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Friction and Force problem! Help!

  1. Jan 24, 2005 #1
    Two blocks are connected by a rope. Block A = 8kg, Block B = 5kg

    Ramp's angle theta is 22 deg. Kinetic friction = .10

    The blocks are held motionless, then released. Block A slides up as Block B falls.

    Block A is on the ramp connected by a rope to Block B hanging off the side of the ramp and only has the force of gravity down and the tension force up acting upon it.

    Can someone please tell me what I am missing/left out of each part and help me set things up right? Thanks

    1. Find Net force of Block A parallel to the ramp
    Uk = coeff of kinetic friction

    mass A * accel A = -mg sin theta + Uk mg cos theta
    mass A * accel A = 8* -9.8 * sin 22 + .1 * 8 * 9.8 * cos 22

    2. Find Net force of Block B vertically

    mass B * accel B = -mg + Ftension
    mass B * accel B = -5 * 9.8 + Ftension

    3. You should have two equations with three unknowns: tension in the string, accel_A, and accel_B. The magnitude of the two accelerations should be equal. Figure out if they have the same sign, or opposite signs.

    You should now have two equations for two unknowns, the tension in the string and the acceleration of the boxes. Solve for the magnitude of this acceleration.

    Check your work: if all is well, then you should be able to solve for the tension in the string using the box A equation. You should also be able to solve for the tension in the string using the box B equation. It should turn out to be the same tension

    I am stuck here, I don't know how to combine them and get them the way it wants. Also, Since I should be able to solve for tension in the Box A equation, I must be missing something in the Box A equation.
  2. jcsd
  3. Jan 24, 2005 #2
    Bump! Someone please help me with this!
  4. Jan 24, 2005 #3


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    Homework Helper
    Gold Member

    That rope is connected both A and B. You should include the tension of the rope among the forces acting on A. Don't forget that the tension is the same at botes ends of the rope.

  5. Jan 24, 2005 #4
    Yea but where do I put the Tension on A exactly and how do I make it into two equations with two unknowns? this is my real problem
  6. Jan 24, 2005 #5

    Doc Al

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    Staff: Mentor

    As ehild points out, you left out the force of the string pulling block A. Also, in what direction does the friction act?
    It is important to relate the motion of blocks A and B, since they are connected by the string. They have the same magnitude of acceleration. So, choose a consistent sign convention. If "up the incline" is positive, the the acceleration of block A is +a; If, for block B, "down" is negative, then the acceleration of block B is "-a".
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