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Suspended object, two rope system

  1. Sep 22, 2009 #1
    1. The problem statement, all variables and given/known data

    1) What is Y-component of the force due to rope 2
    2) What is the magnitude of F2
    3) What are the X components of the force due to rope 1 and rope 2

    mass = 60kg
    [tex]\theta[/tex] = 20 degrees

    2. Relevant equations
    F = ma
    ma = T-W

    3. The attempt at a solution
    1) mass * gravity = 60 * -9.8 = -588
    To find the Y component, take -588 * sin(90-20) (Should the angle be off of horizontal or vertical, currently set to horizontal)

    2) No clue, the actual term 'magnitude' seems to confuse me, so if someone could briefly define what the magnitude of a force is with relevance to the actual force, that would be great.

    3) Same as 1, just substitute sin with cos (for rope 2)
    For rope 1, is it 0? That doesn't seem right though, because the rope offset the block to create the angle of 20 degrees.
  2. jcsd
  3. Sep 22, 2009 #2


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    It would be easier to see what is going on if you resolved F2 into vertical and horizontal components. Then your diagram will look like a cross with four arms at right angles. If the net force is zero on the block, this means that the horizontal and vertical arms are

    1. Irrelevant to the problem.
    2. Equal in size and opposite in direction, in pairs
    3. Don't know.

    What do you think?
  4. Sep 22, 2009 #3
    Obviously, if you assign the entire picture as the system, the net force is going to be zero. What I am trying to figure out is when the system is defined as just the block, and to find the separate external forces as noted in the questions. It's the 'arms' separate force that I need to calculate, knowing the basic rules of equal & opposite (and therefore cancellation) is true, but doesn't help much if at all on the calculations.. other than the fact that on question 3, the numbers should be the same, but one is negative and one is positive.
  5. Sep 22, 2009 #4
    Actually the "equal and opposite forces" is probably the key to solving this problem.

    Understanding that the block is currently stationary, then what can you conclude about the net forces on the block?

    After breaking F2 into vertical and horizontal components you can easily observe how those forces interact with F1 and gravity.

    I'm not sure what you're asking about the relativity of magnitude of a force to the actual force. As far as I'm concerned that is nearly the same thing, the only difference being that since force is a vector, an "actual force" would usually be accompanied by a direction, usually an angle. Your question only cares about the magnitude, or the "scalar part". What is that tension in the rope that is keeping the block stationary?
  6. Sep 22, 2009 #5
    Question: Are my calculations correct? If not, where did I go wrong?

    I already said that they cancel each other out. i.e. the net force acting on the block is zero.

    I know, my question is HOW.
  7. Sep 22, 2009 #6


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