PH 105

Dr. Cecilia VogelLecture 1

OUTLINE Intro & units What is Sound? Mechanics

speed acceleration force

Units In this class, we use SI or mks

units distance measured in meters (m) mass measured in kilogram (kg) time measured in seconds (s)

Get an intuition about size of m and kg meter stick, kg weight my mass: about 60 kg my height: about 1.6 m

Units Conversion Units Conversions

conversion factors in appendix A2 NEVER multiply by anything but 1

For example, App A2 says 1 m is the same as 39.37 in, so

1m 1

in 39.37 1

in 39.37

m 1

Units Conversion Example Take my height of 1.6 m multiply by 1

in such a way that the meters unit will cancel, and inches will be left

1m 1

in 39.37

in 39.37

m 1m6.1 39.37 in

1.6m 63in1 m

1in 39.37

m 1

Units Units and variables:

variable names will be in italics ex: ½ mv2

m is mass variable, v is speed variable

units will not be in italics ex: 9.8 m/s2

m is meters unit, s is seconds unit

Sound Sound in air

is a disturbance of air molecules which propagates (moves)

Anything that disturbs the air suddenly makes a sound anything that pushes air at rest

like guitar string, drum head anything that disturbs the flow of

moving air like bassoon reed, branches in the

wind

(or other material)

Mechanics To study sound, we need the language of mechanics

mechanics is the study of motion Be careful, many physics terms have different

meanings in everyday use. speed, acceleration, force, work, energy, pressure

Speed Speed: how fast an object is moving

variable name, v the distance it travels divided by the time it takes to travel

time

distanceavev

Constant Motion If the speed and direction of motion of object doesn’t change, then

d = vt “distance equals rate times time” Only works if there is a single, constant value of speed, v, to plug in!

Examples A car travels 50 mph for one hour. How far does it go?

d = vt d = (50 mi/hr)(1 hr) = 50 mi

A sound wave travels 343 m/s for one ms d = vt d = (343 m/s)(1 X 10-3 s) = 0.343 m

A car speeds up from rest to 30 mph in 10 s. How far does it go? I don’t know!

Acceleration If an object’s motion is not constant

the object is accelerating do not use d = vt acceleration in physics includes:

speeding up slowing down (also called deceleration) changing direction

If the police officer is a physicist, won’t mind if you say, “I accelerated as I approaches the stop sign.”

Constant Acceleration If an object’s motion is not constant, but the acceleration is constant

speed changing at a constant rate like free-fall

then the change in v (delta-v)

atv va

t

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?

25m/s 1m/s2 m/s

2s

va

t

2(3 kg)(2 m/s ) = 6 NF ma

Unit of force: 1 N = 1 kgm/s2

SummarySound is propagating disturbance of moleculesSpeed is how fast object movesAcceleration is any change in motionAcceleration requires a force

no force no change in motiongreater force provides greater accelerationgreater force required for larger mass

Homework due Friday, Dec 2, before classChapter 1 R1, 4, 6*, 9, 10*, 11, 14, Q3*, 4, E1, 3*, 4, 5*, D1, 3, 4, 6