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Why does height affect the amount of damage an object does?

  1. Oct 26, 2015 #1
    If I were to drop a penny from empire state building it would do more damage than if i dropped it from standing distance. Why? F=MA. neither the penny's acceleration nor mass change when its falling so the forces on both pennies should be the same. The only solution i could come up with is that the acceleration comes from the deceleration caused when the penny hits the ground which would be bigger than the acceleration due to gravity and would be based on the time spent falling or in other words the height . am i correct?
     
  2. jcsd
  3. Oct 26, 2015 #2
    We should make this sentence into a meme.

    The penny dropped from the empire state building is travelling at a higher speed when it impacts whatever it impacts on the ground,
     
  4. Oct 26, 2015 #3
    im aware but why does that affect force?
     
  5. Oct 26, 2015 #4
    F=MA Not F=MV
     
  6. Oct 26, 2015 #5

    russ_watters

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    Staff: Mentor

    You were right that the deceleration when it hits the ground is what causes the impact force via f=ma. So higher speed = larger force (more deceleration needs to happen to make it stop).
     
  7. Oct 26, 2015 #6
    thank you!!
     
  8. Oct 27, 2015 #7
    I'd say the energy here is crucial, which is in turn connected to speed - kinetic energy (hope I don't have to write it down).
    Ever heard that bullet has "that-and-that kJ"? This is how sometimes destructive potential of a weapon is given.

    It is true that the "force" that penny will be applying to the object is connected to the deceleration - problem is, depending on what it will be hitting, the deceleration might differ greatly. Lets assume you're to hit some kind of weak material, that is easy to penetrate. Penny will go through, and "force" that will apply on its road further will be lower, than when it hits, say, a solid wall.

    It is not really good, that in common language "force" is connected with hitting something hard, because force can change as quickly as speed, acceleration, or any other parameter is such experiment. What is almost constant (well, assuming there is energy loss due to friction to air) is the energy of that object, and that is what you should be looking at.

    And while it, there is a speed, at which force of friction between air and the object in question is equal, meaning that you won't accelerate any further. So it doesn't matter from how far you throw that penny from - my wild guess would be, that you could throw it from 20m, and it would reach its "top speed" in air. That said, it won't reach any astronomic amounts of speed, and thus, speed, which means it won't be as destructive, as one might think. :=)

    As a side now - for humans (welp), this height is about 50m, and approximate speed that human can be falling freely is about 200km/h.
     
  9. Oct 27, 2015 #8

    russ_watters

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    Staff: Mentor

    Yes. Because the dynamics of the impact are very complicated and specific, force is rarely used to describe destructive potential; kinetic energy is better.
     
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