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!

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
    a=m2g-m1gsin(angle)/ma1+m2


    3. The attempt at a solution

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

    a=m2g-m1gsin20/2+1



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

    rock.freak667

    User Avatar
    Homework Helper

    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

    rock.freak667

    User Avatar
    Homework Helper


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

    [tex]a=\frac{9.81-(19.62)sin20}{3}[/tex]


    Also, do you know how to get that formula?

    [tex]a=\frac{m_2g-m_1gsin\theta}{m_1+m_2}[/tex]
     
  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

    rock.freak667

    User Avatar
    Homework Helper

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

    m2a=m2g-T

    therefore on the incline, m1 moves up, so that

    m1a=T-m1gsin[itex]\theta[/itex]


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




Similar Discussions: Newton's Law Problems
Loading...