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Physics woes - online course - Rotation

  1. Apr 25, 2005 #1
    Hi, i am glad i have found this site. I am taking an online physics course, the kind where you read the book, work the problems, and take the test. Needless to say it has been very hard on me. I must be really dense at physics because i have purchased supplemental books but to no avail. I have a few questions on rotation and would like to know if i can get a general outline as what to do. I am not posting specific homework problems, but just the general idea of them, so i may work them out myself.

    #1. Given a rotation rate in rad/sec and an accelleration rate in rad/sec^2, find:
    a. Rotation rate after a given time in seconds
    b. Angular displacement in radians during that time.

    #2. Someone is slinging a rock of given mass around his head. Knowing the length of the sling and rotation rate find tangetial velocity of mass, centripital acceleration of rock, and tension in rope.

    #3. A wheel requires a given number of radians of rotation to accelerate from rest to a given angular rotation rate (in rad/sec). Find angular rate of acceleration, time to accelerate to given rotation rate, rotation rate after x seconds.

    #4. A horizontal beam is attached to a frictionless hinge on a wall. Beam weight is given. A weight is placed on the beam at a given location. A VERTICAL cable supports the beam at the outer end. Find

    tension in cable
    x-component of force exerted on the beam by the wall
    y-component of force exerted on the beam by the wall

    (that VERTICAL cable is getting me)

    #5. A solid disc has a frictiionless axle running through its center, allowing it to rotate about this axis. Radius is given. Find tangetial force required to accelerate the disc from rest to a given rotation rate in a given number of seconds.

    These are the main problems i am having. Sorry if these seem somewhat easy to some people, but i do not even know where to start on some of them. Thanks to anyone who helps.
  2. jcsd
  3. Apr 25, 2005 #2
    Do you know the equation for angle displacement under constant conditions?
    Hint: it looks alot like the equations for linear displacement:

    [tex] x(t) = x_0 + v_0t + \frac{1}{2} at^2 [/tex]

    Tangential velocity: There is a very simple relationship between tangential velocity and angular velocity, you should find this in the beginning of the rotation section in your book, or easily online.
    Centripetal acceleration: Centripetal acceleration for uniform circular motion is

    [tex] a = \frac{v_{tan}^2}{r} [/tex]
    Tension: Tension is a measure of the force acting within a rope, use the above definition for centripetal acceleration, and Newton's second law.
    Very similar to #1
    Draw a force diagram of the system. What are the forces in the vertical direction? Hint: Gravity is one. You know the beam isnt moving, so you need to balance the gravitational force, what is doing that?

    This is similar to #1 and #3 except you'll need to figure out a few more things than normally. Mostly rotational equations in conjunction with Newton's 2nd Law.
    Everyone learns sometime right? No ones born knowing physics.
  4. Apr 25, 2005 #3
    Ok i believe i am understanding some of this a bit better. My progress so far:

    deltaTHETA = Wit + 1/2at^2 - angular displacement
    w = Wi + at - rotation rate

    (sorry i do not know how to insert nice looking formulas)

    Vt = rw - tangetiel velocity
    Using above V - alpha = V^2tan/r - centripital acceleration
    <still working on tension, not sure>

    alpha = W/t - rate of acceleration
    w = Wi + alpha t - Some rearranging to get t?
    delta THETA = Wi t + 1/2 alpha t^2 - rotation rate after x seconds

    For this one, wouldnt you just have to find alpha = deltaW/deltaT and then use a = r alpha?

    Thanks for all your help
  5. Apr 25, 2005 #4
    learn latex it aint that hard look for the link in the sticky in the general physics forums
    for the first one you are correct
    [tex] \omega_{2} = \omega_{1} + \alpha t [/tex]
    [tex] \Delta \theta = \omega_{1} t + \frac{1}{2} \alpha t^2 [/tex]

    for the second one

    tangential velocity is simply the angular velocity (vector) times the radial (radius) vector. That is tangential velcoity [itex] v = r \omega [/itex]

    Centripetal Acceleration is simply [tex] \frac{mv^2}{r} [/tex] no tan or sin or whatever if there is no angle with respect to the axis of rotation. here the angle is 90 so you dont have to worry about that

    As for the tension in the string. Since this is a perfectly horizontal circle draw a sideways view of the block going into the page - what forces act on it?? Centripetal and gravitational. The String must balance these forces to keep itself horizontal. Draw a diagram and show us what you got.

    For hte third one
    everything is fine except for the last one wher they ask you the rotation RATE not DISPLACEMENT after x seconds. What is the Rotation Rate?

    For hte Fifth one
    certainly [tex] \alpha = \frac{\omega}{t} [/tex]
    taht s the acceleration required to get it to move. Now to move anything circular you need TORQUE whic his calculated using [tex] \tau = I \alpha [/tex]
    where I is the moment of inertia for the object in question.
    now furtermore torque [tex] \tau = R F \sin \theta [/tex]
    where R is the radius of the tangeital force F, and the theta is the angle the force makes with the radial vector. Since it is TANGENTIAL theta is 90 so [tex] \tau = RF [/tex]
    Last edited: Apr 25, 2005
  6. Apr 25, 2005 #5
    This is a force, and "vtan" means [tex] v_{tan(gential)}[/tex]

    This is correct ,but make sure you know the difference between average angular acceleration/angular velocity and final angular acceleration/angular velocity. These equations are both correct though, the last one is not necessary. It's asking you for a rate of rotation.. a velocity. One of the above already takes care of this.

    Using radius, centripetal acceleration, and the relationship btwn tangential and angular acceleration you can figure this one out. It wants you to accelerate a disk from rest to a certain speed (you know how to do this) by accelerating it tangent to the radius. How does linear acceleration and angular acceleration relate? thats pretty much all you need.
  7. Apr 26, 2005 #6
    Ok, with your help i feel confident about these problems. I have just figured out the horizontal beam one:

    Three equations, leaving sin theta in assuming there is an angle in the cable and not vertical:

    1. [tex] R_(x) - T cos \theta = 0 [/tex]
    2. [tex] R_(y) + T sin \theta - (\mbox{weight of objectt}) - 300 N = 0 [/tex]
    3. [tex] (T sin \theta)(\mbox{length of boardt}) - (300\ N)(\mbox{length to center of gravity}) - (\mbox{weight of object})(\mbox{distance to object}) = 0 [/tex]

    Solve for T on the last one and plug into the first two.
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