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Average force of friction

  1. Jan 16, 2013 #1
    1. The problem statement, all variables and given/known data
    We have to make a roller coaster for our school project. This coaster starts with a spring. In this coaster, there is a collision. Essentially, I need to find the energy lost at the collision, and the average force of friction.


    2. Relevant equations
    Em1=Em2
    W=Fd
    F=ma
    A=DELTA v/ DELTA t

    3. The attempt at a solution
    To find the energy lost, I set:
    EM1-Wf=Em2
    Em1-Wf=Em2
    -Wf=Em2-Em1
    -Fdcos180=0.5mv^2-0.5kx^2 (I sub. in the actual measured value of v in)

    Is this the correct way to solve for the average force of friction? How do I account for the energy lost during the collision? Would it be included in my Wf? In that case, how would i find the energy lost during the collision?

    My other idea for finding the force of friction, is to do a=v/t v= theoretical speed at the end-actual measured speed
    Once I find the a value, I can sub it into my F=ma equation to find F. Would this be valid as well?

    Thank you
     
  2. jcsd
  3. Jan 16, 2013 #2

    haruspex

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    To determine the energy lost you need to know the coefficient of restitution. You can get an upper bound by assuming the colliding masses coalesce and using conservation of momentum.
    Where is the friction you're trying to measure? Are the masses on wheels or just sliding?
     
  4. Jan 16, 2013 #3
    Uhhh the mass is a marble. And I'm trying to measure the average force of friction acting on the marble during its journey. I'm only in 12 U physics so it wouldn't involve that as I haven't learned it yet. I'm pretty sure this should be dealt with from an energy perspective.
     
  5. Jan 16, 2013 #4

    haruspex

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    Since the marble is rolling, it does not lose energy to 'friction'. It will lose energy because of 'rolling resistance' http://en.wikipedia.org/wiki/Rolling_resistance, which is a little different (basically, bouncing up and down on the microscopic scale) and air resistance.
    The only way to figure out how much energy is lost when two marbles collide is by knowing or measuring the coefficient of restitution http://en.wikipedia.org/wiki/Coefficient_of_restitution#Speeds_after_impact. Taking the masses to be the same simplifies the equation.
    http://hypertextbook.com/facts/2006/restitution.shtml gives a glass marble as 0.66.
    You might need to take into account the moment of inertia of the marble. Have you covered that topic?
     
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