Does velocity from rotational Ek add to a total velocity of a body?

[rammer]

If we have a ball rolling down from inclined roof that has bottom part at certain height about the ground. I want to know where does the ball land after it leaves the roof. So at this point potential energy is transferred to kinetic (rotational + translational).

My question is: What is the expression I derive an initial velocity from? Is it EK(transl) or EK(transl)+EK(rot)?

I think it should be only EK(transl), because I'm interested only in motion of a centre of mass, right?
BUT, exercises in my book always counts with EK(rot), so I'm pretty confused.

Can you please make it little more clear to me??

 

[Doc Al]

To find the final velocity of the ball as it leaves the roof you must use conservation of energy. That includes all forms of mechanical energy, which in this case is both rotational and translational KE.

Even though all you care about is the translational speed of the ball, in order to correctly find that speed you must use conservation of energy properly.

 

[rammer]

thank you