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Homework Help: AP Physics Lab: Coefficient of Friction

  1. Oct 5, 2005 #1
    Hi, I'm new to this forum and I hope I can find some help with my AP Physics class! I find the subject really interesting, I just have a hard time applying it. Also, I don't really know the people in my AP Physics class, so it would be awkward to ask them for help.

    Anyway, last week we did this Lab to find the Coefficient of Friction. I have the data and need help on how to find the coefficient of friction.

    The lab goes as follows: We have a car and rail set up a certain degree. At the end of the rail there is a photogate, and we tape a ruler on the top of the car (so that it passes through the photogate). At the top of the rail there is a pulley, and a string that goes through it. One end of the string is attached to the car, and the other end to a certain mass. We are supposed to calculate the friction between the car and the rail.

    The information that I have is the weight of the car (.348kg), the width of the ruler that was taped onto the car (.025m), and 4 sets of angles, masses, and times. (For example, the rail was set up at a 43 degree angle and the mass on the other end of the string from the car weighed .0994 kg. The cart took .018 seconds to go through the photogate 9 times, and .017 twice.)

    Now, I've got this equation from a lab partner, but I keep getting really big numbers for the coefficient. (The equation is: acceleration times the total mass equals mass of the cart times sin theta minus the mass hanging from the string times gravity minus mu (coefficient of friction) times mass of the cart times gravity times cosine theta... a(tm) = (m1 sine theta - mass2 g) - mu(m1 g cosine theta)...). The "m1 g sine theta" is the equation for the downhill force.

    I hope this is enough information for someone to help me. If possible, maybe someone could figure out a coefficient for me and explain while they're doing it, how? Thank you for your time.
  2. jcsd
  3. Oct 5, 2005 #2


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    Let me see if I have this right. A free falling mass over a pulley or something is pulling the car up an incline ?
  4. Oct 6, 2005 #3
    Let me see if I have this right. A free falling mass over a pulley or something is pulling the car up an incline ?

    The car is going down the incline, and there is a string attached to the car which goes over the pulley and has a weight attached to it.

    Like this: http://img.photobucket.com/albums/v285/gothicpie/physicslabfriction.jpg [Broken]
    Last edited by a moderator: May 2, 2017
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