Angular Momentum(Turntable and blob collision)

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A small blob of putty with mass 7.1 kg lands on a rotating turntable with mass 7.5 kg and an initial angular speed of 1.4 rad/s. The conservation of angular momentum is applied to determine the post-collision angular speed. The initial angular momentum is calculated incorrectly using the wrong moment of inertia for the turntable. The correct moment of inertia for a disc is 1/2 MR^2, leading to a revised calculation for the final angular speed. Understanding the correct moments of inertia is crucial for solving this problem accurately.
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A small blob of putty of mass m = 7.1 falls from the ceiling and lands on the outer rim of a turntable of radius R and mass M = 7.5 kg that is rotating freely with an angular speed of 1.4 rad/s about its axis. The postcollision angular speed of the turntable-putty system is ____ rad/s.


L = r x p = Iω
this is the only equation i think i need for this problem


As far as i can tell all i would need to do is...
since R is unknown but will be a constant through out the whole problem, i give it a value of 1 for simplicity sake.
the momentum of the turntable before would be, L = (7.5 x 1^2) x 1.4
L= 10.5
since momentum as to be conserved in this problem the end momentum should also be 10.5
10.5 = [(7.5 x 1^2) + (7.1 x 1^2)] x ω
10.5 = 14.6 x ω
10.5/14.6 = ω
.719 = ω

and since this is a online assignment i can try and answer as many times until i get it correct, but i can't think of where i might be going wrong cause it says i have the wrong answer.
I don't need the problem worked out for me per say...just maybe hint at the error or explain where I am going wrong, and thank you for any help
 
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Hi MRMooneyham! Welcome to PF! :smile:

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MRMooneyham said:
the momentum of the turntable before would be, L = (7.5 x 1^2) x 1.4

nooo … wrong moment of inertia ! :redface:
 
I figured that's where I was wrong at, but I don't understand how, so another hint pot favor :)?
 
you've used MR2 for the moment of inertia of a disc about its axis

it isn't

look it up, you need to learn the common moments of inertia :smile:
 
so then it would be? I am still using the ^ symbol cause i don't understand the x^2 above the reply
L = (1/4 * 7.5 * 1^2) * 1.4
L = 2.625
so after the collosion the equation would look like
2.625 = [(1/4 * 7.5 * 1^2) + (7.1 * 1^2)] * ω
so 2.625 = 8.975 * ω
and ω = .292479?
 
ok thanks, i got confused on that :/
 

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