Determine Velocity for Physics Homework Question

[dvep]

Homework Statement

I am doing a past paper question, it is shown in the attached JPEG.

I have done part A and got 2.74 rads/s.

and in my notes ( as we done the solution in class) it says the elastic potetntial energy is 56.27 J, then it goes on to say that when the bar hits horizontal, it rotates about A so,

KE = 0.5*IAw^2

so,

KE = 0.5*((1/3)*20*(1.2^2))*(Vb/0.6)^2 = 13.33Vb^2

why is w=Vb/0.6? wouldn't the kinetic energy transform into all elastic potential energy when the bar is horizontal?


Thanks!

 

[Redbelly98]

why is w=Vb/0.6? wouldn't the kinetic energy transform into all elastic potential energy when the bar is horizontal?

The bar is not necessarily motionless at the point where it hits the bottom. In fact, you could imagine that if the vertical channel were to continue on downward past this point, the bar could continue falling. The left end of the bar is momentarily at rest at this point, but its center is still moving.