1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What kills you when you fall down?

  1. Jun 10, 2009 #1
    When you fall off a building and then you hit the floor, what actually kills you?
    I mean what energy or mechanism is acting on your organs and bones and damages them?

    The sudden change of momentum your body went through?
    The Kinetic energy you had before the impact?

    Please try to explain this to me without using the concept of force (I do know it is the change of momentum) but I actually want to see how the conservation laws apply to this particular case.

    I want to know this because on a videogame related forum of a PS3 games where you control a superhero who controls electricity (The game is called infamous) and people started asking why the main character was not damaged by high falls and people tried to give a reasonable explanation of that and It just stuck with me.

    Bonus question, if someone were to control all aspects of electromagnetism what could they do to prevent damage from falling down great distances, that is of course assiming that said person could exist.

    Or maybe I should stop worrying about stupid debates on how physics work in a fictional setting right?

    Thanks in advance for those who are willing to reply to this thread.
  2. jcsd
  3. Jun 10, 2009 #2


    User Avatar

    Staff: Mentor

    Short term, the main damage is soft tissue and blood vessel damage from the tearing motion when the outside of your body stops suddenly, and the internal stuff keeps moving until stopped by hitting the ribs, skull, etc. That causes internal bleeding, loss of perfusion to the brain, death. If that trauma is not enough to kill you short term, it can still get you in the longer term (a day to a week), depending on what-all is damaged.
    Last edited: Jun 11, 2009
  4. Jun 10, 2009 #3
    Its rather difficult to explain something when you have veto'd the explination for it.

    What kills you is the sudden stop.

    Energy is a tricky thing to be discussing regarding impacts.
  5. Jun 10, 2009 #4
    Oh well, I just wanted to see if there was an explanation not involving the concept of force, but please explain with the concept of force. I was not trying to veto it
  6. Jun 10, 2009 #5

    Well really what acutally phyiscally kills you is decribed thoroughly by berkeman. The fact that the more rigid parts of you stop suddenly and the squishy bits inside carry on moving.

    You can approximate this to an inelastic collision (so momenum is conserved but not KE as its dissipated) between a rigid body and a soft body.

    What acutally causes damge is always a force which is caused by the deceleration. So the faster you stop the more damage will occur.

    The KE you had before the impact is totally irrelevent to how much damage occurs. Becuase imagine two people jump of a building and fall 50 feet. 1 hits concrete and the other hits a super soft bouncy floor.

    They both have the same KE at impact, but the concrete slows the 1st man very quickly and the impact invovles high forces. The 2nd man decelerates slows so his body is only subjected to low forces.

    This is not to say that energies are totally irrelevent to describing the collision/impact however (they are just not very useful for describing what kills you).
  7. Jun 10, 2009 #6
    As was mentioned previously, you initially have x amount of kinetic energy before the fall, after the fall you have 0 (on average, in the direction of fall). Where did that energy go? Into deforming your body. And I don't think you're going to get much resistance when you state that the human body doesn't take too kindly to large kinetic deformation.
  8. Jun 10, 2009 #7


    User Avatar

    Staff: Mentor

    Excellent point Chris. It's not the delta-KE, it's the Impulse. The Impulse is time interval dependent, while the delta-KE is not.
  9. Jun 10, 2009 #8


    User Avatar
    Science Advisor


    Because force is not the change of momentum - it is the rate of change of momentum.
  10. Jun 11, 2009 #9
    I know that, maybe I just worded it wrong. It does not matter ayways I satisfied my curiosity.
  11. Jun 11, 2009 #10


    Staff: Mentor

    You can look at this through conservation principles also. Consider this to be a plastic collision between the earth and the person.

    Conservation of momentum: due to the mass of the earth there is negligible acceleration of the earth, the deceleration of the person is quite high. The final velocity of the earth and the person is approximately 0 after the collision.

    Conservation of energy: this is a plastic collision where the KE goes almost entirely into deformation of the earth and the person. If the earth is highly deformable (e.g. water or cushion) then the energy will go into deforming the earth, if the earth is very rigid (e.g. concrete) then the energy will go into deforming the person. This is usually a bad thing for the person as blood vessels etc. don't work well when deformed as described by berkman.
  12. Jun 11, 2009 #11
    Does that mean, 0mph to 60mph in 1 second can do the same damage to our body as 60mph to 0mph in 1 second? I don't hear anybody died in Formula One, because they started too fast.
  13. Jun 11, 2009 #12
    Yeah, it's the same. However, I think one second is much too long when you consider someone falling on the ground. I think it would be more towards 0.01 seconds or something like that!

    Think of what would happen if you were hit by a train. That would be more like 0 to 60 mph in a tiny amount of time, and you would probably die.
  14. Jun 11, 2009 #13
    And to add to this discussion, I recall something about Princess Di's death which apparently involved her aorta ripping from her heart(causing massive internal bleeding) during the rapid decceleration from the crash.

    Edit: Apparently this does not happen to everyone in an identical crash. Perhaps some people's internal "connections" are stronger than others.
  15. Jun 11, 2009 #14
    Well the speed at which the kinetic energy change (or rather the momentum) relates to the force. So if you stop very quickly the force on you is great than if you stop slowly. How this relates to damage to the human body is that many structures have elasticity and break points (ow) dependent on the force. Thus if the stopping is done very quickly (like sky diver hits pavement) then the deformation of your body will be a lot of fractures and ruptured membrane. If the fall is very slowly decelerated then the energy lost into deformation will mostly just go to heat and your body will not suffer and real adverse effects.
  16. Jun 11, 2009 #15
    Thats only just over a 6g acceleration (in reality formula 1 cars get to 60 in about 2 - 2.5 seconds so peak acceleration is about 2.8g), which is nothing really.

    To start causing proper damage you are talking 30g average with peak loads much higher.

    For example Kubica's crash was 28g avg with 75g peak loading, its only the well designed crash structure and HANS device that allowed him to walk away pretty much unhurt. Both crash stucture and HANS work by slowing the driver more gradually, thus exposing him to less force.
  17. Jun 11, 2009 #16
    In terms of energy, maverick is correct. Unless you are lucky enough to land on the world's largest pile of cotton, your velocity will reach zero very quickly upon landing, thus your kinetic energy will be transferred into heat and deformation of your body. Since your body isn't exceptionally rigid, deformation will take most of the energy and vital functions will be disabled after they are critically deformed. In any case I would suggest staying away from ledges on bridges and tall buildings or cliffs.
  18. Jun 12, 2009 #17
    Thanks for the clarification.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook