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Math Help - Trig problem

  1. #1
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    Trig problem

    Hi guys. I need some help to solve this problem.
    a) A body of mass m kg is attached to a point by string of length
    1.25 m. If the mass is rotating in a horizontal circle 0.75 m below the
    point of attachment, calculate its angular velocity.

    (b) If the mass rotates on a table, calculate the force on the table when
    the speed of rotation is 25 rpm and the mass is 6 kg
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  2. #2
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    Quote Originally Posted by jo74 View Post
    Hi guys. I need some help to solve this problem.
    a) A body of mass m kg is attached to a point by string of length
    1.25 m. If the mass is rotating in a horizontal circle 0.75 m below the
    point of attachment, calculate its angular velocity.

    (b) If the mass rotates on a table, calculate the force on the table when
    the speed of rotation is 25 rpm and the mass is 6 kg
    part (a)

    r = \sqrt{1.25^2 - .75^2}

    \theta = \arcsin\left(\frac{.75}{1.25}\right)

    if T is the tension in the string ...

    T\sin{\theta} = mg

    T\cos{\theta} = F_c , where F_c is the centripetal force.

    eliminate T in the two equations above and solve for F_c ... then use the equation below to determine \omega

    F_c = mr\omega^2


    part (b) is too easy ... think about the net force in the vertical direction.
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  3. #3
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    Hi it's me again. I tried to find the answer but just got my paper back but got it completely wrong again. I already had the first part answered, it's the second part I'm struggling with.
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  4. #4
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    Quote Originally Posted by jo74 View Post
    Hi guys. I need some help to solve this problem.
    a) A body of mass m kg is attached to a point by string of length
    1.25 m. If the mass is rotating in a horizontal circle 0.75 m below the
    point of attachment, calculate its angular velocity.

    (b) If the mass rotates on a table, calculate the force on the table when
    the speed of rotation is 25 rpm and the mass is 6 kg
    is the string still attached as in part (a) ?
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  5. #5
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    Yeah, it is still attached to the string at the same angle as in part 1.
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  6. #6
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    T\cos{\theta} = m r \omega^2

    T = \frac{m r \omega^2}{\cos{\theta}}

    sub in your known values and calculate T


    let F = force that the table exerts upward on the mass.

    T\sin{\theta} + F = mg

    F = mg - T\sin{\theta}

    calculate F
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  7. #7
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    Thanks for that one, much appreciated
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  8. #8
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    Hi I am also having problems with this exact question, I have the value for the radius and the angle but I am completly lost how to find the other values, all formulas contain m but it is an unknown value?

    Thanks
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