AP Physics - Memorize rotational inertias?

Test Preparation AP Test Preparation
1

<p>Hey there,</p>

<p>Just wondering:</p>

<p>We don't have to memorize these things do we? List</a> of moments of inertia - Wikipedia, the free encyclopedia</p>

<p>I think I've seen on the majority of the FRQs that we are given them.</p>

2

<p>My teacher told us not to memorize them but to put them in our calculator just in case.</p>

3

<p>I would memorize them just in case. Usually for rods rotating at the end, they tell you I = 1/3ML^2. From that you can derive the inertia of a rod rotating in the middle (I = 1/12ML^2) from the parallel axis theorem (I = Icm + MR^2). But some basic ones are obviously point masses (MR^2) and disks (1/2MR^2).</p>

4

<p>just derive them by integrating r^2dm</p>

5

<p>^Easier said than done. I can’t derive any of them. Lol</p>

6

<p>then just memorize them. but just know that dm = density * length</p>

<p>so if you had like a block, you would take a small sliver dm, where dm = M/L (dr)</p>

<p>then you integrate r^2(M/L)(dr) from whatever you wanted.</p>

7

<p>Memorizing them isn’t tough. For rods and flat surfaces, it’s just ML^2 (replace L with a for the surfaces), then add either 1/12 or 1/3 to the front of that if it’s in the middle or on the end, respectively.</p>

<p>For cylinders and spheres, it’s MR^2, then add 1/2 for a solid cylinder, 2/5 for a solid sphere, and 2/3 for a hollow sphere.</p>

8

<p>I’d memorize some of them (rods through CM/end, ring, solid disk, solid sphere, hollow sphere).</p>

9

<p>whats the point of putting it on a calculator when the probability for the problem being in free response is relatively low combined with the fact that you have to show your work</p>

10

<p>Uh oh. This isn’t a good sign.</p>

<p>The hell’s a moment of inertia?</p>

11

<p>^this = gg.</p>

<p>I(a/r) = torque</p>

<p>where torque = rF</p>

<p>thats all you need.</p>

12

<p>A moment of inertia is the same thing as Rotational Inertia. Its symbol is I.</p>

13

<p>Just memorize the big 6 if derivations aren’t your thing.</p>

<p>hoop/ring MR^2
disk 1/2MR^2
rod (at end) 1/3ML^2
rod (at middle) 1/12ML^2
sphere (solid) 2/5MR^2
sphere (hollow) 2/3MR^2</p>

<p>There’re some interesting mnemonics that come with some of these…</p>

14

<p>^^I love mnemonics! Let’s hear em’.</p>

15

<p>well as im sure were all aware of now it would have been very helpful to have had these memorized lol</p>