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: Dynamics question (kinetics and energy)

  1. Jun 10, 2010 #1
    1. The problem statement, all variables and given/known data

    The two blocks are connected by a light inextensible cord, which passes around small

    massless pulleys as shown below. If block B is pulled down 500 mm from the equilibrium

    position and released from rest, determine its speed when it returns to the equilibrium



    2. Relevant equations

    T1 + U1 = T2 + U2

    3. The attempt at a solution

    If it's pulled down below equilibrium and held there, then T1 of the system is zero because both blocks are not moving. At the moment B passes through the equilibrium, there is no more potential energy, only kinetic, then the equation would look like:

    U1 = T2

    The problem I'm running into is that I get a negative value for the left side of this equation, which is impossible because then it would have to go under a square root when solving for the velocity.

    for U1 I had:

    mgha - mghb + 0.5kx^2

    Since 'b' moves down 0.5m, a moves up 0.25 and the spring is stretched 0.25. Is this right since A is attached to the pulley and B is simply hanging? If that's the case then:

    (2)(9.81)(0.25) - (10)(9.81)(0.5) + 0.5(800)(0.25)^2

    Which is negative. Supposedly the answer is 2.16 m/s.
    Last edited by a moderator: May 4, 2017
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted