Angular Momentum/Impulse Problem

[cdbowman42]

1. A 1.8 kg, 20 cm diameter turntable rotates at 160 rpm on frictionless bearings. Two 500 g blocks fall from above, hit the turntable simultaneously at opposite ends of a diagonal, and stick. What is the turntable's angular velocity, in rpm, just after this event?


2. Angular momentum(L)=angular velocity(w)*moment of inertia(I)


3. I'm confused as to whether or not to appraoch this as a conservation of momentum problem or some other way. I think I would need to know the velocities of the blocks before they hit to to this.

 

[olivermsun]

Do the blocks carry with them any angular momentum of their own? (It seems the problem is set up to hint that they do not).

 

[cdbowman42]

Thanks! that's all I needed!

 

[tiny-tim]

hi cdbowman42!

I think I would need to know the velocities of the blocks before they hit to to this.

hint: what was their angular momentum about the axis, before they hit?

(assuming they fell vertically at speed v)

 

[AlexChandler]

I would think about it in the following way

\int \tau dt = \Delta ( I \omega)

where \int \tau dt is the angular impulse and \Delta (I \omega) is the change in angular momentum.

We can see that no vertical torque is exerted on the turntable, so the angular momentum must remain constant.

Then we must have

(I \omega)_{initial} = (I \omega)_{final}