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Interrelated motion problems

  1. Oct 16, 2008 #1
    I'm just going to post a few questions in this post. After missing a day of class I am having difficulty with the homework for the day.

    Problem 1
    1. The problem statement, all variables and given/known data
    The coefficient of kinetic friction between the 2.0 kg block in figure and the table is 0.260.
    [​IMG]
    What is the acceleration of the 2.0 kg block?

    2. Relevant equations
    MkN=ff
    T=m*a


    3. The attempt at a solution
    I went through and calculated the force of friction which gave me 5.096N. However, I'm not entirely sure where to go from there. I know that you need the sum of the forces acting on the 2kg mass in the x direction and divide that by the total mass of the system.



    Problem 2
    1. The problem statement, all variables and given/known data
    The 100 kg block in figure takes 5.20 to reach the floor after being released from rest.
    [​IMG]
    What is the mass of the block on the left?

    2. Relevant equations
    T=m*a


    3. The attempt at a solution
    I know that I can figure out the exact acceleration based on the time and mass and such, but how would I go about solving for the other mass?


    Problem 3
    1. The problem statement, all variables and given/known data
    Two packages at UPS start sliding down the ramp shown in the figure. Package A has a mass of 4.00 and a coefficient of kinetic friction of 0.210. Package B has a mass of 9.00 and a coefficient of kinetic friction of 0.140.
    [​IMG]
    How long does it take package A to reach the bottom?

    2. Relevant equations
    F=m*a

    3. The attempt at a solution
    I'm not even really sure how to attempt this problem. I know you can draw a free body diagram and figure out the equal and opposite forces, but where to go from there?


    Problem 4
    1. The problem statement, all variables and given/known data
    A rope attached to a 20.0 wood sled pulls the sled up a 21.0 snow-covered hill. A 11.0 wood box rides on top of the sled.
    If the tension in the rope steadily increases, at what value of the tension does the box slip?

    2. Relevant equations
    T=m*a

    3. The attempt at a solution
    This is another one i'm not real sure how to start on, any ideas?


    Thanks for any and all help.

    -Chris
     
    Last edited: Oct 16, 2008
  2. jcsd
  3. Oct 17, 2008 #2

    alphysicist

    User Avatar
    Homework Helper

    Hi fronomo,

    This does not look right to me. The net force is equal to ma; but here there are two ropes and therefore two tensions that are acting in opposite direction, as well as the force of friction. What would the net force in the x direction be for the 2kg object?
     
  4. Oct 17, 2008 #3
    You seem to have everything you need to solve this. When you sum the forces, just remember the directions each is acting - it should be fairly obvious for the tensile forces and the frictional force will work opposite the direction of motion.

    Start by finding the net acceleration of the system -- but don't use mass to do so as it has an unknown at this point, just use the distance traveled, initial velocity, and time.

    Once you have the net acceleration, you can find the net force acting on the system in terms of the known and unknown masses.
     
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