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Thread: Determine the perimeter of the stones?

  1. #1
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    Determine the perimeter of the stones?

    In the Strongman competition, one of the competitions is to lift a rock. At a competition, the stones weigh 120, 140, 160 and 180 kg. Suppose the density is 2800 kg/m^3. Determine the perimeter of the stones.

    I really do not know how to solve this, it would be nice if someone could explain!
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  2. #2
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    Re: Determine the perimeter of the stones?

    what shape are the stones? spheres?

    note $V = \dfrac{m}{\rho}$

    circumference of a sphere is $2\pi r$
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  3. #3
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    Re: Determine the perimeter of the stones?

    the stones are spheres. However, we do not know r. So i cant get out the circumference with 2*pi*r ...
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  4. #4
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    Re: Determine the perimeter of the stones?

    The volume of a sphere is given by V= \frac{4\pi}{3}r^3 so if you know the volume, V, you can find the radius, r. You are told that one of the spheres weighs 120 kg and that the density is 2800 kg/m^3. So the volume of that sphere is \frac{120}{2800}= \frac{3}{70} cubic meters.
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  5. #5
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    Re: Determine the perimeter of the stones?

    so

    p = m / v

    v = 120/2800
    V = 3/7 m^3

    v = 4 * pi * r^3/3

    R^3 = 0.10 m
    R = 0.46 m

    Circumference = 2 * pi * r
    2 * pi * 0.46 = 2.93 m

    My answer is wrong, because the correct answer should be one of the answers 136m, 144m, 147m, 150m, and 156m.

    Do not know what I've done wrong.
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  6. #6
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    Re: Determine the perimeter of the stones?

    Quote Originally Posted by selin View Post
    so

    p = m / v

    v = 120/2800
    V = 3/7 m^3
    Sigh. I told you previously that the volume is 3/70 cubic meters, not 3/7!


    v = 4 * pi * r^3/3

    R^3 = 0.10 m
    R = 0.46 m

    Circumference = 2 * pi * r
    2 * pi * 0.46 = 2.93 m

    My answer is wrong, because the correct answer should be one of the answers 136m, 144m, 147m, 150m, and 156m.

    Do not know what I've done wrong.
    Follow Math Help Forum on Facebook and Google+

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