1. Not finding help here? Sign up for a free 30min 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!

Two pulleys and two blocks

  1. Feb 7, 2008 #1
    Hope someone can help me with this one. I have two blocks with m1 and m2 connected to two pulleys. I need to figure out T1, T2, a1, and a2 using only the values of g, m1, and m2. But ive been unable to come up with an equation that will work with it. I have a picture attached.

    I tried using the following expression: ((m2*g)-(m1)*g)/(m1+m2) to find a1, but that didnt work and im stumped. does anyone have any ideas? any help would be great!


    [​IMG]
     
  2. jcsd
  3. Feb 7, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Instead of trying to force fit some derived formula, use Newton's 2nd law. Analyze the forces acting on each mass and set up equations of motion for each. Hint: How does the acceleration of m1 compare to m2? How does the tension T1 compare to T2?
     
  4. Feb 7, 2008 #3
    well i know that a1 = 2 *a2, and i know that T2 = 2 *T1. But i dont know how i can put these together to work with newtons 2nd law.
     
  5. Feb 7, 2008 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Good.
    What forces act on each mass? Set the net force equal to ma! (Careful with signs.)

    You'll get a separate equation for each mass. Combine to solve for your unknowns.
     
  6. Feb 7, 2008 #5
    Would m1 accelerate at 2*g? BEcause of the pulley?
     
  7. Feb 7, 2008 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Don't guess or try to do it in your head. Write the equations and then solve them.
     
  8. Feb 7, 2008 #7
    Because of the frictionless surface, would the net force be just the tension? If so, would a1 = ((m1*m2*(Sin(theta)+1)*g)/(m1+m2))/m1) and Sin(theta) equals 1 becuase theta is equal to 90?
     
  9. Feb 7, 2008 #8

    Doc Al

    User Avatar

    Staff: Mentor

    Net force on what? Which tension?

    (Write the equations.)
     
  10. Feb 7, 2008 #9
    Aha, revelation. Looks like i figured it out!

    T1 = (m1*m2)/(2m1 + .5m2)*g

    then

    T2 = (m1*m2)/(m1 + .25m2)*g

    yes?

    and using the eqautions of tension,

    a1 = t1/m1 = (m2*g)/(2m1 +.5m2)

    and so a2 = half of a1 which is = (m2*g)/(4m1+m2)

    and in the end we get a correct answer for all of them! Brilliant!

    Thanks so much!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Two pulleys and two blocks
Loading...