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Homework Help: Newton's Law Problems

  1. Jul 8, 2009 #1
    1. The problem statement, all variables and given/known data
    A block of mass m1=2.0 kg on a frictionless inclined plane of angle 20 degrees is connected by a rope over a pulley to another block of mass m2=1.0kg. What are the magnitude and direction of the acceleration of the second block?

    2. Relevant equations

    3. The attempt at a solution

    how would i calculate m1g and m2g in the first place?
    other then that...


    Your help will beeee very appreciated seeing as I have a test on it tom!!!
  2. jcsd
  3. Jul 8, 2009 #2


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    g is acceleration due to gravity = 9.81 m/s2
  4. Jul 8, 2009 #3
    so basically i would multiply mass with 9.81
    m2 = 1(9.81) = 9.81
    m1= 2(9.81) = 19.62

    a= -19.62-(-9.81)sin20/3
    a= 3.3m/s^2 (down)

    but the answer is supposed to be -1.03 m/s^2 (down)
  5. Jul 8, 2009 #4


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    Well for one, it seems that you switched around m1g and m2g. It should be


    Also, do you know how to get that formula?

  6. Jul 8, 2009 #5

    No i don't! But the modified version you gave me is right! I got 1.03! :) thank you soo much :cool:
  7. Jul 8, 2009 #6


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    If the resultant force of m2 is downwards, then for the system vertically. (T=Tension)


    therefore on the incline, m1 moves up, so that


    two equations where you can eliminate T by adding them. That is how to derive that formula. It's better to know how to do these kinds of of problems from first principles than to memorize a formula
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