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Homework Help: Momentum to Force

  1. Oct 23, 2014 #1
    << Moderator Note -- thread moved from General Physics to HH/Intro Physics >>

    If a force F is needed to deform or break an object, with what velocity, V, must I hit it with a hammer of mass, m?
    Last edited by a moderator: Oct 23, 2014
  2. jcsd
  3. Oct 23, 2014 #2


    Staff: Mentor

    Welcome to PF!

    This looks like homework. We have a homework template that we ask students to fill out so we know your level of understanding. We also need to see your work before we can give suggestions to you.
  4. Oct 23, 2014 #3


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    From Newton's 2nd law, we have [itex] F=\frac{\Delta p}{\Delta t}=m\frac{\Delta v}{\Delta t} [/itex].
    So its important that what will be the velocity of the hammer after hitting the object. If it stops, you'll have [itex] \Delta v=V [/itex]. But if it rebounds with the same speed, you'll have [itex] \Delta v=2 V [/itex]. In reality, its always somewhere between these extremes which depends on the material from which the object and the hammer are made of and, to some extent, on how hard you're hitting, but that's not as important as the materials I think.
    Another point is the time that takes for the change of velocity to happen. The shorter it lasts, the more force you'll have.
    So at first, you should know in the kind of hitting you're doing and kind of materials you're using, how will be the rebound. Then you should see how fast the velocity changes. This isn't as adjustable as the velocity so you should think of it as a given I think.
    Then you can decide how much should be the velocity of hitting.
  5. Oct 23, 2014 #4
    I have to design a machine and this is one of the sub-components. We have to deform a small pin made of steel, and 800 lbs of force is necessary. the pin is fixed at one end and a hammer on a rotating arm will hit it on the other end. I'm not sure about any of this, so I don't have any work. What material and what speed of the hammer would be necessary to deform the pin?
  6. Oct 23, 2014 #5
    I'm a 3rd year Mechanical Engineering student, if that helps with level of understanding
  7. Oct 23, 2014 #6


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    Staff: Mentor

    Welcome to the PF.

    Well, as a 3rd year ME student, you certainly are familiar with the concepts of stress and strain, correct? And the concepts of elastic versus plastic deformation, correct? How do those concepts apply to this project? It sounds like you must do more than just elastically move the pin with the hammer, but it's hard to tell from your project statement. Can you post the exact text of the project statement?
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