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Homework Help: Some clarification on FORCES!

  1. Oct 15, 2006 #1
    1) Two masses of 3.00 kg and 5.00 kg are connected by a light string that passes over a frictionless pulley as in the figure below. Determine (A) the tension in the string (B) the acceleration of each mass (C) the distance each mass will move up in the first second of motion if both masses start from rest.


    Thats the figure.

    I drew my free body diagram. I worked out my formulas to be:

    F = MA
    F = (3.00)A

    F = MA
    F = (5.00)A

    I know im missing something. I feel like i don't have enough information as the question is stated now. I would very much appreciate it if someone could please point out my mistake. Also, tension is the force the string exerts on the weights correct? So could it also be wrriten as..

    T = MA?

    Would that be ok?

    Help would be very much appreciated thank you!
  2. jcsd
  3. Oct 15, 2006 #2

    Doc Al

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    Staff: Mentor

    Newton's 2nd law tells you that the net force on each mass will equal the mass times the acceleration.
    (1) What forces act on each mass?
    (2) How does the acceleration of one mass compare to that of the other?

    Use the answers to come up with two equations (one for each mass) which you will solve to find the tension and acceleration.
  4. Oct 15, 2006 #3

    Hm. So for the 3.0 kg weight, the string pulls up on weight, while the earth pulls down on it. And wouldn't their accelerations be the same?:confused:

    How could I translate this formulaically?
  5. Oct 15, 2006 #4

    Doc Al

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    Staff: Mentor

    Right. And similar forces act on the other mass.

    The magnitudes would be the same, but one moves up while the other moves down.
  6. Oct 15, 2006 #5
    Thanks Doc, i think i understand the concept now however, how could I translate this information into the equation? My take on it is that..

    the FORCE of one weight would be the variable F

    the MASS would be the same (3.00 kg)

    and the ACCELERATION would be the variable A minus 9.8

    am I correct or an idiot? :(
  7. Oct 15, 2006 #6
    desperate bump.
  8. Oct 15, 2006 #7
    anyone? i just need a little bit of clarification.
  9. Oct 15, 2006 #8
    Just follow the recipe Doc Al gave you in his very first message. For this problem, each free body diagram will show the mass, and each mass will have two arrows attached to it, representing the vectors for the forces. One will be pushing in the positive direction, and one in the negative direction.

    So add these two together, and use Newton's second law to decide what to set this sum, which is the net force, equal to.

    Do the same for the second free body diagram. You'll end up with two equations in two unknowns, which you can solve for the acceleration or the tension in the string.

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