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

Homework Help: Just got owned on this exam question

  1. Dec 9, 2009 #1
    1. The problem statement, all variables and given/known data

    You've got two masses, m1 and m2, hanging from opposite ends of a rope that goes over a pulley with radius R. Both the masses are suspended by the rope alone and the pulley is on a frictionless peg. M2 moves down by x2 meters in 5 seconds. I believe that's all the variables we were given - m1, m2, radius R, and 2 meters in 5 seconds. EDIT: The pulley doesn't slip.

    Now find the accelleration of m1, the tensions of the rope on each side of the pulley, the angular velocity of the pulley, and the inertia of the pulley.

    2. Relevant equations

    3. The attempt at a solution

    I have no idea how to do this without being given the mass of the pulley or its inertia.

    Started out with m1gx + m2gx = (1/2)m1v^2 + (1/2)m2v^2 + (1/2)I(V/R)^2 + m1g*2x.
  2. jcsd
  3. Dec 9, 2009 #2


    User Avatar
    Homework Helper

    Since the question asks for acceleration, you probably don't want to use conservation of energy because conservation of energy equations have no acceleration terms. Instead, use plain old free-body diagrams and Newton's second law.

    Since the acceleration is constant and d=v0t+(1/2)at^2, you can calculate acceleration with the fact that m2 dropped by 5 m in 2 s. See where this takes you.
  4. Dec 9, 2009 #3
    Oh man, duh. I can't believe I didn't see that. I can get the rest of it if I have that a no problem. Guess I'll just have to chalk that one up to a brain fart, hope I'll still get partial credit. :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook