1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Relative Velocity, Rigid Body Kinematics

  1. Jul 11, 2013 #1
    1. The problem statement, all variables and given/known data


    W(oa) = -5k
    α(oa) = 3k
    B = 30 deg
    B(dot) = -2cos 30i + 2sin30j = Vrel
    b(double dot) = 4cos30i - 4sin30j)

    2. Relevant equations
    Va = Woa x Rao + Vrel

    3. The attempt at a solution

    This is such a simple problem and I don't know why im messing it up. Im just trying to figure out the Va part right now. I know the answer from the back is 4.38i + 7.58j

    I did Va = W(oa) x R(ao) + Vrel solving for Va.
    Va = -5k x 1.25(-cos30i + sin30j) + (-2cos 30i + 2sin30j)
    Va = 3.125i + 5.4j - 1.73i - 1j

    Va = 1.395i - 4.4j

    This is what im getting for Va but the answer is wrong. Can anyone point me in the right direction please?
  2. jcsd
  3. Jul 11, 2013 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Please you explain your reasoning at each step?

    So - by "absolute velocity" the mean the linear velocity.
    What does the rotation of the disk have to do with the motion of the sphere? Are the beta figures supposed to be wrt the rotating frome of the disk?

    In your equations you appear to be making angular velocity and angular acceleration equal to linear velocity and acceleration. What is the relationship between angular velocity and tangential velocity?

    Note: sin(30°)=1/2, cos(30°)=(√3)/2
    Last edited: Jul 11, 2013
  4. Jul 11, 2013 #3
    yes by absolute velocity i mean linear velocity.

    well I know that if point A was fixed, than Va = Woa x Rao. But since it is moving, Va = Woa x Rao + Vrel. I assume that Vrel is tangent to point A in the direction: 1.255i + 2.18j. and yes angular velocity is clockwise.
  5. Jul 11, 2013 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Please answer all the questions.

    Did you mix up the sin and cos?
    Consider, an object on a circular path 5" from the center, angular velocity 3rad/s clockwise, has a tangential velocity 15in/s right? When the angle is position 30deg anticlockwise from the horizontal, the velocity vector makes an angle of 60 degrees clockwise from the horizontal. sin(60)=cos(30).
    (draw the pic)

    hence: v= [-15sin(30)i+15cos(30)j]in/s = [-(7.5)i+(7.5√3)j]in/s
    note: always include the units and you don't have to expand the √3 as 1.7321 until the very end.
    Last edited: Jul 11, 2013
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted