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Why is it harder to hammer in a nail the deeper it gets?

  1. Mar 28, 2012 #1
    Why is it harder to hammer in a nail the deeper it gets?

    Classical physics says

    F k or s = μ k or s N

    the normal force is constant right? or is the nail's normal force increasing as it enters the piece of wood?

    or is it one of those exceptions like why wider tires on snow work better than thinner ones? (b/c of pressure exertion over an area)

    could someone please explain this its not making any sense
  2. jcsd
  3. Mar 28, 2012 #2
    Think qualitatively for a moment. As the nail gets deeper in the material, what has changed?
  4. Mar 28, 2012 #3


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    Science Advisor

    There's more friction with the wood on the sides of the nail.
  5. Mar 28, 2012 #4
    the pressure exerted by the wood onto the nail in this case the normal force is increasing?

    is that why? was i just overlooking the fact that N is increasing in this case and N ≠ mg
  6. Mar 28, 2012 #5

    Philip Wood

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    Gold Member

    The normal force (pushing radially inwards) is indeed increasing as more and more of the nail is gripped by the wood.

    Whereas, to a very rough approximation, the area of contact doesn't matter for the frictional force between a body and the surface it's resting on, because the pressure goes down if the area goes up, there's no such effect with the nail, because the normal contact force isn't equal to a fixed weight but arises in response to the wood being strained.
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