# CHECKLIST • Be able to define angular

l_10_angular_momentum_and_moment_of_inertia.ppt

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CHECKLIST • Be able to define angular momentum in its 3 D and 2 D form • Be able to define the moment of inertia of a rigid body or system of point particles • Understand the link between angular momentum, moment of inertia and angular speed • Be able to define the work done by a torque moving through an angle and the power delivered by this torque • Know an equation for the kinetic energy of a rotating body moving with angular velocity, • Have a knowledge of the correspondence between linear equations of motion and angular equations of motion • Be able to perform calculations using any of the equations you have met

Angular momentum. Moment of inertia and more properties of angular motion. Recall: Vector product, e. g. , We can also define angular momentum: ˆ sin ˆ where the direction of is perpendicular to the plane of and r F L r p n n r p r r r

Moment of inertia, I For linear momentum, p = mv Similarly, for angular momentum, L = “something” x ω That is, we seek a property of the body that measures ‘angular inertia’. This is defined as the moment of inertia , I. Thus: L = I ω

One way to get a formula for I Consider a body rotating with angular speed ω. Then consider the body is divided into many small masses, m 1 , m 2 , etc at distances r 1 , r 2 , etc, respectively, from a pivot.

Moment of inertia 2 2 2 2 TOT Now, the KE of each point mass is 1 where is mass of ith particle 2 But and _________ 1 1 2 2 Total KE of body KE 1 1 KE 2 2 i i i N N i i i KE m v r KE m r m r is the angular speed

Moment of inertia 2 2 If we define I = then we have an expression for the KE due to angular mot ion 1 2 Note: I measures the ‘distribution of ma ss about the axis of rotation’. N i i i ang m r KE I

More on I Note: we know that 1) if no external forces act on the system, linear momentum is conserved. We now know that 2) if no external torques act on the system, angular momentum is conserved. This explains why, e. g. , an ice skater rotates faster when she pulls in her arms.

Conservation of angular momentum Since L = I ω, if no external torques act, L is constant. So, when the skater moves her arms in, closer to her body, I decreases and therefore ω increases.

Conservation of angular momentum In (1) I = m 1 a 2 +m 2 a 2 In (2) I = m 1 b 2 + m 2 b 2 For L = I ω to remain constant, the skater rotates faster in (2).

Please don’t try this! A crazy physics professor stands on a swivel chair with weights in his outstretched arms. A student causes him to rotate. The professor brings in his arms and…

Comparison of equations for linear and angular motion Linear Angular displacement velocity acceleration momentum force r dr v dt dv a dt p mv r r r r dp F ma dt r rr

Example 1. Conservation of angular momentum A horizontal disc rotates around a vertical axis at 90 rev/min. A small piece of chewing gum of 20 g falls vertically onto the disc and sticks to it at a distance of 5. 0 cm from the axis. If the number of revs/min is reduced to 80, find the moment of inertia, I, of the disc.

Work done by torque Note: We can also define work done by a constant torque in moving through an angle θ as: W = The power dtd. W P

Example 2 An electric motor supplies a power of 500 W to drive a flywheel of I = 2. 0 kgm 2 at a speed of 600 rev/min. How long will it take the flywheel to come to rest after the power is switched off, assuming the retarding torque, due to friction, is constant?

CHECKLIST READING Adams and Allday: 3. 30, 3. 31 At the end of this lecture you should • Be able to define angular momentum in its 3 D and 2 D form • Be able to define the moment of inertia of a rigid body or system of point particles • Understand the link between angular momentum, moment of inertia and angular speed • Be able to define the work done by a torque moving through an angle and the power delivered by this torque • Know an equation for the kinetic energy of a rotating body moving with angular velocity, • Have a knowledge of the correspondence between linear equations of motion and angular equations of motion • Be able to perform calculations using any of the equations you have met