Transcript Physics 102 Introduction to Physics

Chapter 8
Rotational Motion, Part 2
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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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Center of Mass/Center of Gravity
A baseball follows
a smooth parabolic
trajectory.
A baseball Bat
doesn’t’! It follows
a wobbly 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
Question 1
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 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
The force exerted
radially inward by
friction on the tires
makes the car
move in a circle.
Question 2
Does a centripetal force do work on an object?
A. Yes
B. No
C. Only on Tuesdays
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
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