Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Frictional toruq of a brake disc

  1. Jan 13, 2010 #1
    im struggling abit with this question. i have a formula but it doesnt really go with my question

    a brake disc has the following specification
    mean radius 0.16m
    force applied to each pad 5045N
    U = 0.35
    brake disc rotational speed is 550rev/min

    work out its frictional torque
    work done a min by this torque
    heat energy generated pers second

    i science for motor vehicle engineers by peter twigg and the book has a very similar example. but it has a brake cycliner in the equation which i dne have and hydraulic oil pressure.

    i have the formula total braking force per disc = n x U x P x A (n)

    in my case i take that to be 550 x 0.35 x 5.045Kn x 0.16

    this gives me 155N of frictional torque per disc.
    am i going along the right sort of lines or am i using the completely wrong forumla?
    looking around alot fo other examples have time bt as i dnt have this im abit lost.
    any help would be very much appriated :)
  2. jcsd
  3. Jan 13, 2010 #2
    you might need an angular momentum formula.
  4. Jan 14, 2010 #3
    ive been looking for a forumula to use bt im nt sure what one.
  5. Jan 14, 2010 #4
    well would you like the answer or guidance?
    Basically you want to plot out what it is you are trying to use your formula for.

    basically we have the force of friction given to us with the applied force and the U value.

    so we need to figure out the deceleration of the spinning wheel. The deceleration will be a negative angular acceleration. Its currently travelling 9 revolutions per second, so you want to find the change in revolutions per second.

    This is where it might get a little bit tricky. I believe you will have to use a kinematics formula to find out the distance(in a straight line) that the wheel has travelled. This is because Work = Force x Distance. So what I would do is fine the angular acceleration using your knowledge of torque and hopefully the Moment of Inertia(which you should be aware of). Then with that acceleration figure out the distance travelled if you have an intial speed of 9 rps, and the perimeter of the disk is 2*pi*r, the speed in metres per second will be the rps * perimeter. You are also going to have to find the deceleration of the wheel and plug in to d = v(initial)*t + (at^2)/2 where your time is 60 seconds your initial speed is 9 * 2*pi*r and your acceleration is your angular acceleration * perimeter.

    This sounds like I'm not explaining it very well. So let me know if something is unclear

    Remember you want to find the frictional torque which is just Force of Friction * radius
    You want to find the work done in a minute which is Force * distance
    And you want to find energy lost per second to the friction, which is equal to the heat energy if we assume the sound energy is negilgible(which it pretty much is)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook