1. Physics 2211: Lecture 38 Rolling Motion
Mass rolling down incline plane
Energy solution
Newton’s 2nd Law solution
Angular Momentum
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

2. Translational & rotational motion combined
For a solid object which rotates about its center or mass and whose CM is moving:
VCM 
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

3. Rolling Motion Rolling without slipping R vcm R But
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

4. Rolling Motion
Objects of different I rolling down an inclined plane: find their speed at the bottom of the plane and their acceleration on the plane. h R
v = R M
v = 0
= 0
K = 0
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

5. Rolling Motion Use v = R and I = cMR2
1 hoop
1/2 disk
2/5 sphere
c =
The rolling speed is always lower than in the case of simple sliding
since the kinetic energy is shared between CM motion and rotation.
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

6. Rolling Motion R M h v 1 hoop 1/2 disk 2/5 sphere c =
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

7. Rolling Motion (alternate approach)
An object with mass M, radius R, and moment of inertia I rolls without slipping down a plane inclined at an angle  with respect to horizontal. What is its acceleration?
Consider CM motion and rotation about the CM separately when solving this problem (like we did with the last problem)... I M R 
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

8. Rolling Motion (alternate approach)
Static friction f causes rolling. It is an unknown, so we must solve for it.
First consider the free body diagram of the object and use FNET = MaCM . In the x direction R M  y x
Now consider rotation about the CM and use  = I realizing that
 = Rf and a = R
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

9. Rolling Motion (alternate approach)
We have two equations:
We can combine these to eliminate f:  a R I M
For a sphere, c = :
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

10. Example Rotations
Two uniform cylinders are machined out of solid aluminum. One has twice the radius of the other.
If both are placed at the top of the same ramp and released, which is moving faster at the bottom?
(1) bigger one
(2) smaller one
(3) same
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

11. Example Solution Answer: (3) same speed does not depend on size,
1 hoop
1/2 disk
2/5 sphere
c =
Answer:
(3) same
speed does not depend on size,
as long as the shape is the same!! h
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

12. Direction of Rotational Motion
In general, the rotation variables (w, a, t , etc.) are vectors (have direction)
If the plane of rotation is in the x-y plane, then the convention is x y z
CCW rotation is in the + z direction x y z
CW rotation is in the - z direction
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

13. Direction of Rotation: The Right Hand Rulex y z
To figure out in which direction the rotation vector points, curl the fingers of your right hand the same way the object turns, and your thumb will point in the direction of the rotation vector!
We normally pick the z-axis to be the rotation axis as shown.
= z
= z
= z
For simplicity we omit the subscripts unless explicitly needed. x y z
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

14. Angular momentum of a rigid body about a fixed axis:
Consider a rigid distribution of point particles rotating in the x-y plane around the z axis, as shown below. The total angular momentum around the origin is the sum of the angular momenta of each particle:
(since and are perpendicular) i j m2 m1 m3 
We see that is in the z direction.
Using vi = ri  , we get
Analog of !
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

15. Angular momentum of a rigid body about a fixed axis:
In general, for an object rotating about a fixed (z) axis we can write LZ = I 
The direction of LZ is given by the right hand rule (same as ).
We will omit the Z subscript for simplicity, and write L = I   z
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

16. Conservation of Angular Momentum
where
In the absence of external torques
Total angular momentum is conserved
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

17. Example: Two Disks i f
A disk of mass M and radius R rotates around the z axis with angular velocity i. A second identical disk, initially not rotating, is dropped on top of the first. There is friction between the disks, and eventually they rotate together with angular velocity f. z f z i
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

18. Example: Two Disks
First realize that there are no external torques acting on the two-disk system.
Angular momentum will be conserved!
Initially, the total angular momentum is due only to the disk on the bottom: i z 2 1
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

19. Example: Two Disks
First realize that there are no external torques acting on the two-disk system.
Angular momentum will be conserved!
Finally, the total angular momentum is due to both disks spinning: f z 2 1 or
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

20. Example: Two Disks Since Li = Lf
An inelastic collision, since K is not conserved (friction)! z f Li Lf
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

21. Example: Rotating Table
A student sits on a rotating stool with his arms extended and a weight in each hand. The total moment of inertia is Ii, and he is rotating with angular speed i. He then pulls his hands in toward his body so that the moment of inertia reduces to If. What is his final angular speed f? i f Ii If
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

22. Example: Rotating Table
Again, there are no external torques acting on the student-stool system, so angular momentum will be conserved.
Initially: Li = Iii
Finally: Lf = If f i f Ii If Li Lf
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

23. Example Angular Momentum
A student sits on a freely turning stool and rotates with constant angular velocity w1. She pulls her arms in, and due to angular momentum conservation her angular velocity increases to w2. In doing this her kinetic energy:
(1) increases (2) decreases (3) stays the same w1 w2 I1 I2 L L
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

24. Example Solution (using L = I) L is conserved and I2 < I1
K2 > K K increases! w1 w2 I2 I1 L
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

25. Example Solution
Since the student has to force her arms to move toward her body, she must be doing positive work!
The work/kinetic energy theorem states that this will increase the kinetic energy of the system! w1 w2 I1 I2 L L
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

26. Angular Momentum of a Freely Moving Particle
We have defined the angular momentum of a particle about the origin as
This does not demand that the particle is moving in a circle!
We will show that this particle has a constant angular momentum! y x
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

27. Angular Momentum of a Freely Moving Particle
Consider a particle of mass m moving with speed v along the line y = -d. What is its angular momentum as measured from the origin (0,0)? y x d m
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

28. Angular Momentum of a Freely Moving Particle
We need to figure out
The magnitude of the angular momentum is:
Since and are both in the x-y plane, will be in the z direction (right hand rule): y x d  
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

29. Angular Momentum of a Freely Moving Particle
So we see that the direction of is along the z axis, and its magnitude is given by LZ = pd = mvd.
L is clearly conserved since d is constant (the distance of closest approach of the particle to the origin) and p is constant (linear momentum conservation). y x d
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

30. Example: Bullet hitting stick
A uniform stick of mass M and length D is pivoted at the center. A bullet of mass m is shot through the stick at a point halfway between the pivot and the end. The initial speed of the bullet is v1, and the final speed is v2.
What is the angular speed F of the stick after the collision? (Ignore gravity) M F D m
D/4 v1 v2
initial
final
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

31. Example: Bullet hitting stick
Conserve angular momentum around the pivot (z) axis!
The total angular momentum before the collision is due only to the bullet (since the stick is not rotating yet). M D m
D/4 v1
initial
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

32. Example: Bullet hitting stick
Conserve angular momentum around the pivot (z) axis!
The total angular momentum after the collision has contributions from both the bullet and the stick.
where I is the moment of inertia of the stick about the pivot. F
D/4 v2
final
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm

33. Example: Bullet hitting stick
Set Li = Lf and using M F D m
D/4 v1 v2
initial
final
Physics 2211 Spring 2005
© 2005 Dr. Bill Holm