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Homework Help: Acceleration and Force question - Thanks!

  1. Oct 2, 2011 #1
    1. The problem statement, all variables and given/known data

    Image below shows the questions. I need help with B and C - Finding the acceleration and the force...


    2. Relevant equations

    3. The attempt at a solution

    I am not sure how to find acceleration going down a ramp. So, B and C
  2. jcsd
  3. Oct 2, 2011 #2
    The starting point is to draw the two FBDs, one for A and one for B. Then someone can help you.
  4. Oct 2, 2011 #3
    umm...yes I know. I already drew the diagrams. As I stated, I need help with B and C.

    Box A has a force of Gravity, normal force, friction force, and force from box b.

    Box B has a force of gravity, normal force, friction force, and force from box a

    Is that correct?
  5. Oct 2, 2011 #4
    Now use F(net) = ma twice. Once for A and once for B.
  6. Oct 2, 2011 #5
    That cannot be correct. You are not taking account of the 20 degree ramp ??
  7. Oct 2, 2011 #6
    Use f=ma in the direction parallel down the plane. You have to use component of weight.
  8. Oct 3, 2011 #7
    Can anyone else help me with B and C ?

    I am not sure how to find the acceleration of the system with the friction and 20 degree slope of the ramp, as pointed out in my original post.

  9. Oct 3, 2011 #8
    see the attachment to see how one can use F = Ma along the plane.

    Attached Files:

  10. Oct 3, 2011 #9
    Now try to do the same for block A.
  11. Oct 3, 2011 #10
    I am not trying to find acceleration of one block, but both as a sytem...

    Thanks for the diagram!

    How do I use F = Ma to find the acceleration in the diagram you gave me?

    F would be 100n correct? M would be 15kg correct? a would be ?
  12. Oct 3, 2011 #11
    I think that you were asked to draw the FBD for EACH block so that you can enter the force required in part (c).
    If you treat both as one block, then the force required in (c) will not enter.
    That is why I suggested that you use F = Ma for each block seperately.
  13. Oct 3, 2011 #12
    Oh ok. So, lets say I do that, and start with block A

    How do I use F = Ma ? while including the angle and friction? How do I find a?

  14. Oct 3, 2011 #13
    Let us start with block B so that we can use the diagram in the attachment.
    What are the forces acting on B downwards along the inclined plane?
  15. Oct 4, 2011 #14

    Forces acting on B are Gravity, friction in opposite direction of acceleration, and normal force from the ramp, and applired force from block A ?
  16. Oct 4, 2011 #15
    But the force due to gravity is vertically downwards and we want forces downwards ALONG the inclined plane.

    So what do you think we need to do?
  17. Oct 4, 2011 #16
    The forces along the plane are just friction?

    So, what do we do with that?

    for acceleration ?
  18. Oct 5, 2011 #17
    Part (a) of the question was to draw the FBD for EACH block because this is the starting point of the solution.
    You said that you drew these FBDs.
    If you look at the FBD for block B there are THREE forces parallel to the plane:
    the push of A on B up the plane,the friction of plane on B up the plane and a COMPONENT of the weight of B down the plane.
    Now find this component of the weight of B.
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