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Thread: Basic HP question

  1. #1 Basic HP question 
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    Hey everyone. Just taking a basic physics class and we were given a problem to solve for our own hp. So heres the numbers: I went up a flight of stairs for a total distance of 6.2 ft. I weigh 190lbs. It took me 2.2 seconds. I know 1 hp = 550 ft lbs/sec. So..... I would think that I could set up 6.2ft over 190 lbs. and 1 ft over x and solve for it. So I get 30.69 ft lbs. Then I could set up 30.69 ft lbs over 2.2 seconds and x over 1 second to get 13.59 ft lbs/sec. Then set up 550 ft lbs/sec over 1 hp and 13.59 ft lbs/sec over x and solve for x. So I get .025HP. Does this sound right?
    Thanks,
    Scott
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  2. #2  
    mvb
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    Quote Originally Posted by scott.t.wetter View Post
    Hey everyone. Just taking a basic physics class and we were given a problem to solve for our own hp. So heres the numbers: I went up a flight of stairs for a total distance of 6.2 ft. I weigh 190lbs. It took me 2.2 seconds. I know 1 hp = 550 ft lbs/sec. So..... I would think that I could set up 6.2ft over 190 lbs. and 1 ft over x and solve for it. So I get 30.69 ft lbs. Then I could set up 30.69 ft lbs over 2.2 seconds and x over 1 second to get 13.59 ft lbs/sec. Then set up 550 ft lbs/sec over 1 hp and 13.59 ft lbs/sec over x and solve for x. So I get .025HP. Does this sound right?
    Thanks,
    Scott
    Something happened to the units in your first step, so I think you should check your algebra. It is usually better to work a problem analytically first anyway: that is less complicated and you are less likely to make a mistake. So I would write out your starting point as P=F d / t, "power is force times distance over elapsed time." Then there is actually no algebra to do, you just plug in the numbers and convert to the units you want. There is not much difference in this problem, although what you have done will be clearer to a reader. In later, more complicated problems, proceeding systematically makes the effort much simpler.
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  3. #3  
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    Ok I can see that. Thank you. What units would the p be then? I'm assuming hp and from there I can divide by 550 to get a fractional hp. Thanks again
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  4. #4  
    mvb
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    Quote Originally Posted by scott.t.wetter View Post
    Ok I can see that. Thank you. What units would the p be then? I'm assuming hp and from there I can divide by 550 to get a fractional hp. Thanks again
    In English units, P would most naturally be in foot-pounds/second, which you could change to horse-power using the factor you quote above and in your OP.
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  5. #5  
    KJW
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    In problems like this, one shouldn't just plug in the numbers, but the units as well. That way, you can algebraically manipulate the units along with the arithmetic.
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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