Angular Momentum(Turntable and blob collision)

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Homework Help Overview

The problem involves a collision between a blob of putty and a rotating turntable, focusing on the conservation of angular momentum. The subject area is angular momentum and rotational dynamics.

Discussion Character

  • Conceptual clarification, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to apply the conservation of angular momentum using a specific equation but questions the correctness of their moment of inertia calculation. Participants discuss the correct moment of inertia for a disc and provide hints for clarification.

Discussion Status

Participants are actively engaging in clarifying the moment of inertia and its application in the problem. Some guidance has been offered regarding the correct formula for the moment of inertia of a disc, but there is no explicit consensus on the final answer yet.

Contextual Notes

There is a mention of the original poster's confusion regarding the notation used in the equations and the implications of using a simplified radius for calculations. The discussion also reflects the constraints of an online assignment format, where multiple attempts are allowed.

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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:

(try using the X2 button just above the Reply box :wink:)
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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