Mass Moment of Inertia, IG

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Mass Moment of Inertia, IG IG is the “mass moment of inertia” for a body about an axis passing through the body’s mass center, G. 2 2 2 IG is defined as: IG = ∫ r dm Units: kg-m or slug-ft IG is used for several kinds of rigid body rotation problems, including: (a) F=ma analysis moment equation ( ΣMG = IGα ). 2 (b) Rotational kinetic energy ( T = ½ IGω ) (c) Angular momentum ( HG = IGω ) IG is the resistance of the body to angular acceleration. That is, for a given net moment or torque on a body, the larger a body’s IG, the lower will be its angular acceleration, α. IG also affects a body’s angular momentum, and how a body stores kinetic energy in rotation. Mass Moment of Inertia, IG (cont’d) IG for a body depends on the body’s mass and the location of the mass. The greater the distance the mass is from the axis of rotation, the larger IG will be. For example, flywheels have a heavy outer flange that locates as much mass as possible at a greater distance from the hub. If I is needed about an axis other than G, it may be calculated from the “parallel axis theorem.” Parallel Axis Theorem (PAT) for I about axes other than G. Parallel Axis Theorem If you know I about the G axis, and need P G G IP about another axis (parallel to the G axis) y use the “parallel axis theorem.” d IG = I about center of mass, G P G x IP = I about an axis passing through P (parallel to the G axis) md2 = “transfer term”; m = mass of body, I = I + md2 PG d = distance between axes Important: This equation cannot be used between any two parallel axes. One axis must be G, about the center of mass. IG’s for Common Shapes Thin y Ring: Thin y Sphere: Disk: y x x x R G R G G R 2 IG = mR 2 I = 1 mR2 I = mR2 G G 5 (All mass is at 2 the same radius, R) Slender Rod: y Rectangular y Plate: L a x x L P 2 G G b I = 1 mL2 I = 1 mL2 P 3 G 12 I = 1 m(a22 + b ) About P About G G 12 (end of rod) (center of rod) Radius of Gyration, kG, for Complex Shapes Some problems with a fairly complex shape, such as a drum or multi-flanged pulley, will give the body’s mass m and a radius of gyration, kG, that you use to calculate IG. 2 If given these, calculate IG from: IG = mkG As illustrated below, using kG in this way is effectively modeling the complex shape as a thin ring. Radius of Gyration, kG Complex Thin Ring, Shape: kG Model: = kG G G R 2 IG = mkG Some problems involving a complex shape with mass, m, and an outer radius, R, will give a “radius of gyration”, kGG , that can be used to determine I for that shape. The 2 equation, IGG = mk , indicates that the complex shape is being modeled dynamically by a thin ring with mass, m, and a radius, kG..

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