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

Relative weight when falling

  1. Jan 26, 2015 #1
    Mostly for self interest:
    If I drop a 1lb weight from 15 ft (5m), what does it weigh when it hits the floor.
    If I have a 10lb weight and it drops from the same 5m shelf, does it weigh 10 times as much as my 1 lb weight from the 1st question?

    I may have the wrong terminology, but basically the 1lb weight in my hand weighs 1lb. If I through it in the air and catch it, it is heavier when it hits my hand, because of the force of gravity pulling it back down. If that fall is 5 meters, what is the weight the floor will feel when the object hits the ground.
     
  2. jcsd
  3. Jan 26, 2015 #2

    NascentOxygen

    User Avatar

    Staff: Mentor

    Hi Theatre_Kid. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    Without knowing the muscle strength you can adeptly repeatedly exert, I don't think it is possible to make a comparison the way you are asking.

    A comparison that could be made is: if a 1lb weight when gently sat on a spring depressed it by x inches, how many inches would the spring depress if that weight were dropped on it from 15 ft.
     
    Last edited by a moderator: May 7, 2017
  4. Jan 26, 2015 #3

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    When the 1lb weight hits the floor it will be travelling at some velocity V. You can work that out using...

    V = Sqrt(2as)

    where
    a is the acceleration due to gravity (9.81m/s2)
    s is the displacement or height it fell from (eg 5m)

    The problem is working out the force with which the weight hits the ground. Suppose the ground was soft and the weight sank in (s) 5cm (0.05m) before coming to rest. You could run the equation in reverse to work out the average deceleration (d)...

    d = V2/2s

    Then you could use Newtons laws to estimate the force (F) ...

    F = md

    where
    m=mass of the object.

    Note that the harder the ground the shorter the stopping distance. If the 1lb weigh somehow hit the ground and stopped dead the deceleration would be very large, potentially infinite. Therefore the force on the object/ground would also be very large. That's why things dropped onto concrete tend to break.

    (Aside: The above makes some assumptions that aren't always valid but that's another story).
     
  5. Jan 26, 2015 #4
    Thank you, both for the welcome and the answer.
    Is there a way to know the stopping distance? I get that concrete would be shorter than wood or foam, but is there a property (density? hardness) of a given material that would let me know how to calculate that, or something I just make an assumption for and move on?

    Also, am I right in believing that weight (pounds) is a measure of force? IE it measures the force of gravity on a given object?

    Finally, does it matter if I'm mixing imperial and metric units (a 1lb weight falling 5 meters)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Relative weight when falling
  1. Falling weight (Replies: 1)

Loading...