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Momentum, force, energy and impact to break something.

  1. Jul 25, 2013 #1
    Ok ive been trying to figure out the difference between dropping something very heavy onto glass breaking the glass and throwing a penny at glass fast enough to cause it to shatter.

    specifically what im trying to figure out is how does energy, and change in momentum relate to force required to break or shatter glass.

    for example 100kg weight moving at 1m/s would quite easly smash through glass.
    momentum = 100kg -m/s
    energy = .5x100x1 = 50j

    it has not much change in momentum so what i dont understand is shouldnt that mean it has little force?

    on the opposite side

    say for example a weight 1kg is moving at 100m/s bounces of the glass but shatters it in the process ( i know must be really thick glass but stick with me)

    momentum = 100kg-m/s
    energy = .5x1x10000 = 5000j

    it has alot more energy and a massive change in momentum but doesnt manage to go through. why is this?

    (both weights have the exact same size and shape)
    Forgive me for if any of my calculations are wrong and im completely missing something here, i havent done physics since gcse level :/ was just pondering one day and it started bugging me that i didnt understand :*(
    Last edited: Jul 25, 2013
  2. jcsd
  3. Jul 25, 2013 #2


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    Staff: Mentor

    It is indeed an issue of energy, but the energy can be different for different speeds because of the way the material reacts to the impact (faster impact means higher force and less time for the material to react to the impact). Liik into the Charpy impact test and fracture energy/toughness.

  4. Jul 25, 2013 #3
    Let's say we have a 100 kg bullet moving at 1 m/s and 1 kg bullet moving at 100 m/s shooting at thick glass with the same size and shape. Both have the same momentum (meaning force required to change the direction of the moving object is same). In this case, the force required to bounce back the heavier object is equal to the force required to bounce back the lighter one. F = ma.
    So we can calculate that the 1 kg bullet hits with a force of 5000 N and the 100 kg bullet with a force of 0.5 N. Therefore, I think your numbers are fine, but the glass is not the same in both cases.
    It's like saying 100 kg man at walking speed broke a glass, but a 1 kg ham moving at 360 kmph just bounced off.
  5. Jul 25, 2013 #4


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    Science Advisor
    Homework Helper

    You didn't say how you calculated that, but I don't believe the answers. One Newton is about equal to the weight of an apple. So you claim a "bullet" the weight of an adult human (100kg) will "hit" with a force of half a Newton? I don't think so....
  6. Jul 25, 2013 #5
    My mistake, I meant 50 N.
    Okay so Energy is force applied over a certain distance. Since the "certain distance" in this case is the same, we use the equation Work done = Force . Distance
    Distance is not a factor in the case because we are simply comparing the two forces. Therefore, the proportion of the force of the two bullets is same as the proportion of the kinetic energies in this case.
  7. Jul 25, 2013 #6


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    Staff: Mentor

    The "certain distance" is not the same. When you apply the strain to the glass slower, it bends further before breaking.
  8. Jul 25, 2013 #7
    In this case, the distance is close enough that the numbers he provided be proved inaccurate
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