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Homework Help: Questions on Circular Motion

  1. Sep 9, 2006 #1
    1. When the radius and mass are held constant, what do you expect to happen to the centripetal force if the frequency of rotation is increased?

    2. What will happen if a rotating platform was set up slightly off the horizontal? Would anything change? What in particular?

    3. A ball rotates in a horizontal circle at a constant speed of 10m/s. what are the tensions in the upper and lower strings? The mass of the ball is 3kg.
    http://www.freewebs.com/mcroldan08/problem.jpg [Broken]

    4. What angle of bank is necessary for a car to make it around a 130m curve at a speed of 60kph without relying on friction.

    5. Determine the speed of the satellite orbiting at a height of 700km above the Earth's surface.
    mass of earth = 5.98 x 10^24 kg
    radius of earth = 6.38 x 10^6 m


    I would greatly appreciate the help...

    Oh, and btw, I also need Free Body Diagrams for problems 3 - 5 (so that i won't get confused :bugeye: )

    Thanks again....
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Sep 9, 2006 #2
    asteg, what are your thoughts so far?
    Especially about the free body diagram--have you come up with anything at all?
  4. Sep 9, 2006 #3
    So far, I had only done number 4, but i am not very sure of what i did...

    I had made a FBD of this and it showed a vector at a certain angle (WRT the y-axis) and towards the right... this vector represents the Normal force... there is also a vector downwards, and it is W=mg

    and from there, i'm stuck...

    With regards to the other problems, I always get stuck on the FBD, so I can't really figure the problem out...

    Hope you could help me...
  5. Sep 10, 2006 #4
    Ok, so far, I had answers to number 3 & 4, However, I'm not sure if they're right...

    Here's what i did, on number 3, I made an FBD and here it is...
    http://freewebs.com/mcroldan08/problem2.jpg [Broken]
    Can anyone verify this.... This is my equation

    Letting the angle be A

    the Summation Fy=0=T'sinA - T''sinA-w
    the summation Fx=m*a=(mv^2)/R=T''cosA+T'cosA

    then i simply did algebra and got the answer

    T'' = 325.62N and T' = 360.9N

    ~~~ In need of verification ~~~

    and for number 4
    http://freewebs.com/mcroldan08/problem3.jpg [Broken]
    where the angle is A

    i got the summation Fx=m*a=n*sinA where a is a-rad
    and summation Fy=0=n*cosA-m*g
    I have yet to solve this, and i had noticed the lot of conversion of units in this problem

    However, i still need to verify if what I did was correct...

    ~~ in need of verification ~~

    and with regards to 1, 2 and 5 , I still have no answers... especially in the numbers 1 & 2 (I'm bad at theory)

    Anyways... hopefully, someone will provide help....

    Thanks in advance...
    Last edited by a moderator: May 2, 2017
  6. Sep 10, 2006 #5


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    Homework Helper

    The centripetal force increases, since [tex]F_{cp}=m\omega^2r[/tex], where [tex]\omega[/tex] is the angular speed of the ball (frequency of rotation).

    You have to set the centripetal force and the gravitational force to equal, which is a condition for a satellite to orbit around the Earth.
  7. Sep 10, 2006 #6
    Uhmm... is the gravitational force the constant of GM earth??

    Is there any other way of solving this other than this method... coz we still haven't discussed GM yet...

    I think the method is somewhat involving frequency..
    I'm not really sure...
    Last edited: Sep 10, 2006
  8. Sep 10, 2006 #7
    well i'm not very sure it Frequency can be used here, but what is the point of giving u the mass of the earth if Gravitational Force will not be used.
  9. Sep 10, 2006 #8
    hmm... true...

    I guess I will use the Gravitational force...

    Last edited: Sep 10, 2006
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