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Homework Help: Newton's Laws Problems.

  1. Nov 10, 2009 #1
    I'm having a lot of trouble trying to figure out some of these problems. If people could help me, it'd be greatly appreciated!

    1. A 50.0-kg person takes a nap in a backyard hammock. Both ropes supporting the hammock are at an angle of 15.0° above the horizontal. Find the tension of the ropes.

    -I used Fy2 = Ft2 sin (°). I got 1895.3 for an answer, yet I don't think this is right at all.

    2. Your friend's 12.6-g graduation tassel hangs on a string from his rear-view mirror.
    (a) When he accelerates from a stoplight, the tassel deflects backward toward the rear of the car. Explain.
    (b) If the tassel hangs at an angle of 6.2° relative to the vertical, what is the acceleration of the car?
    (c) Find the tension in the string holding the tassel.
    (d) At what angle to the vertical will the tension in the string be twice the weight of the tassel?

    -I found that the tassel is .0126-kg.
    -For (a) I said "the sudden stop in acceleration causes it to go back, because there is nothing to stop it's movement."
    -For (b) I wrote the equation Fnet = Fg - Fn

    Thanks for any help, I am just very lost. If you can even reccomend any detailed websites, that'd be great as well. :)
  2. jcsd
  3. Nov 10, 2009 #2


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    Homework Helper

    You're very close, but you forgot that the person's weight is supported by two ropes, not one. If you divide 1895 by 2, you'd get the right answer.

    For a: What is a "sudden stop in acceleration"? The car's just accelerating; there's no stop in its acceleration. Otherwise, your explanation is correct: the thing has inertia and doesn't want to change its speed.

    For b, what's Fn? Try drawing a free-body diagram of the tassel, labeling the external forces, and writing out Newton's second law for both the x and y directions. After that, you just have to solve the equations.
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