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Homework Help: Average Force Of Friction Please Help (attempted Question)

  1. Jun 12, 2007 #1
    1. The problem statement, all variables and given/known data
    find the average of a block being propelled propelled by an elastic band.
    We know it is uniform acceleration.

    my attempt on this problem
    Measure the force applied of elastic band with a newton scale. The more you strecth the farther the distance the block will travel after you let it go. In my case my Fa was 6N. i proposed this idea to my teacher but he said the as the elastic band destreches after you let it go the force become 6,5,4,3,2. I am wondering if this is a correct way of finding the Fa. Then i would perfrom three trials to find the average.To find the average i would perform trials 2 times and divide them by three to find the average of d,t,forceapplied. i would find the distance,and time and assume V1 is 0. Can i assume V1 is 0 since it is in rest? my teacher looked puzzled i and i sure v1 is not 0. so then using v1,d,t, i would find v2 when it reaches the final velocity. then i would find the Acceleration acting on the block by using v1,v2,d,t. then i would find force normal by entering this fn=Mag
    which would be mass x acceleration acting on block x acceleration due to gravity. i don't know how to go about finding force of friction because i can use this equation. FF=uFn.
    One more question
    can i find Fnet=Ma since i am going to find acceleration. will finding Fnet help me out?
    thanks. Any suggestions ar welcome
  2. jcsd
  3. Jun 13, 2007 #2


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    I assume that you are shooting a block with the elastic so that it slides along a surface. The problem is then to determine the average frictional force that acts on the block during the release and sliding up to the point that it comes to rest?

    Is the elastic initially stretched between two posts before it is pulled back with the block against it?
  4. Jun 13, 2007 #3
    no a wooden block with a slight cut in it is used to grip the elastic and propell the other wooden block.
  5. Jun 14, 2007 #4


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    .... which slides along a table top?

    Do you have access to a photogate? That way you could measure the inital speed of the (free) block at the point of release.

    Another way is to determine the work done by the elastic on the (free) block is to calculate the area under the restoring force vs displacement graph. This means that you have to note at which displacement mark you release the elastic to shoot the block (maybe a scale on the table).
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