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A simpe force question

  1. Mar 28, 2013 #1
    A value of 737 foot pounds per second is the same as saying 1.34hp (horsepower).

    If I divide 737 by 7 pounds, what is the answer 105? Is it then feet per second?

    in less words

    737lbf/s divided by 7lb = 105f/s

    is that right?

    Is that calculating the speed for the 7 pound object that had 1.34hp applied to it for one second?

    It's not passing the "smell test" for me, though as it would mean 0-115km/h in one second...11.63g(force)
     
    Last edited by a moderator: Mar 29, 2013
  2. jcsd
  3. Mar 29, 2013 #2
    you are almost right , the 7lb force is needed to move some object at a speed 105f/s (its not the mass of the body ) the 7lb force arises becasue of the resistance (friction , air drag, etc)
     
  4. Mar 29, 2013 #3

    Doc Al

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    You really need to describe the problem you are trying to solve.

    If you provide an object with 737 foot-lbs of energy, you can calculate the resulting change in velocity by calculating the change in kinetic energy.
     
  5. Mar 29, 2013 #4
    Imperial units are very confusing here, because pounds can refer to either force or mass, which are totally different quantities.

    Foot-pound is a unit of energy, because it is length * force.
    Whereas, if you are talking about a 7 pound object, you really are talking about the mass of the object, so this is a different kind of pound. So you can't divide 737 foot pounds per second by 7 pounds simply.

    To convert from pounds (mass) to pounds (force), you need to multiply by the Earth's gravitational acceleration g.

    So, the correct answer
    737 ft*(lb*g)/s / (7 lb) = 105 ft/s*g = 3360 ft^2/s^3
     
  6. Mar 29, 2013 #5
    with a remote control car toy, i measured a value of 90 amps at 11.1 volts. The remote control car weighs about 7 lbs.

    Using just these values I'm trying to learn some simple physics. the "real" world connection to these straight bananas calculations is just to help reinforce what the values are....if that makes any sense.


    so from 90amps * 11.1v = 1,000w approximately.

    that's about 1.34hp or 737 foot-pounds force per second.

    so I was wondering what the value 737 foot-pounds force per second divided by the 7lb RC car would be representing...if anything at all. (do the "pounds" or weight "cancel" leaving just the time and distance units)

    here is what wiki says a foot-pound force is a measure of

    "A slug has a mass of 32.174049*lbm. A pound-force is the amount of force required to accelerate a slug at a rate of 1*ft/s2"
     
  7. Mar 29, 2013 #6

    Doc Al

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    Foot-pound force is a unit of energy.
     
  8. Mar 29, 2013 #7
    Thanks Khashishi!

    what is 3360 ft^2/s^3 ?
     
  9. Mar 29, 2013 #8
    I'm not very smart or clever Doc Al.

    Thought I was applying some amount of foot-pound force to a 7lb object. and could calculate an acceleration. In this case 105f/s^2.

    If that's not what was calculated I'd like to know what it actual is. Think that's what malemdk posted.
     
  10. Mar 29, 2013 #9

    HallsofIvy

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    Doc Al just told you that "foot-pound" is NOT a measure of force but you repeat the mistake: if you apply a pound force, not "foot-pound" force.

     
  11. Mar 29, 2013 #10

    Doc Al

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    Note that in your first post you never once mentioned that you were trying to calculate an acceleration.

    To get an acceleration: Divide the net force (measured in pounds, not foot-pounds) by the mass (measured in slugs).

    You divided a power (737 ft-lb/s) by a force (7 lbs). That will give you a speed (105 ft/s). That's the speed you must be moving at if want to deliver 737 ft-lb/s of power by pushing with a force of 7 lbs. (Power = force * speed)
     
  12. Mar 29, 2013 #11
    Thanks Doc Al! :smile:

    in my first post I did say;

    "Is that calculating the speed for the 7 pound object that had 1.34hp applied to it for one second?"

    I see that as an acceleration, but may be wrong. idk
     
  13. Mar 29, 2013 #12

    sophiecentaur

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    OMG. SI, where are you when we need you?
     
  14. Mar 29, 2013 #13
    Oh it's worse; I'm metric raised Canadian. :smile:
     
  15. Mar 29, 2013 #14

    SteamKing

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    You can't blame this on Imperial units. The OP started out with volts, amps, and watts, and then veered out of control into hp and pounds.

    I think the OP might be trying to figure out what is the value of lbs/hp for his model car. Lbs/hp is often used for different vehicles (cars, planes, boats, etc.) to gauge how quickly they can accelerate.
     
  16. Mar 30, 2013 #15
    not trying to calculate anything specific for the remote control car toy, of course there are much easier ways to do that, such as with a clock and ruler. it's about 7lbs/1.35hp or 5.2lbs per hp. Better than a Dodge viper, but worse than a Ferrari Enzo. :rofl:

    Maybe it wasn't clear, "Is that calculating the speed for the 7 pound object that had 1.34hp applied to it for one second?", is what i was trying to calculate.

    The OP started out with volts, amps, and watts, and then veered out of control into hp and pounds.

    amps * volts = watts
    1hp = 745 watts just different units.

    I went out of control thinking pound force had something to do with weight, so that I could just divide the foot-pound value by the 7lb mass. And that`s where the OMG SI where are you comment comes in.

    Looks like pound can refer to a mass or pound(force) refer to gravity acting on a mass.

    Khashishi provided the conversion, but I didn't get a reasonable answer.
     
    Last edited: Mar 30, 2013
  17. Mar 30, 2013 #16
    Yup, I can read, just can't understand, 'cause I've read elsewhere...


    Wanna know what a foot-pound is?

    " It is the energy transferred on applying a force of one pound-force (lbf) through a displacement of one foot. The corresponding SI unit is the joule." wiki

    wanna know what a pound force is?

    "A slug has a mass of 32.174049*lbm. A pound-force is the amount of force required to accelerate a slug at a rate of 1*ft/s2" wiki
     
  18. Mar 30, 2013 #17

    Doc Al

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    All of that is perfectly consistent.

    I recommend that you forget about pound-mass for the moment. Then things are easy. Force is measured in pounds and mass in slugs. (1 slug is about 14.6 kg.)

    Weight and mass are related using W = mg. Here g ≈ 32.2 ft/s^2.
     
  19. Mar 30, 2013 #18

    sophiecentaur

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    The daft thing about the Imperial system is that the word "pound" appears in the context of forces and masses. I know that there is the "force" word, just after it but why keep such nonsense going? When I did the fps system at School, we also had Poundals (force to accelerate a mass of one pound by 1 ft /s/s). They were obviously floundering, even in the 1950s and the UK made the smart move of getting out PDQ (via the cgs system, which was a bit nasty in some ways, with its ergs and dynes).
     
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