General Physics I. Lecture 10: Rolling Motion and Angular Momentum. Prof. WAN, Xin ( 万歆 )

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1 General Physics I Lecture 10: Rolling Motion and Angular Momentum Prof. WAN, Xin ( 万歆 ) xinwan@zju.edu.cn

2 Outline Rolling motion of a rigid object: center-of-mass motion + rotation around Generic rotational motion in the vector language Kinematics/Energy/Torque Angular momentum of A particle A rotating rigid object Conservation of angular momentum

3 A Cycloid One light source at the center of a rolling cylinder and another at one point on the rim illustrate the different paths these two points take. The center moves in a straight line (green line), whereas the point on the rim moves in the path called a cycloid (red curve).

4 Pure Rolling Motion condition for pure rolling motion Frictional force?

5 Same Angular Speed At any instant, the part of the rim that is at point P is at rest relative to the surface because slipping does not occur. In other words, all points rotate about P.

6 Translation + Rotation

7 Parallel-Axis Theorem

8 Parallel-Axis Theorem Suppose the moment of inertia about an axis through the center of mass of an object is I CM. The parallel-axis theorem states that the moment of inertia about any axis parallel to and a distance D away from this axis is See Resnick/Halliday/Krane, 5th ed., p. 182 for a proof.

9 Kinetic Energy The total kinetic energy of the rolling cylinder rotational kinetic energy about the center of mass translational kinetic energy of the center of mass

10 Example: Rolling Sphere For the solid sphere shown below, calculate the linear speed of the center of mass at the bottom of the incline and the magnitude of the linear acceleration of the center of mass. Accelerated rolling motion is possible only if a frictional force is present between the sphere and the incline to produce a net torque about the center of mass. Despite the presence of friction, no loss of mechanical energy occurs because the contact point is at rest relative to the surface at any instant.

11 Moment of Inertia Now, try it yourself.

12 Appendix: Spherical Shell First we calculate the moment of inertia of a spherical shell about a diagonal axis. I z = dm(x 2 + y 2 ) By the rotational symmetry, I x =I y =I z = 1 3 (I x + I y + I z ) = 2 3 dm(x 2 + y 2 +z 2 )= 2 3 dm R 2 = 2 3 M R2

13 Appendix: Solid Sphere By dimension analysis, the moment of inertia of a solid sphere (density r) is I (R) = c M R 2 cρ 4π 3 R5 Since a spherical shell can be written as the difference of two solid spheres I (R+Δ R) I ( R) = 2 3 (ρ4 π R2 Δ R) R 2 Δ R 0 di = 8π ρ 3 R4 dr I = 8 πρ 15 R5 = 2 5 MR2

14 Conservation of Energy Kinematic equation

15 Comments All homogeneous solid spheres experience the same speed and acceleration on a given incline, independent of mass radius of the sphere Try hollow sphere, solid cylinder, or hoop, you will find only the dimensionless factor would differ. The factors depend only on the moment of inertia about the center of mass for the specific body. In all cases, the acceleration of the center of mass is less than the value the acceleration would have if the incline were frictionless and no rolling occurred.

16 Still Prefer the Newton's Laws? The frictional force does not change the total kinetic energy, but it contributes to F and decreases the acceleration of the center of mass.

17 Right-Hand Rule For rotation about a fixed axis, the only direction that uniquely specifies the rotational motion is the direction along the axis of rotation. Therefore, the directions of w and a are along this axis. right-hand rule

18 Angular vs Linear Speed when a rigid object rotates about a fixed axis, every particle of the object moves in a circle whose center is the axis of rotation. v = ω r

19 Angular vs Linear Acceleration when a rigid object rotates about a fixed axis, every particle of the object moves in a circle whose center is the axis of rotation.

20 Acceleration: Vector Version For rotation of a rigid object about a fixed axis v = d r dt a = d v dt = ω r = d dt ( ω r ) = d ω dt r + ω d r dt The vector formulas have no requirement that r is perpendicular to the axis of rotation. = α r + ω ( ω r )

21 Rotational Kinetic Energy The total rotational kinetic energy of the rotating rigid object is the sum of the kinetic energies of the individual particles

22 Rotational Kinetic Energy K = 1 2 i m i ω r i 2 = 1 2 i m i( ω r i ) ( ω r i ) r = r ( r ω) ω ω = 1 2 i m i [ω 2 r i 2 ω 2 ( ω r i ) 2 ] r = 1 2 ( 2 i m i r i, )ω 2 = 1 2 I ω2 Identity: ( a b) ( c d)=( a c)( b d) ( a d)( b c)

23 Parallel-Axis Theorem

24 Parallel-Axis Theorem Define r '= r r CM K = 1 2 i m i( ω r i ) ( ω r i ) = 1 2 i m i[ ω ( r i ' + r CM )] [ ω ( r i ' + r CM )] = 1 2 i m i[ ω r i '] [ ω r i '] + i m i [ ω r i '] [ ω r CM ] i m i[ ω r CM ] [ ω r CM ] (1) (2) (3)

25 Parallel-Axis Theorem Term (1): The rotational energy about the parallel axis through the center of mass K ' = 1 2 i m i[ ω r i '] [ ω r i '] Term (2): i m i [ ω r i '] [ ω r CM ]=0 i m i r i '= i m i r i ( i m i ) r CM =0 Term (3): as if the object shrink to the center of mass K CM = 1 2 ( i m i)[ ω r CM ] [ ω r CM ]

26 Torque The torque t involves the two vectors r and F, and its direction is perpendicular to the plane of r and F. We can establish a mathematical relationship between t, r, and F, using a new mathematical operation called the vector product, or cross product Will discuss more next week.

27 Angular Momentum: Motivation Newton's second law v (m v) = 0 (rotational analog) Angular momentum

28 Angular Momentum: Definition The instantaneous angular momentum L of the particle relative to the origin O is defined as the cross product of the particle s instantaneous position vector r and its instantaneous linear momentum p. Magnitude: SI Unit: L is zero when r is parallel or antiparallel to p.

29 Dependence of the Origin The net torque acting on a particle is equal to the time rate of change of the particle s angular momentum. Both the magnitude and the direction of L depend on the choice of origin. The expression is valid for any origin fixed in an inertial frame, as long as L and t are measured about the same origin.

30 A System of Particles The time rate of change of the total angular momentum of a system about some origin in an inertial frame equals the net external torque acting on the system about that origin. The moment arm d from O to the line of action of the internal forces is equal for both particles. Recall

31 A Rotating Rigid Object

32 The Scalar Version Now extend to a rigid object of arbitrary shape rotating about a fixed axis. I: The moment of inertia

33 Exercise A rigid rod of mass M and length l is pivoted without friction at its center. Two particles of masses m 1 and m 2 are connected to its ends. The combination rotates in a vertical plane with an angular speed w. (a) Find an expression for the magnitude of the angular momentum of the system. (b) Find an expression for the magnitude of the angular acceleration of the system when the rod makes an angle with the horizontal.

34 Solution

35 Conservation of Angular Momentum The total angular momentum of a system is constant in both magnitude and direction if the resultant external torque acting on the system is zero. If the system is an object rotating about a fixed axis, such as the z axis,

36 Figure Skating Anything wrong?

37 Kepler s Second Law When a force is directed toward or away from a fixed point and is function of r only, it is called a central force.

38 Kepler s Second Law The radius vector from the Sun to a planet sweeps out equal areas in equal time intervals.

39 Isolated Systems The (mechanical) energy, linear momentum, and angular momentum of an isolated system all remain constant.

40 Example: Ball and Stick

Chapter 10 Rotational Motion. Copyright 2009 Pearson Education, Inc.

Chapter 10 Rotational Motion. Copyright 2009 Pearson Education, Inc. Chapter 10 Rotational Motion Angular Quantities Units of Chapter 10 Vector Nature of Angular Quantities Constant Angular Acceleration Torque Rotational Dynamics; Torque and Rotational Inertia Solving Problems