Lecture 04

WEB

What’s Happinin?

One Ball:

Time

For

ce o

n th

e “W

all”

What is the “AverageAverage” Force??

One Ball:

Time Interval

For

ce o

n th

e “W

all”

1 2 3 4 5 6

F

Time Interval

For

ce o

n th

e “W

all”

F

AveragingInterval Force

1 0

2 0

3 F4 0

5 0

6 0

66

00000 FFAverage

Last time we defined pressure

• The force is created by molecules “bouncing” off of the wall or surface.

• We need to quickly look at why there is a force.

A

Fp

area

forcepressure

Movement

PRESSURE: HIGH LOW

Remove the Wall

High Low Pressure

FLOW

Weather Map

Molecules Move Around!

WALL

An Aside …States of Matter

As the temperature of a material increases, the

TEMPERATUREincreases as well. (Definition of Temperature shortly)

STATES OF MATTER

• SolidsSolids

• LiquidLiquid

• GasGas

SOLIDS• Hold their shape.

• Relatively heavy

• Examples– rocks– houses– mountains– computers– salt, sugar

LIQUIDS

• Conforms to the shape of the container

• Can be light or heavy

• Water is the most important example

GAS

• Will conform to any container.

• If we increase the size of the container, the gas will EXAPAND to fill the new container.– Liquids or solids will not do this.

dilutedLIQUID GASLIQUID GAS

0/0 Cross-Tab Label

26% 26% 26%

22%

1 2 3 4

Which is Heavier?

1. A cubic foot of a solid.2. A cubic foot of a liquid.3. A cubic foot of a gas.4. You can’t tell without

more information.

WHY DO WE WE CARE ABOUT GASSES??

• We breath air which is a gas.

• Sound travels from the source to our ears through the air.– It also can travel through solids and liquids.– Whales can “talk to each other” over a

distance of hundreds or perhaps thousands of miles of water!

– Note: Sound does NOT travel over wires from one phone to another. VERY DIFFERENT

States of Matter – ExampleWATER

• Below 32 degrees F … SOLID– ice

• Between 32 and 212 degrees F … LIQUID– water

• Greater than 212 degrees F … GAS– steam

We need two more concepts

•Momentum

•Energy

Consider a mass m with a “velocity” v

mv

IMPORTANT DEFINITION

momentum = (mass) x (velocity)or

p=mv

Remember??Remember??

onacceleratimassForce

or

maF

time

velocity)initial- velocity(finalonaccelerati

initialfinal

initialfinal

initialfinal

initialfinal

momentimmomentumFt

velocitymvelocitymFt

velocityvelocitymtimeF

time

velocityvelocitymF

)()(

)(

DEFINITIONS

• IMPULSE = (Force x time it acts)=F x t

• IMPULSE = CHANGE IMPULSE = CHANGE IN MOMENTUMIN MOMENTUM

Time

For

ce o

n th

e “W

all”

How Big is this force??

Average Force

One Crash

v

v

pinitial=mv

pfinal= - mv

pfinal – pinitial = 2 mv

F x t = 2mv

To Calculate the Pressure

• Use the previous equation to estimate F– Estimate the time of the “bounce” … short!– Look up the mass.– Check out the velocity

• Then, find out how many collisions take place per second.

• Multiply all these things together to find pressure.

• We ain’t gonna do dat! We ain’t gonna do dat!

An Aside on an “inelastic” crash

INTERNALForces

Conservation of Momentum

initialfinal momentimmomentumFt

Ext

erna

l For

ce =

0

initialfinal momentimmomentum

M

MM

M

Initial Momentum = M vinitial

Final Momentum = (2M) x vfinal

So

• Initial Momentum = M vinitial

• Final Momentum = 2M vfinal

initialfinal

finalinitial

vv

MvMv

2

1

2

Summary

• Pressure is caused by the presence of molecules.

• When the molecules crash into the walls of a container, they push on it and supply an “impulse” which is the force multiplied by the time that the force (crash) exists.

• The wall applies a FORCE to the molecule as well as an impulse.

• The molecule changes its momentum. • The force exerted on the wall by the billions of

molecules that hit the wall each second creates the pressure.