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Welcome back to Physics 215 Today’s agenda: Moment of Inertia Rotational Dynamics Angular Momentum

Current homework assignment • HW9: • Knight Textbook Ch.12: 70, 74, 82 • 2 Exam-style problems from course website • due Monday, Nov. 12th in Don’s mailbox

Exam 3: Tuesday (11/13/12) • Material covered: • Textbook chapters 9-12 • Lectures up to and including 11/8 (slides online) • Workshop Activities • Homework assignments • As with Exams 1 & 2, Exam 3 is closed book, but you may bring calculator and onehandwritten 8.5” x 11” sheet of notes • Practice versions of Exam 3 posted online

Extended objectsneed extended free-body diagrams • Point free-body diagrams allow finding net force since points of application do not matter. • Extended free-body diagrams show point of application for each force and allow finding net torque.

Discussion Dw/Dt)Smiri2 = tnet a - angular acceleration Moment of inertia, I Ia = tnet compare this with Newton’s 2nd law Ma = F

Moment of Inertia * I must be defined with respect to a particular axis

Dm r Moment of Inertia of Continuous Body Dma0

Tabulated Results for Moments of Inertia of some rigid, uniform objects (from p.299 of University Physics, Young & Freedman)

D D Parallel-Axis Theorem CM *Smallest I will always be along axis passing through CM

Practical Comments on Calculation of Moment of Inertia for Complex Object • To find I for a complex object, split it into simple geometrical shapes that can be found in Table 9.2 • Use Table 9.2 to get ICMfor each part about the axis parallel to the axis of rotation and going through the center-of-mass • If needed use parallel-axis theorem to get I for each part about the axis of rotation • Add up moments of inertia of all parts

Beam resting on pivot CM of beam N r r rm x m M = ? Mb = 2m SF = Vertical equilibrium? S = Rotational equilibrium? M = N =

Suppose M replaced by M/2 ? SF = • vertical equilibrium? • rotational dynamics? • net torque? • which way rotates? • initial angular acceleration? S =

Moment of Inertia? I = Smiri2 * depends on pivot position! I = * Hencea=t=

Rotational Kinetic Energy • K = Si(1/2mivi2 = (1/2)w2Simiri2 • Hence • K = (1/2)Iw2 • This is the energy that a rigid body possesses by virtue of rotation

Spinning a cylinder Cable wrapped around cylinder. Pull off with constant force F. Suppose unwind a distance d of cable 2R F • What is final angular speed of cylinder? • Use work-KE theorem • W = Fd = Kf = (1/2)I2 • Mom. of inertia of cyl.? -- from table: (1/2)mR2 • from table: (1/2)mR2 •  = [2Fd/(mR2/2)]1/2 = [4Fd/(mR2)]1/2

cylinder+cable problem -- constant acceleration method extended free body diagram N F * no torque due to N or FW * why direction of N ? * torque due to t= FR * hence a= FR/[(1/2)MR2] = 2F/(MR)  wt = [4Fd/(MR2)] 1/2 FW radius R  = (1/2)t2 = d/R; t = [(MR/F)(d/R)]1/2

Angular Momentum • can define rotational analog of linear momentum called angular momentum • in absence of external torque it will be conserved in time • True even in situations where Newton’s laws fail ….

Definition of Angular Momentum * Back to slide on rotational dynamics: miri2Dw/Dt = ti * Rewrite, using li = miri2w Dli/Dt = ti * Summing over all particles in body: DL/Dt = text L = angular momentum = Iw w ri Fi pivot mi

An ice skater spins about a vertical axis through her body with her arms held out. As she draws her arms in, her angular velocity • 1. increases • 2. decreases • 3. remains the same • 4. need more information

Angular Momentum 1. q r p O Point particle: |L| = |r||p|sin(q) = m|r||v| sin(q) vector form  L = r x p – direction of L given by right hand rule (into paper here) L = mvr if v is at 900 to r for single particle

Angular Momentum 2. w o rigid body: * |L| = Iw(fixed axis of rotation) * direction – along axis – into paper here

Rotational Dynamics • t=Ia DL/Dt = t • These are equivalent statements • If no net external torque: t=0  • * L is constant in time • * Conservation of Angular Momentum • * Internal forces/torques do not contribute • to external torque.

Bicycle wheel demo • Spin wheel, then step onto platform • Apply force to tilt axle of wheel

Force Acceleration Momentum Kinetic energy Torque Angular acceleration Angular momentum Kinetic energy Linear and rotational motion

A hammer is held horizontally and then released. Which way will it fall?

General motion of extended objects • Net force  acceleration of CM • Net torque about CM  angular acceleration (rotation) about CM • Resultant motion is superposition of these two motions • Total kinetic energy K = KCM + Krot

Three identical rectangular blocks are at rest on a flat, frictionless table. The same force is exerted on each of the three blocks for a very short time interval. The force is exerted at a different point on each block, as shown. After the force has stopped acting on each block, which block will spin the fastest? 1. A. 2. B. 3. C. 4. A and C. Top-view diagram

Three identical rectangular blocks are at rest on a flat, frictionless table. The same force is exerted on each of the three blocks for a very short time interval. The force is exerted at a different point on each block, as shown. After each force has stopped acting, which block’s center of mass will have the greatest speed? 1. A. 2. B. 3. C. 4. A, B, and C have the same C.O.M. speed. Top-view diagram

Reading assignment • Rotational dynamics, rolling • Remainder of Ch. 12 in textbook

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