Experiment 08: Physical Pendulum. 8.01t Nov 10, 2004

Transcription

1 Experiment 08: Physical Pendulum 8.01t Nov 10, 2004

2 Goals Investigate the oscillation of a real (physical) pendulum and compare to an ideal (point mass) pendulum. Angular frequency calculation: Practice calculating moments of inertia, using them, and solving the τ = I a equation of motion.

3 Equipment setup uspend 1m ruler so it can swing over edge of table. Measure the period of oscillation with the Datatudio motion sensor. et motion sensor on narrow beam, aim it to just miss support rod and hit ruler about 25 cm away. Place a chair about cm from motion sensor to intercept ultrasound beam when ruler swings out of beam.

4 Understanding the graphs Position vs. time data from the motion sensor. What is happening: 1. Along the top plateaus marked by A? 2. At the downward peaks marked by B? How do you use this graph to find the period of oscillation of the pendulum?

5 tarting Datatudio Create a new experiment. Plug motion sensor into the 750 and drag their icons to inputs in the etup window. Double-click the Motion ensor icon, set trigger rate to 120. Plot position vs. time.

6 Ruler pendulum Click Delayed tart = None. Automatic top = 10 sec. Pull ruler aside and release it to swing at the same time you start Datatudio. Measure periods filling in the table below. Displace ment 0.10 m 0.25 m 0.50 m θ Period

7 Modified ruler pendulum Clip a 50g brass weight to the ruler at positions in table in order to change the moment of inertia. (Clip is 8.6 g.) Measure the period of oscillation filling in the table below: Displace ment Weight Position Period 0.20 m 58.6 g 0.25 m 0.20 m 58.6 g 0.50 m 0.20 m 58.6 g 0.90 m

8 Angular Momentum and Fixed Axis Rotation 8.01t Nov 10, 2004

9 Dynamics: Translational and Rotational Motion Translational Dynamics Total Force Torque Angular Momentum about Dynamics of Rotation F ext Momentum of a ystem p Dynamics of Translation dp Fext = dt Rotational Dynamics of point mass about τ = r, F L = r, m p d τ = L dt m m

10 Angular Velocity Vector and Angular Acceleration Vector for Fixed Axis Rotation Fixed axis of rotation: z-axis Angular velocity vector ω = dθ kˆ dt Angular acceleration vector α = 2 d θ kˆ 2 dt

11 Angular Momentum of a Point Particle point particle of mass m moving with a velocity v momentum p= mv Fix a point vector r m, from the point to the location of the object angular momentum about the point L = r p, m

12 Cross Product: Angular Momentum of a Point Particle Magnitude: L = r, p sin a) moment arm r = r L m θ, sinθ m = r p b) Perpendicular momentum p = p L = r sinθ p, T L = r p, m

13 Cross Product: Angular Momentum of a Point Particle Direction Right Hand Rule

14 Angular Momentum for Fixed Axis Rotation Fixed axis of rotation: z- axis Angular velocity v = r ω =rωkˆ angular momentum about the point L = r p = r v, m m z-component of the angular momentum about, ˆ ˆ 2 L ˆ = r m m v= rmvk = rmrωk = mr ωk,

15 Fixed Axis Rotation Angular Momentum about z-axis 2 Lz, = mr ω = I Rotational Dynamics τ dl ω z, dω z, = = I = I dt dt α

16 PR Question A person spins a tennis ball on a string in a horizontal circle (so that the axis of rotation is vertical). At the point indicated below, the ball is given a sharp blow in the forward direction. This causes a change in angular momentum dl in the 1. x direction 2. y direction 3. z direction

17 PR Question A dumbbell is rotating about its center as shown. Compared to the dumbbell's angular momentum about its center, its angular momentum about point B is 1. bigger. 2. the same. 3. smaller.

18 Time Derivative of Angular Momentum for a Point Particle Time derivative of the angular momentum about : dl d = ( r ) m, p dt dt Product rule dl d dr ( ) m, d = rm, p = p+ rm, p i dt dt dt dt Key Fact: = drm, d m, m m dt r dt = = v v v v 0 Result: dl d = rm, p= r i m, F= τ dt dt

19 Torque and the Time Derivative of Angular Momentum for a Point Particle Torque about a point is equal to the time derivative of the angular momentum about. τ d = L dt

20 Angular Momentum for a ystem Treat each particle separately L = r p i, m, i i Total Angular Momentum for ystem about L L r p i= N i= N =, i =, m i i i= 1 i= 1 of Particles

21 Angular Momentum and Torque for a ystem of Particles Total torque about is the time derivative of angular momentum about dl dt dl i= N i= N i= N i, = = rm, F i i= τi, = τ i= 1 dt i= 1 i= 1

22 Angular Momentum of a Rigid Body for Fixed Axis Rotation Fixed axis of rotation: z-axis angular momentum about the point L = r p = r m v i, i, i i, i i z-component of the angular momentum about, L = r m v ( ) i, Oi, i i i z r = r + r i, O, O, i i i

23 Z-component of the Angular Momentum about Mass element radius of the circle momentum m i r,i mv i i z-component of the angular momentum about ( ) L = r mv i, z, i i i Velocity v i = r ω, i ummary: ( ) = = ( ) 2 L r mv m r ω i, z, i i i i, i

24 Z-component of the Angular Momentum about um over all mass elements Continuous body Moment of Inertia Main Result ( ) ( ) ( ) 2 =, i = i, i L L m r ω z i z i ( ) ( ) 2 L = dm r ω z I body, = dm( r ) z body ( ) =, L I ω z z 2

25 Torque and Angular Momentum for Fixed Axis Rotation torque about is equal to the time derivative of the angular momentum about d = L dt resolved in the z-direction ( τ ) τ ( ) ( ) 2 d L d I ω dω d θ dt dt dt dt z z, = = = I, z = I z, z = I 2, z α

26 Conservation of Angular Momentum about a Point Rotational dynamics No external torques τ d = L dt d = = L 0 τ dt Change in Angular momentum is zero L L L = 0 ( ) ( ) 0 f Angular Momentum is conserved ( ) ( L ) = L f 0

27 PR Question A figure skater stands on one spot on the ice (assumed frictionless) and spins around with her arms extended. When she pulls in her arms, she reduces her rotational inertia and her angular speed increases so that her angular momentum is conserved. Compared to her initial rotational kinetic energy, her rotational kinetic energy after she has pulled in her arms must be 1. the same. 2. larger because she's rotating faster. 3. smaller because her rotational inertia is smaller.

28 Conservation Principles Change in mechanical energy No non-conservative work Change in momentum nc No external forces 0 W = E = K + U nc mechanical = W = E = K + U mechanical F external = = N d i p i= 1 dt 0 = x = 0 = F = d dt d dt d p dt ( ) Fexternal ( p ) ( ) external ( p ) y x y

29 PR Question A streetcar is freely coasting (no friction) around a large circular track. It is then switched to a small circular track. When coasting on the smaller circle the streetcar's 1. mechanical energy is conserved and angular momentum about the center is conserved 2. mechanical energy is not conserved and angular momentum about the center is conserved 3. mechanical energy is not conserved and angular momentum about the center is not conserved 4. mechanical energy is conserved and angular momentum about the center is not conserved.

30 Total Angular Momentum about Total for translation and rotation about point Orbital angular momentum a Fixed Point L = r m v + L spin, cm T cm cm L = r p orbital, cm pin Angular Momentum for fixed axis rotation L = I ω spin cm cm spin

31 Class Problem A meteor of mass m is approaching earth as shown on the sketch. The radius of the earth is R. The mass of the earth is me. uppose the meteor has an initial speed of ve. Assume that the meteor started very far away from the earth. uppose the meteor just grazes the earth. The initial moment arm of the meteor ( h on the sketch) is called the impact parameter. The effective scattering angle for the meteor is the area. This is the effective target size of the earth as initially seen by the meteor. π h 2

32 Class Problem a) Draw a force diagram for the forces acting on the meteor. b) Can you find a point about which the gravitational torque of the earth s force on the meteor is zero for the entire orbit of the meteor? c) What is the initial angular momentum and final angular momentum (when it just grazes the earth) of the meteor? d) Apply conservation of angular momentum to find a relationship between the meteor s final velocity and the impact parameter. e) Apply conservation of energy to find a relationship between the final velocity of the meteor and the initial velocity of the meteor. f) Use your results in parts d) and e) to calculate the impact parameter and the effective scattering cross section.