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Homework Help: Circular motion graph of a ball

  1. Oct 19, 2007 #1
    Sorry for posting 3 problems, but I have 25 due and im just not grasping the material...

    [[[[[[ #1 ]]]]]]

    1. The problem statement, all variables and given/known data

    A 100 g ball on a 60 cm long string is swung in a vertical circle about a point 200 cm above the floor. The tension in the string when the ball is at the very bottom of the circle is 4.0 N. A very sharp knife is suddenly inserted, as shown in the picture, to cut the string directly below the point of support. Where does the ball hit the floor? (Measure from the point where the string was cut and consider right to be the positive direction.)

    http://img151.imageshack.us/img151/6631/p751fl2.gif [Broken]

    2. Relevant equations


    and the constant accel kinematic equations

    3. The attempt at a solution



    then i use y(final)=y(initial)+1/2at^2
    0=(.2-.06) + 1/2(-9.81)t^2

    plugging in for x


    not getting the right answer tho...

    i suck at physics

    [[[[[[ #2 ]]]]]]

    1. The problem statement, all variables and given/known data

    A conical pendulum is formed by attaching a 400 g ball to a 1.0 m long string, then allowing the mass to move in a horizontal circle of radius 14 cm. The picture shows that the string traces out the surface of a cone, hence the name.

    http://img151.imageshack.us/img151/8415/p739hd6.gif [Broken]

    2. Relevant equations


    3. The attempt at a solution

    I found the tension force F = 3.924N, which was part a

    anyways its right, but I need the second part which says:(b) What is the ball's angular velocity, in rpm?

    I dont know how do do that, I used w=sqrt(f/mr) and then multiplied by 60sec/1min and then divided by 1revolution/2pi radians, but the answer is wrong.

    [[[[[[ #3 ]]]]]]

    1. The problem statement, all variables and given/known data

    Two wires are tied to the 320g sphere pictured. The sphere revolves in a horizontal circle at a constant speed of 6 m/s. What is the tension in each of the wires?

    http://i9.photobucket.com/albums/a71/bk12321/p7-61.gif [Broken]

    2. Relevant equations


    t=netforce, if gravity doesnt come into play

    since they are both acting on the ball, 2t=the net force on the x axis

    3. The attempt at a solution

    You just have to find the tension force in each wire. I solved it but i'm getting the wrong answer :(. This one seems to be the easy one but I just can't get the right answer so im dumb.

    net force=mv^2/r
    from there I took cos(theta) and multiplied it and did some other stuff, and then divided the total tension force by 2 since the tensions should be equal, but the answer is wrong.... so some help with the part after finding the net force would be awesome

    thanks a ton
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Oct 19, 2007 #2
    I wish I could get physics like I do calc II...
  4. Oct 19, 2007 #3
    no help?
  5. Oct 19, 2007 #4


    User Avatar
    Homework Helper

    For the first problem you need to consider the tension, and the gravity...

    net force = mv^2/r

    what is the net force (using gravity and tension)...

    also for the kinematics part... 200cm = 2m. 60cm = 0.6m.

    for the second problem... w = v/r, where r is the radius of the circular path... get v using centripetal motion... then you can get w...

    For the third,

    I think you need to consider gravity... suppose T1 is the force in the top string. T2 is the bottom.

    sum of the vertical forces = 0

    plug in the appropriate forces above

    sum of horizontal forces = mv^2/r

    2 equations... 2 unknowns. you can get both the tensions.
  6. Oct 20, 2007 #5


    User Avatar

    For your first question,

    Since tension is given, and you know the gravitational force at the bottom and you know that these difference is equal to the centripetal force. The solution to this equation gives you the velocity in the x direction.

    With the velocity in the x direction and an accelaration due to gravity, can you find the position of the ball??

    The scenario after the string is cut is similar to throwing a stone on a cliff in x direction.
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