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Inertia and static friction confusion

  1. Apr 8, 2014 #1
    In a recent physics class I took, my teacher explained how friction is not affected by surface area, but by the "bumpiness" of the two objects and the mass, as well as other things, but not the surface area. But this made me wonder how nails get harder to pull out the deeper they are embedded in a surface. Since the only thing apparently increasing is the contacting surface area of the object, why does it become harder to remove?

    Also, the definition of inertia is "a tendency for an object to remain in motion while in motion or to stay still while still", would the nail technically have a larger inertia in the previous example? I know that Inertia is solely dependent on mass, but shouldn't this example warrant a better definition? Thanks in advance.
  2. jcsd
  3. Apr 8, 2014 #2

    Simon Bridge

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    Note: the friction model taught in schools is a simple one in which there is no dependence on the surface area of contact. This need not be the case but it is often enough to make this a reasonable thing to do.

    However - it is not strongly related to the bumpiness of materials.

    Have you actually hammered a nail to different depths into something and then measured how hard it is to pull out or are you going by a personal impression or intuition or "because everyone says so" something? Do you have a reference of anyone doing this experiment? (It strikes me as a good one to do.)

    i.e. how do you know it takes more force to get the nail moving the deeper it is?

    The experience is that it take more effort ... that's different.

    Considering that something like wood springs back after the initial drive to grip the nail tightly, it is plausible that deeper nails require more force. The nails are not free to jounce about on the surface they have to slide against - which is required in the class model. In terms of your class model, this would be the same as the normal force depending on depth.

    No better definition needed - the inertia relates the object to the force required to do something - friction is defined as a force. Encorporating some forces into the definition of inertia and leaving some out would just overcomplicate things. Leave it as it is - it has worked for centuries.
  4. Apr 9, 2014 #3
    To answer your question, no I have not done the experiment (although I agree I should), and I posted this question on a poor assumption. I could have easily just confused Force for Work, which certainly would increase due to the increased distance needed to pull the nail out. However, I do have a potential way increased surface area could appear to increase necessary force. Alternatively, I could be confusing force for impulse, as it just might take a longer continuous force to pull it out. Most likely, my confusion caused this problem. However, that doesn't mean the experiment should not be done (I cannot, because I do not have access to advanced physics technology).
  5. Apr 9, 2014 #4

    Simon Bridge

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    The experiment is very low tech.

    Aim: determine relationship between force-to-move, and penetration-depth, of a nail in wood.

    hammer, nail, wood, ruler, vice-grip pliers, wire hook, bucket, scrap metal, scales.

    The highest tech devices there are the scales.
    All equipment can be found in a department store.

    Rough Method:
    (This will need to be refined after the first few trials depending on exact equipment used.)
    1. Measure the length of the nail.
    2. Hammer the nail into the wood and measure how much is sticking out.
    3. Fasten wood securely overhead so nail is pointing downwards - so you want a framework: clamping to ceiling beams for example (use your imagination)
    4. Securely fasten the vice-grip to the nail head. (Take care not to move the nail about.)
    5. Hang bucket off vice-grip.
    6. Add scrap metal to bucket until you see the nail just start to move.
    7. Weigh bucket.

    Repeat lots of times. Try different nails and depths.

    Variations: different kinds of wood
    different materials (i.e. concrete - which would involve drilling a small guidehole).

    Graph: weight vs exposed length for each nail and find the slope.

    Physics does not have to involve "advanced physics technology" - though it can help.

    If you want quick results, use panel pins for the nail and balsa wood.
    You'll need a lighter way to fix the weights to the end of the nail than a vice grip and bucket but I'm sure you can work it out.
    Last edited: Apr 9, 2014
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