Conceptual question - relative accleration of rigid bodies

[prettydumbguy]

I saw this problem in my dynamics class: http://d2vlcm61l7u1fs.cloudfront.net/media/a22/a2261e7a-0e85-45e7-8b63-f23157d623bc/php37ql1a.png

I know how to solve it, but I don't understand why A(bd) has both a normal and tangential component to acceleration. If it is rotating at a constant angular velocity, how is it accelerating tangentially? This seems to defy what I learned in my physics classes.

 

[Sirsh]

To my knowledge, link AB should only have a normal component of acceleration. Whereas, link BD could have a combination of both normal and tangential. Also, point P will only a tangential acceleration.

Even though link AB has constant angular velocity, doesn't mean that BD will also have constant angular velocity. As if you were to do the relative velocity diagram of this problem you'd notice that there is both translation and rotation of link BD.

 

[prettydumbguy]

To my knowledge, link AB should only have a normal component of acceleration. Whereas, link BD could have a combination of both normal and tangential. Also, point P will only a tangential acceleration.

Even though link AB has constant angular velocity, doesn't mean that BD will also have constant angular velocity. As if you were to do the relative velocity diagram of this problem you'd notice that there is both translation and rotation of link BD.

Hmm, so if I have theta=0, AB and BD are both going straight up and D is at its highest point, what happens then? The normal acceleration in AB doesn't change, but then the acceleration of BD would be (alpha x rBD) + w x (w x rBD). So the tangential acceleration would be k x j = i. Then the normal would be j x k = i, k x i =j? I think I get it now.