# Pure Rotation - Example Problem 4.2-8

The square plate rotates about the fixed pivot O. At the instant represented, the direction of the angular velocity, ω, and angular acceleration, α, of the plate are shown in the figure. Determine the velocity and acceleration of points A and B in the x-y coordinate frame.

Given:

• ω
• α
• Dimensions of the plate.

Find:

• vA, aA
• vB, aB

So far we have used the n-t coordinate equations to calculate the velocity and acceleration of a point on a rigid body undergoing pure rotation.

vA = rAωet           aA = rAαet + rAω2en

#### Draw the n-t coordinate axes for points A and B. Click on the figure to see these coordinates.

Notice that equating the nA-tA coordinate axes to the x-y coordinate axes is easy, but it is not so easy to equate the nB-tB coordinate axes. Therefore, the above equations may not be simple to use. For situations where we are working in a coordinate system other than the n-t coordinate system, it may be advantageous to use the cross product versions of the velocity and acceleration equations.

vA = ω x rA/0          aA = α x rA/0 - ω02rA/0