 
Much appreciated! 
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12/15/15

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