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PH 105
Dr. Cecilia Vogel
Lecture 2
OUTLINE
Mechanics
Force
Pressure
energy
power
Oscillation
period
frequency
amplitude
Force
Any acceleration,
i.e. any change in motion,
requires a force (push or pull).
Demo: Object at rest tends to stay at
rest. How do you get it moving?
Object moving tends to keep moving.
How do you speed it up? slow it down?
stop it? change its direction?
Equilibrium
Equilibrium means there is no force.
If there’s no force, there’s no
acceleration,
there is no change in motion.
Demo: Pendulum at bottom feels no
force.
What happens to it if it is at rest
there?
What happens to if it is moving
there?
Mass
How much force does it take?
depends on the size of acceleration.
depends on object’s mass
 F = ma
Demo: Spring compressed more exerts a
larger force, so it will cause a greater
acceleration (masses same).
What effect does that have on speed?
Springs exerting same force on different
masses.
Which will be speeded up more?
Example
What is the acceleration of a 3-kg cart, if
it speeds up from 1 m/s to 5 m/s in 2
seconds?
What force is required to do this?
Unit of force: 1 N = 1 kgm/s2
Pressure
The total force determines how an
object accelerates,

Pressure is
F  PA
pressure greater if
demo
my weight spread on flat shoes vs high
heels -- which will damage floor?
Fluid Pressure
The pressure of a fluid pushes on all
sides of an object in the fluid
The air around us is under pressure,
because
about 100,000N/m2
Under water has additional pressure,
The deeper,
Floating
What force makes an object float
toward the surface of water?
Under water, pressure on all sides
the pressure on the
because it’s
more force ___ on the ______ than
_____ on the ____
net effect is an
Work
If I push an object, I can speed it up
the farther I push,
Define the work done by me as
force times distance
W = Fd
unit of energy is Nm = J (Joule)
Energy
Work changes
Types of energy:

Kinetic energy is
work done object
KE = ½ mv2
the ______ it goes, the more _____ it has
the more ______ it has, the more ______ it has
Potential Energy
Sometimes you do work and the
object doesn’t speed up.
The energy you gave it might be
that’s
Examples:
compressing or stretching spring
stretching or tensioning string
lifting an object
Energy Again
An object has energy, if it has the ability to
do work
Does moving object (even air) have ability?
wind
movinghigh
oject have ability?
Does a object lifted

Does a compressed spring have ability?

Does a stretched string have ability?

Power
You have to do work to speed something
up or lift something up.
You can do that work
Which is harder, going up stairs (i.e. lifting
yourself) walking or running?
Power is
W=Pt
or E=Pt
Power
Power is work/time or energy/time
W=Pt
or E=Pt
The more quickly the work is done,
Unit of power is J/s = W (Watts)
A 50 Watt bulb left on for 2 hours uses
________ as 100 W bulb left on for one
hour.
Oscillation
An object oscillates if it
examples
vibrating string
pendulum
mass on a spring
Simple Harmonic Oscillator
Suppose a system has an equilibrium
point
where there is
but on either side of equilibrium,
there is a force
force always pushes system
If you displace this system from eqlb,
it will
Analyze Oscillation
The system is initially away from eqlb,
the force pushes it
It will move back toward eqlb
will it stop when it gets there?

Now it’s going away from eqlb
the force pushes
 what happens when force is opposite
motion?
Properties of Oscillations
Amplitude is
how far it gets from
larger amplitude,
Period is the time it takes to
time before
Frequency is how
the _____ the period, the _____
frequently it repeats.
f = 1/T
Period and Frequency
Period and frequency of a system
depend on properties of system
like, ______ of spring, _____ of
pendulum, ______ in string
does ____ depend on ______ for
simple systems
Demo:
What is the period of pendulum for small
amplitude oscillations? For large
amplitude?
What will change the period?
Damping
Will the oscillation go on forever

all oscillations lose


Because of
Damping is due to factors like
Summary
Pressure is force divided by area
Work changes the energy of an object
kinetic energy is motion energy
potential energy is stored energy
Power is how quickly energy is gained or
lost
Oscillations are repeated motion
due to forces pulling back toward eqlb
Properties of oscillations: period, frequency,
and amplitude