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Homework Help: Angular displacement, velocity and acceleration

  1. Oct 29, 2009 #1
    I have a few problems that I am very confused on. If someone could walk me through them that would be a big help.

    1. An antique spring-driven Victrola phonograph plays recordings at 78 rpm. At the end of each record the arm hits a lever that activates a brake that brings the record to rest in 1.3 s. Through how many radians does it turn in the process of stopping?

    2. Placed on a long incline, a wheel is released from rest and rolls for 30.0 s until it reaches a speed of 11.0 rad/s. Assume its acceleration is constant, what angle did it turn through?

    3. What is the average angular speed of the Earth in its orbit? Take a year to be 365.24 days. Give your answer to three significant figures.
  2. jcsd
  3. Oct 29, 2009 #2
    Made a mistake on this one...see the post below instead
    Last edited: Oct 29, 2009
  4. Oct 29, 2009 #3
    I think you're placing these in the wrong forum, these should be in the Introductory Physics...

    You have an angular velocity, [itex]\omega=78[/itex] rpm and a time [itex]t=1.3[/itex] s, so then either convert [itex]\omega[/itex] into revolutions per second or [itex]t[/itex] into minutes, multiply the two then convert from revolutions to radians.

    You're given an initial and final angular velocity, [itex]\omega_i=0[/itex] and [itex]\omega_f=11[/itex] rad/s respectively. If you subtract the initial angular velocity from the final angular velocity, [itex]\omega_f-\omega_i[/itex] and then divide by the time given, you will have the angular acceleration, [itex]\alpha[/itex]. Using [itex]\alpha[/itex], [itex]\omega_i[/itex], [itex]t[/itex], you should put it into the rotational kinematic equation

    \theta=\omega_it+\frac{1}{2}\alpha t^2

    then you can find the total angle through which it rotated in the motion.

    The Earth travels [itex]2\pi[/itex] radians per year. If you convert days into hours into minutes, you can divide the angle, [itex]2\pi[/itex], by the time to get radians per minute. Then convert radians into revolutions so that you'll have revolutions per minute.
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