Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Converting to lbs of force

  1. Sep 28, 2006 #1
    Im trying to find out how much i would weigh (in pounds) if I were to land on a scale from a 10 foot drop, in a vacuum of course. Does anyone know the equation that would solve this? I know the scale would level back out to my original weight, but I want the weight it would initially display.
  2. jcsd
  3. Sep 28, 2006 #2
    try finding your momentum when youve reached your final velocity
  4. Sep 28, 2006 #3


    User Avatar

    Staff: Mentor

    It depends on how fast you decelerate when you hit the ground.
  5. Sep 28, 2006 #4
    I would round it off to instantly, no? Just imagine my legs being perfectly straight as i hit the ground..
  6. Sep 28, 2006 #5
    Thing is, when you lose momentum like this, you exert what is called an impulse, or force * time. If you decelerate instantly, that means you exert an infinitely large force for an infinitely small period of time.
  7. Sep 28, 2006 #6
    3.048m/s * 68.04kg = 207.38 kg m/s

    so the scale would read 207.38kg?
  8. Sep 29, 2006 #7


    User Avatar

    Staff: Mentor

    No. Notice that your units don't match...
  9. Sep 29, 2006 #8


    User Avatar

    Staff: Mentor

    If it takes you one second to slow to a stop, the average force that the ground exerts on you is 207.38 N. If it takes 0.1 second, the average force is 2073.8 N. If it takes 0.01 second, the average force is 20738 N. And so forth.
  10. Sep 29, 2006 #9
    i guess its alot more complicated than i thought it would be, actually it seems somewhat impossible to even come up with even a good estimate. thanks for all the help anyways
  11. Sep 29, 2006 #10
    A physics book could present such a problem and include the number of springs in the scale and the spring constants of each spring....
  12. Sep 29, 2006 #11

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    You would need to know the spring constant of the scale, then use:

    [tex]mgh = \frac{1}{2}k(\Delta x)^2[/tex]

    where [itex]\Delta x[/itex] is the displacement of the spring from its equilibrium position when you are just standing on it. This gives:

    [tex]\Delta x = \sqrt{2mgh/k}[/tex]

    You would then add mg/k (the spring extension due to your weight when just standing on it) to that to find the maximum displacement.


    [tex]F = k\Delta x + mg = \sqrt{2mghk} + mg[/tex]

    The higher the spring k, the higher the force.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook