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: Kinetics of rectilinear

  1. Jan 30, 2010 #1
    1. The problem statement, all variables and given/known data

    Refer to the diagram.

    can someone explain why is the 300lb block moves only one-half the distance of the 200lb then the acceleration and velocity also to one half.

    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Jan 30, 2010 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi darkmagic! :smile:

    velocity = dx/dt and acceleration = d2x/dt2, so if you halve x, you halve velocity and acceleration also.

    (and of course x is halved because the pulley moves only half as far as the string)
     
  4. Jan 30, 2010 #3
    how can you know that the pulley moves only half of the string?
     
  5. Jan 31, 2010 #4

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    When the pulley goes down 1 cm, that makes the string above it 1 cm longer on both sides, which requires 2 cm of string, so the block on the slope moves 2cm. :smile:
     
  6. Jan 31, 2010 #5
    ok thanks. another question, how can I know to which the blocks will move and how their acceleration are computed? look at the new attachment.
     

    Attached Files:

  7. Jan 31, 2010 #6

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Just ask yourself, does the pulley move? if it does, how many strings are attached? (because each string will have to get longer or shorter by the same distance)
     
  8. Jan 31, 2010 #7
    yes the blocks will move. my problem is in which direction will they move?
     
  9. Jan 31, 2010 #8

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    To find out, write the F = ma equations for each of the three masses.

    (and, if the heights of the masses are x y and z, you'll need a linear equation connecting x y and z)
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook