PH 105

Dr. Cecilia VogelLecture 2

OUTLINE Mechanics

Force Pressure energy power

Oscillation period frequency amplitude

ForceAny 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?

EquilibriumEquilibrium 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?

MassHow much force does it take?

depends on the size of acceleration.depends on object’s mass

F = maDemo: 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?

ExampleWhat 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 pressure greater if demo my weight spread on flat shoes vs high

heels -- which will damage floor?

PAF

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 Does a object lifted high have ability?

Does a compressed spring have ability?

Does a stretched string have ability?

moving oject

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

SummaryPressure is force divided by areaWork changes the energy of an object

kinetic energy is motion energypotential energy is stored energy

Power is how quickly energy is gained or lostOscillations are repeated motion

due to forces pulling back toward eqlb Properties of oscillations: period, frequency, and amplitude