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

Pulley question

  1. Jan 10, 2007 #1
    1. The problem statement, all variables and given/known data
    Two objects of mass 0.5kg and 0.8kg respectively are connected by a light string that passes over a frictionless pulley. Find:
    a) the acceleration of the 0.5kg mass
    b) the net force acting on the 0.8kg mass
    c) the time taken for the masses to reach a velocity of 2.8m/s

    2. Relevant equations
    Newton's laws

    3. The attempt at a solution
    The question was given without a diagram. Is it possible to do this problem just from the data above without a diagram?
  2. jcsd
  3. Jan 10, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    A diagram would be helpful. I believe the problem assumes a pulley attached to a ceiling, with the masses hanging down from either side. The 0.8 kg mass will move downward, and the 0.5 kg mass moves up. Draw free body diagrams of each mass and identify the forces acting on each, then apply Newton 2 to determine the acceleration of each from the net force acting on each.
  4. Jan 11, 2007 #3
    Object 1: 0.5 kg
    Object 2: 0.8 kg

    Tension relates the two objects =).

    There needs to be a force body diagram to get an idea of how the forces are working. But, without it you can get a basic idea of how it looks in your head and be able to solve it.

    Acceleration of the system = [ (object 1 - object 2) / (object 1 + object 2) ] * gravity

    1/2mv^2 and mgh are your tools when dealing with energy =). They can also be tied with vectors.

    Using the equations: x = xo + vot + 1/2at^2, v2^2 = v2o^2 + 2a(x2 - x1) and v = vo + at.

    It is possible.
  5. Jan 12, 2007 #4
    The answers did assume that the weights were spread from both sides downwards. Each mass has the same acceleration and tension because they are connected by strings so are considered linked. We assume the string cannot be stretched nor dangled do we?
  6. Jan 12, 2007 #5
    Right, and we assume the pulley does not actually spin. When the pulley rotates, you have to factor in friction and moment of inertia of the wheel
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook