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      09-25-2008, 02:41 AM   #39
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Drives: E92 M3
Join Date: Sep 2006
Location: San Diego, CA USA

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Originally Posted by AZE92M3 View Post
A couple people on here have mentioned density. Can someone explain to me why density matters and not just mass? It doesn't matter how dense something is. For this example it only matters what it's mass is. Isn't it like that old kids game....Which is heavier a 1lb of bricks or a 1lb of feathers? At the end of the day it's the mass that affects the rotation right? Sorry if that is a stupid question. I'm obviously not an engineer.

Taking the "stick" example someone else mentioned I have another question. If 2 sticks are 13.365 inches long and 1 stick has an extra 2 lbs attached to the last 3 inches wouldn't that stick have greater "I", or inertia and be more difficult to rotate and/or stop rotation? Please don't answer with sidewall stiffness, lap times, predictable behavior of at the limit adhesion, etc. I understand all that, don't necessarily disagree, but quite frankly don't care.

This is a street setup on a car that is not my daily driver. I drive my Escalade during the week and race a 125cc Tony Kart on weekends. Other than practicing my heel-toe downshifting I don't race my M3 on the streets. My stock wheels/tires will be used for track duty. I'm merely just trying to satisfy a curiousity. Thanks.
Sure, I'll help. Mass and distance from the center of rotation are really the only things that matter, for both acceleration and angular acceleration. However, if in a given structure you replaced an equivalent volume of material A with a more dense volume of material B you would be altering both the mass and moment of inertia. It also helps when trying to do a mental estimation of the relative I between two structures, if you imagine sort of morphing one structure into another you can imagine the transformation being possible by replacing a less dense portion of it (say a section of tire) with a more dense (say aluminum) you can guesstimate the change in I. Again mass affects the resistance to linear (or rectilinear) acceleration just as moment of inertia resists spin up (angular acceleration).

You are right on with the stick example. In your example though one had a greater m and greater I. Here is how to take the understanding one step further. If you placed the extra 2 lb right at the center of the stick, call this condition A, and the extra 2 lb at the end of another identical stick, call this condition B. Both A and B would accelerate equally if you placed the same force on them (in such a way that you did not cause them to spin!). However, one will take more more torque to obtain a given angular acceleration.