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I How to calculate force

  1. Jul 31, 2016 #1
    Hello, I am curious about how to calculate lbs of force like in those martial arts shows where they measure martial artists and professional mma fighters' punching power, and how it would translate to psi. This is because I saw a man at a youth hostel hit a safety glass on a door and shattered it. Its breakage pattern was that it broke around so I assume it was safety glass and not toughened glass. If the cheapest type of tempered glass is 10 000 psi( correct me if i'm wrong), then how much lbs of force does that translate into? And please could you show me your calculations and the formula for it!

  2. jcsd
  3. Aug 1, 2016 #2
    Psi standard for "pounds per square inch". This is a measure of pressure. So actually, breaking something depends on the size of the impacting object. This is why people can lay on beds of nails without getting hurt. The pounds are spread out, but if you try laying on just one nail, it's concentrated, and you will probably have to go to the hospital.
  4. Aug 1, 2016 #3


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    There is no simple formula for that sort of thing. What is needed to break something is a combination of pressure, force, direction of force, duration of force and distance over which the force acts. In the case of a brittle material like glass, there is definitely no easy way to predict 'strength' under all circumstances. A small impact from a sharp tool like a metal worker's punch can shatter a car windscreen that could withstand the impact of a body, flying through the air at it.
    This recent thread shows the way this sort of question can go. There is seldom a satisfactory answer. When it is necessary to have a glass sheet reliably 'strong enough', I think the technique is just to make it very thick with loads in hand.
  5. Aug 1, 2016 #4


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    The fact you can shatter a car window with a pebble sized bit of ceramic is a great example of this;
  6. Aug 2, 2016 #5
    The formula would be something like:
    Pressure = (Force of impact)/(Size of impacting object's contact)

    Then whether or not you get a break is dependent on some threshold of the material:
    if Pressure < Threshold, then no break
    if Pressure > Threshold, then break

    This is the simplest case. There's a lot more nuance to this kind of thing.
  7. Aug 2, 2016 #6


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    Re-read #3. There is no simple formula.
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