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: Weights and Tension help

  1. Jan 27, 2009 #1
    Hello PhysicsForums

    I need help with a certain question which has been bugging me for a while.

    a pulley is suspended by a cord with two masses attached. one mass is 1.2 kg. the second mass is 3.2 kg. the two masses are attached on different sides.

    the question: determine the tension in this cord after the masses are released and before one hits the ground. ignore the mass of the pulley and cords.

  2. jcsd
  3. Jan 27, 2009 #2


    User Avatar
    Homework Helper

    Welcome to PF.

    Draw a force diagram for each of the weights.

    The heavier one will be showing a Tension and a net force from the weight greater than the tension. The opposite will be true for the lighter one.

    From your 2 equations Tension must be the same. Solve for acceleration and plug back into one of the equations.
  4. Jan 27, 2009 #3
    Thank you for replying, but the thing is for this question i don't think the acceleration is needed.. because were trying to find C is attached to the wall and the pulley. Then I thought of Fnet = t-mg and since Fnet = ma which has no acceleration. So I figure it would be 4.4*9.8 but I was wrong.. here is a picture of what I'm talking about.

    Thanks again

    Attached Files:

    Last edited: Jan 27, 2009
  5. Jan 27, 2009 #4


    User Avatar
    Homework Helper

    But there is a net force. 3.2 - 1.2 masses create a net force.

    The system is accelerating.

    If it was constant speed, the tension on one side would equal the weight and the tension on the other would equal that weight.
  6. Jan 27, 2009 #5
    ok thanks again ... I figured for 17.12N for tension ... and what im now suppose to do now? Do I make basically sum up what I know that Fnet = mg-t? where mg is (3.2+1.2)(9.8) and Fnet is is (3.2+1.2)(4.45)?
  7. Jan 27, 2009 #6


    User Avatar
    Homework Helper

    I thought tension was what you were solving for?
  8. Jan 27, 2009 #7
    we are solving tension, but its not the tension of the string with the weights on it. we are solving the tension of the pulley carrying the string& weight. I believe i sent an image on post#3. Does this change how the concept of how we are solving it?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook