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: Simple incline problem.

  1. Oct 19, 2011 #1
    1. The problem statement, all variables and given/known data
    A 6kg block "B" rests as shown(refer attachment) on the upper surface of a 15 Kg wedge "A". Neglecting friction, determine immediately after the system is released from rest,
    a)The acceleration of A
    b)The acceleration of B relative to A.

    2. Relevant equations
    F = ma


    3. The attempt at a solution

    For the first part,

    The force on A due to gravity, parallel to the incline will be Mg sin30° = 75N
    And the force of the block B on A is mg= 60N, and the component parallel to the incline will be 60 sin30° = 30N, so the total force parallel to the incline is 30+75= 105N
    Thus, a=Net force/ total mass = 105/(15+6) = 5 m/s^2 .
    The answer should be 6.63 m/s^2. Where have i gone wrong?
     

    Attached Files:

    • A.bmp
      A.bmp
      File size:
      27.6 KB
      Views:
      138
  2. jcsd
  3. Oct 20, 2011 #2
    I think you might have two problems, first you use the total mass when calculating the acceleration but I think it should only be the 15kg mass, the two blocks accelerate differently. Secondly I think the force of the upper block acting on the lower block is reduced because the upper block accelerates downward. There is a geometric relationship between the accelerations. See,
     

    Attached Files:

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook