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Homework Help: Finding Coefficient of Friction - Relationship b/w Normal Force and Static Friction

  1. Nov 7, 2011 #1
    1. The problem statement, all variables and given/known data
    So I have this lab I have to do about finding the relationship between static friction and normal force. So basically I need to find find how the Static Friction and Normal Force relate by doing a lab experiment.

    The materials I have are:
    Spring balance / scale
    Different surfaces of textures (probably something to do with friction)
    and some sort of Cube weight.

    So I just don't understand how we would conduct an experiment to find the relationship between normal force (something that goes up) and static friction (Something that is at rest).

    2. Relevant equations
    Fnet = ma

    My teacher also said something about Coefficient of Friction = Static friction / Normal force but I don't see how I can apply that in this lab overall.

    3. The attempt at a solution

    I really am confused about what to do for this lab. I would really appreciate if you could give me guidance as to how I could approach this lab to find the relationship.

    I know it has to do with something about measuring the mass of the weights in different frictions and I think calculating f=ma but am confused...

  2. jcsd
  3. Nov 7, 2011 #2


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    Homework Helper

    Re: Finding Coefficient of Friction - Relationship b/w Normal Force and Static Fricti

    A common method of demonstrating static friction and kinetic friction is as follows.

    A selected mass is placed on a rough surface [doesn't have to be too rough] and the spring balance is attached to one end/side. The balance is then pulled horizontally.
    The balance will measure the force applied to the mass. The force is gradually increased
    At a certain point, the mass begins to slip. The maximum reading, just as the mass begins to move, equals the static friction. The reading on the spring balance will immediately reduce once the block starts moving. If the operator then continues to pull on the spring balance so as to keep the mass moving at [almost] constant speed. This new [lower] reading will show you the kinetic friction.

    By the sound of it, you are mainly interested in the first part - the static friction.
    Perhaps next weeks lab will be about kinetic friction?

    To see how Static friction varies with Normal force, you can load up the block that touches the surface and see how the static friction varies with weight. SInce you will use a flat surface, the Normal Reaction force will be the same size as the weight.
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