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Ch 13: Rotational Inertia & EnergyWorksheetSee all chapters
All Chapters
Ch 01: Intro to Physics; Units
Ch 02: 1D Motion / Kinematics
Ch 03: Vectors
Ch 04: 2D Kinematics
Ch 05: Projectile Motion
Ch 06: Intro to Forces (Dynamics)
Ch 07: Friction, Inclines, Systems
Ch 08: Centripetal Forces & Gravitation
Ch 09: Work & Energy
Ch 10: Conservation of Energy
Ch 11: Momentum & Impulse
Ch 12: Rotational Kinematics
Ch 13: Rotational Inertia & Energy
Ch 14: Torque & Rotational Dynamics
Ch 15: Rotational Equilibrium
Ch 16: Angular Momentum
Ch 17: Periodic Motion
Ch 19: Waves & Sound
Ch 20: Fluid Mechanics
Ch 21: Heat and Temperature
Ch 22: Kinetic Theory of Ideal Gases
Ch 23: The First Law of Thermodynamics
Ch 24: The Second Law of Thermodynamics
Ch 25: Electric Force & Field; Gauss' Law
Ch 26: Electric Potential
Ch 27: Capacitors & Dielectrics
Ch 28: Resistors & DC Circuits
Ch 29: Magnetic Fields and Forces
Ch 30: Sources of Magnetic Field
Ch 31: Induction and Inductance
Ch 32: Alternating Current
Ch 33: Electromagnetic Waves
Ch 34: Geometric Optics
Ch 35: Wave Optics
Ch 37: Special Relativity
Ch 38: Particle-Wave Duality
Ch 39: Atomic Structure
Ch 40: Nuclear Physics
Ch 41: Quantum Mechanics

Concept #1: Moment of Inertia & Mass Distribution


Hey guys. So, in this quick video I'm going to show you how the moment of inertia of a system has to do with how the mass is distributed, how the masses are spread out around the axis of rotation, let's check it out, so it says, here, the moment of inertia I has to do with how mass is distributed, how its spread out around an axis of rotation. So, here we have a solid disk that has small masses, so this is the disk and the masses are the black dots, the four black dots, and they're arranged in three not two, three different ways and I want to know in which of these will the moment of inertia be greater, in which of these will the moment of inertia should be greater. Now this is a composite system with a bunch of different masses, so the total moment of inertia of this system would be the moment of inertia of the solid disk, which is a solid cylinder plus the moment of inertia of the four masses, okay? So, something like i1 plus i2 plus i3 plus i4. Now, these three situations have the same disk with the same mass with the same radius. So, for all of them this is going to be the same, the only thing that will change is this, so the difference will be in how the tiny masses are arranged around the disk. Now, if these are point masses, which they should be treated as point masses because it says your small mass, the equation for them is m, r squared. So, you have a bunch of m, r squares, right? m, r squared, m, r squared for times. Now, if you have the same four masses everywhere, these m's will also be the same. So, it's going to come down to the r's for each mass, in other words, how far from the axis of rotation they are, okay? So, basically the farther the masses are the greater their individual moments of inertia will be and the greater the total of inertia of the system will be. So, this one has to be the one with the greatest I, okay? So, I'm going to call this A, B and C and B is the greater one. Now, and that's because the masses are farther out from the center, C is the smallest, the lowest value of I because the masses are congregated in the middle, here you can see four masses really close to the center. Here you'll see for masses really far from the center and this guy's somewhere in the middle, two are far and two are close. So, I'm going to say that the moment of inertia B is the greater and the moment of inertia of C is the smallest, okay? So, greatest smallest and A is in the middle, this means that you can think of B as being the heaviest of the three, okay? Even, if the masses are the same it's got the most inertia, another way that this question could be asked is, you know, if you apply the same force to it, who's going to rotate faster, right? Well, this guy's the heaviest so, it's also going to be the slowest, okay? Alright, so that's it for this one, let's keep going.

Practice: The objects below all have the same mass and radius. Mass is distributed evenly in all objects. Rank the objects according to the Moment of Inertia they each have about a central axis perpendicular to them, highest to lowest. (From left to right, the objects are A, B, C, and D.)