Fundamentals of Rotational Motion Dynamics

1. Rotational motion (chapter ten) • Angular speed, acceleration • Rotational kinematics • Relation between rotational and translational quantities • Rotational kinetic energy • Torque • Rigid body in equilibrium, under a net torque • Angular momentum and its conservation • Precession • Rolling of rigid bodies

3. Angular speed Relation between linear and angular displacement The angle here must be measured in radians (conversion:  radians = 180°) s r  The average angular speed can be defined from the angular displacement in some time interval as And the instantaneous angular speed as Units: rad/s (rad is not a “real” unit)

4. Angular acceleration Likewise, we can define average and instantaneous accelerations as For a rigid body (points remain fixed with respect to each other) all parts have same angular speed and acceleration These quantities are vectors – their direction is found using the right hand rule

5. Rotational kinematics Using the same derivations as we used to find the equations of linear kinematics, we find For constant angular acceleration

6. Relation between rotational and translational quantities The angular kinematic variables will be related to the linear ones for points on a rigid body according to: Note this is for the tangential acceleration We also know the expression for the centripetal acceleration

7. Therefore, the total acceleration vector at any instant is Note: rotational variables depend on choice of axes (location) • ConcepTest • Examples

8. A ball of glass will bounce higher than a ball of rubber. • Thomas Edison was afraid of the dark. • A U.S. Quarter has 119 grooves on its circumference. A dime has 118 grooves. • It is impossible to lick your elbow. • It is physically impossible for pigs to look up into the sky. • Over 75% of people who read this will try to lick their elbow.

9. Rotational kinetic energy The total kinetic energy of an extended object can be found by adding up the energy of its constituents. For a collection of discrete particles: Where I is a quantity named the moment of inertia

10. Rotational kinetic energy The resulting equation has a similar form to the one for the linear (translational) kinetic energy Note that this is not a new type of kinetic energy – only an expression for the translation KE for a rotating rigid body For a continuous object • where  is the density and V the volume, i.e., the second integral is over the spatial dimensions of the object. • ConcepTest • Examples

11. Moments of inertia of common objects Solid cylinder, radius R: I=½MR2 Hollow cylinder, radii R1 and R2: I=½M(R12+R22) Thin cylindrical shell, radius R: I=½MR2 Solid sphere: (2/5)MR2 Thin spherical shell: (2/3) MR2 Thin rod, length L, about center: ML2/12 Thin rod, length L, about end: ML2/3

12. Torque Torque is the rotational quantity corresponding to force – it represents the tendency of a force to cause an object to rotate. The expression for torque is: This is a vector product. The magnitude of the vector product is The direction of the vector product is perpendicular to the plane formed by the two vectors forming the product, in a direction given using the right hand rule.

13. Torque We can think about the magnitude in two ways. One is that Fsin is the component of the force perpendicular to the position r. The second is that rsin (the “moment arm”) is the perpendicular distance from the rotation axis to the line of action of the force. For extended objects, equilibrium implies both zero acceleration and zero angular acceleration. Since torque is the tendency of a force to cause rotation, the net torque on an object in equilibrium must be zero about any axis!!! ConcepTest, Example

14. Rigid body under a net torque If the torque on a rigid object about any axis is not zero, the points making up the object will be experiencing a net force, and so accelerate. The tangential acceleration of the ith mass will be governed by Multiplying by the position gives Summing over all particles gives

15. I m1 m1 m2 m2 Rigid body under a net torque Example: Atwood machine with a massive frictionless pulley +y 3 unknowns: T1, T2, and a

16. Rigid body under a net torque

17. Work and energy in rotational motion The work done by an external force on a point P on a rigid rotating body is Where  is the angle between the direction of the force and the position of its application, and d is the infinitesimal angle over which the object rotates. Examples

18. Work and energy in rotational motion If we rewrite the expression for torque as Integrating over  between an initial and final angular speeds, we find the rotational version of the work-kinetic energy theorem

19. Donkeys kill more people annually than plane crashes. • 23% of all photocopier faults worldwide are caused by people sitting on them and photocopying their buttocks. • Walt Disney was afraid of mice. • The average human eats eight bugs or spiders in their lifetime while sleeping.

20. Angular momentum Each point on a rotating object has a velocity and a mass, and so even if the object does not move (translate) it has a momentum associated with it. We call this the angular momentum, defined as The vector angular momentum is a cross product of the radius and the linear momentum Note that it depends on the choice of axis! Also note that the cross product is not commutative:

21. Angular momentum Rewriting the total angular momentum of a system of particles as We can see that the angular momentum is the analog of the linear momentum in equations of dynamics This the rotational version of Newton’s 2nd law

22. Angular momentum Likewise, for a system of particles This means that the angular momentum of a system about a specific axis will be constant if the external torque about that axis is zero For an isolated system, then ConcepTest Examples

23. Precession A top spinning on a horizontal surface with angular speed  will have an angular momentum L with magnitude I, pointing in a direction along the axis of rotation If it makes some angle with the vertical, the gravitational force will give a torque about the point of contact with the surface. This torque will cause the angular momentum to change direction, tracing out a circle perpendicular to the vertical direction – this behavior is called precession. From Serway & Jewett, Principles of Physics, 3rd ed., Harcourt 2002

24. Rolling motion As a cylinder or sphere or radius R rolls through an angle , its center of mass moves s=R , so But note that the object is not rotating about its center – the bottom is stationary at any point, while the top moves at a speed 2vCOM

25. Rolling motion We can write the total kinetic energy of the object as Parallel axis theorem: The moment of inertia through any axis parallel to the axis through the COM follows the following relation: where D is the distance between the axes

26. Rolling motion Using the parallel axis theorem for a cylinder, we see that In other words, the kinetic energy is equal to that of a purely rotating cylinder about the point of contact with the ground. where D is the distance between the axes