# Relative Velocity, Rigid Body Kinematics

1. Jul 11, 2013

### allyfranken

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. Jul 11, 2013

### Simon Bridge

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
3. Jul 11, 2013

### allyfranken

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.

4. Jul 11, 2013