1. Chapter 8 Rotational Motion, Part 2
Circular Motion
Rotational Inertia
Torque
Center of Mass and Center of Gravity
Centripetal/Centrifugal Force
Angular Momentum
Conservation of Angular Momentum
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2. Center of Mass/Center of Gravity
A baseball Bat doesn’t’! It follows a wobbly trajectory.
A baseball follows a smooth parabolic trajectory.
But, the bat’s Center of Mass follows a smooth parabolic trajectory.
Center of Mass: The average of the positions of all the mass in an object.
The point of a body at which the force of gravity can be considered to act
A crescent wrench sliding across a frictionless surface follows a complicated motion, but its center of mass follows a straight line path.
Center of Mass Applet 1
Center of Mass Applet 2
Physics Place Videos

3. Question 1

4. Centripetal Force
Centripetal Force is the force that makes an object move in a curved path.
Newton’s First Law: An object in motion will stay in motion with constant speed and direction unless acted on by an external force.
If a force doesn’t act, an object in motion will stay in motion in a straight line.
Examples:
The force exerted radially inward by friction on the tires makes the car move in a circle.
The centripetal force that makes the airplane move in a circle is exerted on the wings by air moving over them.
The can moves in a circle because of the force exerted on it by the rope

5. Question 2 Does a centripetal force do work on an object? A. Yes B. No
C. Only on Tuesdays

6. Angular Momentum
We have learned that if an object has inertia and it’s in motion, we can describe it’s “momentum” as its mass times its velocity (momentum = mv).
Similarly, a rotating object has rotational inertia and also has a rotational momentum, called “Angular Momentum”.
Angular momentum depends on an object’s rotational inertia and rotational velocity:
Angular momentum = Rotational Inertia x Rotational Velocity
- Or Angular momentum = I
If an object is in revolution around an external point, its angular momentum can be expressed as: v
Angular momentum = mvr r m

7. Conservation of Angular Momentum
In linear motion, to gain or lose momentum, an impulse must be delivered to an object. If no impulse acts, the momentum of the object (or system) doesn’t change … momentum is conserved.
In rotational motion, Angular Momentum is conserved.
If no external net torque acts on a rotating system, the angular momentum of that system remains constant.
Large Rotational Inertia
Small Rotational Velocity
Small Rotational Inertia
Large Rotational Velocity