1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Pulleys and weights - finding acceleration and tension

  1. Sep 26, 2011 #1
    1. The problem statement, all variables and given/known data
    A 24 kg block with a pulley attached slides along a frictionless surface. It is connected by a massless string to a 6 kg block via the arrangement shown.
    The acceleration of gravity is 9.81 m/s2 .

    The first part wanted you to find the horizontal distance traveled by the 24 kg block if the 6 kg blocked descended 10.1 cm. I found the answer, which is just half of that distance, 5.05 cm.

    However, I don't know how to find the other two questions:

    1. Find the acceleration of the 24 kg block. Answer in units of m/s2

    2. Find the tension in the connecting string. Answer in units of N


    2. Relevant equations


    3. The attempt at a solution

    I'm not asking for answers - but could someone point me in the direction I should take? I'm not sure about how to start this...
  2. jcsd
  3. Sep 26, 2011 #2
    If the 6 kg mass moves twice as far as the 24 kg mass, you get a multiplier of 2 in the force on the larger mass due to the weight of the smaller mass. Remember force times distance equals force times distance.

    Realizing that, you can determine the force sliding the 24 kg mass on the table.
  4. Sep 26, 2011 #3
    So if

    (Smaller) = (Larger)
    Fd = Fd
    F(2x) = F(x)

    Wouldn't the F cancel out? I'm a bit confused about what you said - could you perhaps elaborate a little bit more?
  5. Sep 26, 2011 #4
    The F's are not the same. One F is twice the other......

    This is how block and tackle works - I'm not talking football here.

    It's F1 * d1 = F2 * d2. If you move twice as far, you double the force. However, the work done is the same.
  6. Sep 26, 2011 #5
    Think of a little gear turning a larger gear. Torque gets multiplied by the ratio.

    If you use a longer pry bar, you can lift more. It's force(1) times distance(1) equals force(2) times distance(2).
  7. Sep 26, 2011 #6
    Must sign off now.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook