1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Angular velocity question

  1. Mar 22, 2016 #1
    1. The problem statement, all variables and given/known data

    Hi Everyone would somebody please be able to give some advice on the following questions:
    Part (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.


    I have attached a solution does it look right or is there a more efficient way to calculate it?

    Part (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.

    I'm a little bit stuck any idea what formula I would use to begin to solve it? Thanks in advance


    2. Relevant equations

    Fc=mω^2r

    3. The attempt at a solution
    Attached as a photo
     

    Attached Files:

  2. jcsd
  3. Mar 22, 2016 #2
    part A--pl. check your solution - as the speed is very large! you may draw a diagram to represent forces
    in part b also draw a free body diagram.
     
  4. Mar 22, 2016 #3
    Hi Thanks for the reply.

    In relation to my solution there is limited info in the question. Would a diagram be necessary to solve the problem as I believe (but I am probably wrong) that I have calculated the relevant numbers to enter into the formula.

    Is there a more efficient way of calculating Angular velocity? given that I only have two pieces of information. The length and the height.
     
  5. Mar 22, 2016 #4
    In your answer you must define the angle carefully- for example you write cos of theta = h/L -so you are taking theta angle as the angle made by the string with the vertical drawn at point of suspension.
    you have two forces working -
    1. the tension in the string pulling along length-you are calling it F
    2. the weight of the bob acting vertically down ward
    for the motion in circular path to be sustained you need one centripetal force acting horizontally towards the centre , which must be provided by the unbalanced force available to the system .
    therefore resolve the weight m.g in the direction along length opposite to the F and these two balance each other-must be equal.
    the other resolved component along the horizontal radial direction of m.g will give you the centripetal force and calculate ang. velocity-see if it changes your answer- then we may tackle part (b).
     
  6. Mar 22, 2016 #5
    you just liked my previous post but i will request to work it out so that a check can be done with numbers!
    now coming to the part -b-

    the ball is rotating on the table so figure out the forces -
    1. ball's weight acting vertically downward.
    2. the normal reaction perpendicular to table passing through the centre of the ball--....
    3. as it is rotating the required centripetal force pointing towards the centre- must be provided by the string
    you have not said anything about the friction -so one can not do more !
     
  7. Mar 25, 2016 #6
    I have had another go and have attempted the second part.

    How do you think it looks now?
     

    Attached Files:

    • 1a.pdf
      1a.pdf
      File size:
      1.4 MB
      Views:
      107
    • 1b.pdf
      1b.pdf
      File size:
      1.5 MB
      Views:
      112
  8. Mar 26, 2016 #7
    now part a seems to be good -numbers may be checked.
    part b i have a question?
    Is the ball hanging and rotating on a horizontal table ?
    or the ball is rotating on the table about a centre 0 attached with the string?
    If the case of first one the additional force is normal reaction of the table on the ball. so your work out seems to be good.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted