1. College Physics, 7th Edition
Lecture Outline
Chapter 8
College Physics, 7th Edition
Wilson / Buffa / Lou
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2. Units of Chapter 8 Rotational Motion and Equilibrium
Rigid Bodies, Translations, and Rotations
Torque, Equilibrium, and Stability
Rotational Dynamics [Not tested on AP Physics B]
Rotational Work and Kinetic Energy [Not tested on AP Physics B]
Angular Momentum [Not tested on AP Physics B]
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3. 8.1 Rigid Bodies, Translations, and Rotations
A rigid body is an object or a system of particles in which the distances between particles are fixed (remain constant).
Rigid bodies only exist in a perfect, IDEAL world!
In other words, a rigid body must be solid (but not all solid bodies are rigid).
A quantity of liquid water is not a rigid body, but when water freezes into ice it becomes a rigid body.
In reality, the particles of a solid vibrate constantly, and undergo elastic and inelastic collisions.
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4. 8.1 Rigid Bodies, Translations, and Rotations
A rigid body may have either two types of motion or both:
1. Translational
2. Rotational
What’s the difference between the two?
You can combine both types of forces and that’s where we get the term ROLLING!
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5. 8.1 Rigid Bodies, Translations, and Rotations
For an object that is rolling without slipping,
s = rθ
a = rα
An object can roll with slipping, but none of the equations will hold when there is slipping.
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6. 8.1 Rigid Bodies, Translations, and Rotations
A cylinder rolls on a horizontal surface without slipping. The cylinder has a radius of 12 cm and a center of mass speed of 0.10 m/s as it rolls without slipping. If it continues to travel at this speed for 2.0s, through what angle does the cylinder rotate during this time?

7. 8.2 Torque, Equilibrium, and Stability
The perpendicular distance from the line of force to the axis of rotation is called the lever arm or the moment arm. [symbol r, (like radius)
The product of the force and the lever arm is called the torque.
It takes a force to start an object rotating; that force is more effective the farther it is from the axis of rotation, and the closer it is to being perpendicular to the line to that axis.
S.I. Unit: m N
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8. 8.2 Torque, Equilibrium, and Stability
Torque in rotational motion can be thought of as the analogue of force in translational motion.
Torque (rotational) is similar to Force (linear)
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9. 8.2 Torque, Equilibrium, and Stability
Torque is a vector or scalar????
Counterclockwise is positive, and clockwise is negative.
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10. 8.2 Torque, Equilibrium, and Stability
In the human body torques produced by the contraction of muscles cause some bones to rotate at joints. With the axis of rotation through the elbow joint and the muscle attached 4.0 cm from the joint, what are the magnitudes of the muscle torques for cases a and b if the muscle exerts a force of 600 N.

11. 8.2 Torque, Equilibrium, and Stability
In order for an object to be in equilibrium, the net force on it must be zero, and the net torque on it must be zero as well.
Equilibrium means that forces and torques are in balance.
Can be at ______ or at _________ __________.
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12. 8.2 Torque, Equilibrium, and Stability
Two types of equilibrium:
Translational linear; forces
Rotational circular; torques
Mechanical Equilibrium
Static Equilibrium

13. 8.2 Torque, Equilibrium, and Stability
Concurrent Forces vs. Couple
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14. 8.2 Torque, Equilibrium, and Stability
A picture hangs motionless on a wall. If the picture has a mass of 3.0 kg, what are the magnitudes of the tension forces in the wires?

15. 8.2 Torque, Equilibrium, and Stability
Three masses are suspended from a meter stick. How much mass must be suspended on the right side for the system to be in static equilibrium? [Neglect the mass of the meter stick]
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16. 8.2 Torque, Equilibrium, and Stability
If an object is in stable equilibrium, any displacement from the equilibrium position will create a torque that tends to restore the object to equilibrium. Otherwise the equilibrium is unstable.
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17. 8.2 Torque, Equilibrium, and Stability
Whether equilibrium is stable or unstable depends on the width of the base of support.
© 2010 Pearson Education, Inc.