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Grade 12 physics - one mass on incline and other hanging

  1. Nov 1, 2005 #1
    Body B weighs 440 N and body A weighs 140N. The coefficients of friction between B and the incline are Us = 0.56 and Uk = 0.25.
    a) Find the acceleration of the system if B is initially at rest
    b) Find the acceleration of the system if B is moving up the incline
    c) Find the acceleration of the system if B is moving down the incline.

    (diagram: mass B is on the incline, and a is hanging)

    For a, my teacher showed us the following solution:
    a) mass b (g)(sin theta) ?> mass a (g)
    a = ((mass b)(g sin theta) - (mass a)(g) - Ffriction) / (mass B + mass A)
    = 0

    Could someone explain this solution to me? I think I will be able to figure out parts b and c once i understand part a.

    Thanks in advance and sorry I couldn't scan the diagram.
     
    Last edited: Nov 2, 2005
  2. jcsd
  3. Nov 1, 2005 #2

    daniel_i_l

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    Gold Member

    You find the components of each force acting on B in the direction of the plane (tension , gravity ,friction) then use f = ma.
     
  4. Nov 2, 2005 #3
    hm.. sorry i tried that and i still couldn't figure out how to do this question
     
  5. Nov 2, 2005 #4
    urgent... please help!
     
  6. Nov 2, 2005 #5

    verty

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    Homework Helper

    First you calculate the resultant force, then you calculate the acceleration.

    The resultant force is: m_b*g*sin(theta) - m_a*g +/- friction. I put +/- there because the friction always opposes motion. What don't you understand?

    Show your working out please.
     
    Last edited: Nov 2, 2005
  7. Nov 2, 2005 #6
    I don't understand why the answer to part a is 0...

    Thanks I got parts b and c now
     
  8. Nov 2, 2005 #7

    verty

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    The answer to part a will be 0 if friction overcomes the stronger of the applied force and gravity. You haven't given the angle of the incline so I don't know if it does or not. I imagine m_b*g*cos(theta)*Us > m_b*g*sin(theta) - m_a*g, which would mean a = 0.
     
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