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B Regarding the measurement of horsepower

  1. Oct 12, 2018 #1
    I teach aerodynamics. One horsepower is 550 lbs moved one foot in one second.

    My question is this. Does that assume that the 550 lbs is being lifted vertically against gravity? As opposed to moving, say, a 550 lb chunk of concrete across a floor.

    And if so, does the gravity value of 32.2 ft per second per second effect the measurement when gravity changes?

    Tex
     
  2. jcsd
  3. Oct 12, 2018 #2

    mjc123

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    Yes, it is lifted vertically.
    And no, the 550 lb force is constant. If g varies, the mass required to give 550 lb weight will vary, but the definition remains 550 lb weight.
     
  4. Oct 12, 2018 #3

    russ_watters

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    No, the definition of horsepower is a force of 550 lb applied over a distance of 1 ft in one second. I think that should be sufficient to clarify the rest you were confused about.
     
  5. Oct 12, 2018 #4

    russ_watters

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    Note that historically (in the wiki) the first reference, prior to James Watt, was indeed of lifting water. However Watt defined it in terms of pull force and linear motion, which is more general/accurate.
     
  6. Oct 12, 2018 #5

    anorlunda

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    That's true, but we should also point out that the force needed to pull a 550 pound block horizontally across the floor depends on friction, and in most cases will be less than 550 pounds.
     
  7. Oct 12, 2018 #6

    russ_watters

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    Of course; that's why horsepower is defined according to the force, not the mass or weight of the object it is being applied to.
     
  8. Oct 12, 2018 #7
    So, in effect...

    If I apply a measured force of 550 lbs to some unknown something and the work produced is one foot of movement in one second, I will note that the total opposing force including drag or friction plus inertia will be 550 lbs. that could be a 1 lb nerf ball with an additional 549 lbs of opposing force.
    Tex
     
  9. Oct 12, 2018 #8

    anorlunda

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    Now you took something very simple and made it complicated enough that I can't understand the above.

    If you make a 550 pound force (Measure it by tension on the rope. That includes a force pulling and a force resisting.) over a distance of one foot in one second, that is 1 HP.

    SLT_MSAN13208010000.jpg
     
  10. Oct 12, 2018 #9

    phinds

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    No, that is not correct. If the nerf ball experiences no friction then it would not enter into your calculation so the result would be 549/550ths of a HP. Listen to what people are telling you. It is NOT the weight/mass that enters into the calculation, it is just the force. If your 1lb nerf experienced 10lbs of friction that had to be overcome then your example would result in 559/550ths of a HP
     
  11. Oct 12, 2018 #10

    sophiecentaur

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    I have read this thread and it seems a further good reason for switching to SI Units. A Force is a Force and a Mass is a Mass in SI. If you are lifting a load or pushing it across a rough horizontal surface the Force will be in Newtons and the Power will be the Force times the Speed.
    Pounds Mass, Pounds Weight and Slugs is a world I would rather not venture back into. I left that world in about 1959, iirc and took the long route to SI via cgs and mKs during my education years.
    I found this as confusing as the post you are responding to. This can't go far without some diagrams with labels, numbers and arrows on.
     
  12. Oct 12, 2018 #11
    My point is this.

    If there is a huge slab of concrete weighing several thousand lbs. and I apply a measured force of 550 lbs and move that block one foot in one second then I only did the work of one horsepower. In other words...it doesn’t depend on the weight of the object...it depends on how much force is applied and the distance and time.

    We can argue or debate WHY or HOW a two thousand lb block moved a foot but what matters is that it DID move under the force of 550 lbs, it moved one foot, and did so in one second. So one horsepower of work was done.

    Correct?
     
  13. Oct 12, 2018 #12
    Perhaps another way to clarify (or confuse) the issue is that one horsepower is (roughly) equivalent to 746 Watts. No reason to look at them really as anything other than Power defined in terms of different units/quantities. i.e. It is pretty much just an expression of a quantity of Power. The historical definitions are just a way of quantifying a consistently describable amount. The definition of power remains the same (Work / time) no matter how we quantify the units.

    diogenesNY
     
    Last edited: Oct 12, 2018
  14. Oct 12, 2018 #13

    Dale

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    Yes, exactly
     
  15. Oct 12, 2018 #14
    FYI..

    This comes up in aerodynamics all of the time.

    If there is 60 excess BHP available, determined between the power required at a given airspeed and the power available, then a aircraft of a certain weight can be expected to climb at a rate calculated from that excess BHP and weight. This is also used in cruise performance calculations.

    Tex
     
  16. Oct 13, 2018 #15

    sophiecentaur

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    This bare description could be more precise because it suggests that the block starts of stationary. What should be included is that you would be maintaining the speed of the block at one ft per second. A motor with less than 1hp (or a weak, feeble human) can sustain the speed of a canal boat of many tons so that it does one foot per second. However, it can take several minutes to reach that speed from stationary.
    In most of Physics, it is essential to state the conditions precisely if you want to avoid the wrong implication. :smile:
     
  17. Oct 13, 2018 #16

    jbriggs444

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    An average of one horsepower of power was provided by the 550 pound force over the course of that second. 550 foot-pounds of work was done by the force.

    Other forces such as friction, gravity, normal force and three other helpers also pushing may have acted. But what matters for the work done by the 550 pound force is that it was applied and that the object moved one foot in the direction of the force while it was applied. The elapsed time then matters for the "power" -- the rate at which work is supplied.
     
  18. Oct 13, 2018 #17

    sophiecentaur

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    Yes. You often need to provide appropriate extra conditions for the force to achieve the Power input to the system.
     
  19. Oct 14, 2018 #18

    Tom.G

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    Sure looks good to me!
     
  20. Oct 16, 2018 #19

    anorlunda

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    Don't you use both kgf and kgm? When buying meat by weight in Europe, does the scale indicate kg or newtons? Is the price quoted in kg or newtons?
     
  21. Oct 17, 2018 #20

    sophiecentaur

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    NEVER.
    The unit of Force is the Newton in SI. When we buy meat or veg, then we, of course, want to buy it by Mass (how much of it there is) and we usually measure that by the effect of the Weight force on a spring or piezo crystal. We also, sometimes, compare the weight force of the leg of lamb against the weight force of some standard brass masses.
    PS Can you find and quote any authority that implies this?
     
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