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Answer this please

  1. May 1, 2005 #1
    If a body is in freefall and has reached its terminal velocity say 56 metres/s, its acceleration is zero, how do you work out with what force it would hit the ground
     
  2. jcsd
  3. May 1, 2005 #2
    you know the terminal velocity,that velocity divided by the time the body took to come to rest on hitting the ground would be the force with which it would hit the ground
     
  4. May 1, 2005 #3

    Danger

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    Should be just mass of falling object times speed with which it impacts. You should probably, for the sake of simplicity, assume that the landing area has no elastic properties. The force would be the same even if the thing lands in a bowl of Jell-O, but it would be distributed a lot differently.
     
    Last edited: May 1, 2005
  5. May 2, 2005 #4

    Chi Meson

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    Beg your pardon, but the last two responses are incorrect.

    If you have the initial speed of impact, you will need the mass of the object PLUS either the distance that the object went onto the ground during impact, or the time interval of the impact with the ground.

    IF you have the time of the impact, F = mv/t , where v is the "change in velocity" (final velocity is zero). This is using the impuolse-momentum theorem.

    If you are given d, the distance of impact (depth of crater), F = (mv^2)/2d
    This is using kinematics and dynamics
     
  6. May 2, 2005 #5
    well hello!your first option is what I wrote!
     
  7. May 2, 2005 #6

    Chi Meson

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    Your response is missing the mass of the object. The initial velocity divide by time of impact (assuming final velocity is zero) gives the acceleration. Multiply acceleration times mass to get force.
     
  8. May 2, 2005 #7
    oh!I made a mistake
     
  9. May 2, 2005 #8

    Chi Meson

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    Shhh! It happens.
     
  10. May 2, 2005 #9
    it better not happen often
     
  11. May 2, 2005 #10

    Chi Meson

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    Well, if I had a nickel for every mistake I've made in responding here, heck, I'd have fifteen cents by now.
     
  12. May 2, 2005 #11

    Danger

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    I cannot believe that I confused 'force' and 'energy'. :redface: Sorry, guys. The terms just kind of swapped places in my head and I didn't notice. It's like when I'm stuck on a crossword puzzle answer for hours, then suddenly realize that I've misread the clue eight times in a row. (In my defense, I was on my 7th beer at the time.) :biggrin:
     
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