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A question for Math/Physics/Engineering gurus

R

rootaxs

Platinum Member
I tried reading up on Google but eventually got lost in all of it.

If i wanted to spin a 15lbs., 10" disc around a central pivot point. How would i:

a) Determine the amount of force needed to start spinning it
b) Determine the weight distribution of the disc
c) Calculate for the change in force that may be required should i shift the weight distribution of the disc

By weight distribution, i'm referring to how much certain sections of the disc weight. (e.g. areas closer to the center vs. areas closest to the outer edge of the disc).

Thanks! 🙂
 
i think the amount of force you should use angular momentum and energy equations

for weight distribution, i think you need to find the center mass

then to calculate for change in force, you gotta change the center of mass..

it's really been quite long since i've had to do these problems.. good luck
 
Your going to want to read up on the moment of inertia, a triple intigral if you want to do it right (You disk is not infinitly flat, or a perfect cylinder). Wiki has a lot about it and it is quite useful, I would look up the equations for you but I'm lazy

 
One other thing, Torque is basicly force (isn't it F * radius?) So look up some torque equations and youll be good to go.
 
a) Assuming no friction, the force to start spinning it would be anything > 0N
b) Chop it up and measure it on a scale.
c) The force to start spinning it would still be anything > 0N (assuming no friction)

Now..... let me know if your question changes to:

If i wanted to spin a 15lbs., 10" disc around a central pivot point. How would i:

a) Calculate the force that I need to exert on the outside rim of the disc to obtain a certain angular acceleration
b) Calculate the new moment of inertia given a change in weight distribution (some metal plates added here and holes cut out there)
c) Calculate the new force that I need to exert on the outside of the rim of the disc to obtain the same angular acceleration.

Then the answers are going to be drastically different. 🙂
 
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