newton's first law allday and adams

7/24/2019 Newton's First Law Allday and Adams

1/20

In 169 6 Newt

on

beca

me

wa rde n o f th e Royal Min t a nd

was

m ade

Master

of

the Royal Min t in 1699 . He res ig ne d hi s Ca mbr idge

po

s t in

1701 an d wa s

knig

h te d in 1705. He b ec

am

e president of th e Royal

Soc

ietv in 1703 b u t d id virt ua lly no new sci ence

from then

on.

Ne wton is o ften h eld responsib le fo r th e d evelo

pme

n t

of

a me c

hanis

tic

philos op hy th

at regar

ds e veryt

hin

g as dete r ministi c

an

d pre dic tab le, w ith

li

vin

g

thin

gs as co

mp

lex a u t

om

a ta. Ho wever, N

ewton

wa s a lso fascinated

by religious a n d myst ical qu esti

on

s. He left over a th

ou

s

and

m

an u

s

cript

pa ges con tai n i ng hi s ow n r

esea

rc h in to bibl ical ch rono logi es an d o

ver

600 000 wo rds o n alc h emy. H is library co n t a ined 138 bo o ks on alchemy.

As the

wo

rld s grea tes t

ph

ysicist New t

on

cla

ime

d th

at

h e simply w ro te

d ow n laws

a nd d er

ived results from them

but

th at he would no t

sp eculate id ly ab o u t th e u nderlyin g causes of gr a

vity

or th e reasons for

the laws of m ot ion . Ho wever: in p riv

ate

he indu lg ed in wild specul a tions,

lo oked for s i

gns

in the Book

of

Revelat i

on

s,

and

believed

tha

t m

an y of

h is di sco ve rie s w

ere

already k

nown

to the a nc ients.

fter ewton

Newt

on

s t heo r ies we re ap p lie d to as

tro

n

om

ical a nd t

err

estrial

ph

ysics

w it h u npreceden ted s

uccess

in the two cen tu r ies follow ing hi s de ath.

They

w e re

also dev eloped m a the m

ati

cally, partic

ularl

y bv Lagr

an

ge

an d

Ha milto n , w

ho

p

rodu

c ed ge

nera

l ize d e

qu at i

on s of

great

beaury a nd

ma t

hemat

ical elegance. T here ha d never be

en

a theo ry lik e it , a n d the

mec

hanica

l m odel was co pied in th e o th

er

sc i

ence

s

an

d eve n a d

apte

d for

di sciplines like

econo

mics

an

d p olitics . H owever, th e re is p hysic s b eyon d

Newton.

Tow

ard

s th e e nd of the n in et e

enth

ce n tury it seeme d as if

ph

ysics

wo uld

pres en t a fin al c

oher

e

nt

a nd co mpl et e th eory of ev

erything.

T h is

th e

ory wo u

ld inv o lve a toms as fu n

da

m e

nt

al mech

anica

l

pa

rticles

in t

er

acti n g by grav ita t

iona

l an d elec

tromagne

tic forces .

Un fo

rtunatel

y:

Th e re

wer

e

pr o

b lems

accoun

ting fo r

the

sp

ectrum

o f heat ra diation

g iven off by a h ot b ody b l ack - b o d y radiation ). Accord ing to classical

ph

ysics th is sh o uld depend o n m echan ica l v

ib r

a tions

of at

om s cau sing

ele ct r o m a gn e tic wa ves o f th e

sa

me fre q ue ncy, bu t ca reful calcu la t ion s

bas

ed o n th es e ide as led to the

predictio

n of

infinite energ

y radia te d a t

ve ry hi gh frequen

cie

s the ultraviolet

catastrophe

).

J. J.

Th oms on s d iscovery of the elect ron sh owed th at

at

om s a re no t

fu

ndamenta

l,

an

d subseq u

en t

de v

elop

men

ts in

pa

r ticl e p hysics

ge nera ted a host o f new a nd p reviously u nexpected particles.

Th e n ew l y d i

scovered

photoelectric

effect

in

whi

ch ele

ctrons

a re

kn ocked off the s

ur

face

of

a m e tal by li

gh t

was a ls o p roving

im p os si ble to e

xpla

in us ing th e w ave t

heory

o f light.

Att

empt

s to exp la i n li

gh t

as a

mech

a

nic

al wa ve m

ade

o f vib rations in

some

a ll-pe

rvas

ive m

edium

the lurnife ro us ae t

he r

) fail ed w

he

n th e

medium

proved

unde

tecta

ble

th e Mi

chelson Morl

ey experi

ment

).

New p h ys ics was

ne e

ded ,

an

d several revo lu tiona r v the o rie s w

er

e cre

at e

d

in th e early p

art

o f th e twentieth cen tury : .

Einsteins specia l theory of relativity 1905), dealing w it h the

m e ch a ni c s o f rapi d ly moving

ob

jec ts;

Ein stein s

general

theory of

relativity

1915), de al i ng w ith the

geo

met

ry of

sp

ace

an d tim

e an d the

natu

re o f gravity;

qu

antum

theory , de velop ed by m a ny ph ysicists includin g Pl anck,

Ein

stein , B oh r,

S ch ro

ding

e r,

a nd He is

en berg

be tween 1900 a n d 1925 ),

de a l in g wi th the n

atur

e

of

m atter o n a sub a tom ic sc a le.

eossncea PHYSICS

Ul

l

u nU naLeJy:

1 1.--.

,_

...........

.

l.-..l

_ _

.. ..L

MECHANICS

To exp lain all natu re

is 100 dill/c/tll

a task

o r I II

V

one inau or

C\ do a lill ie vcitlt

certaintv and leave the res t or others that

CO ie a

{ 1

.I O U ,

th an 10 exp lain all ih ines.

Isa ac Newton

I

Alb

er t Ei nstein

1879- 1955).

Do

no t

worr

y abou t yo ur p rob lems w it h

mathematics, I

assure

y ou min e ar e far

gr

eater.

,

Quantum

theory ca nno t be attributed t o

on e ph ysi

cist

alo n e . Planck 1858-194 7)

top le ft ) wa s th e fi

rs t

to in

tro

duce

qu an tizat ion 1900). E

inste

in ab ove)

propose d the

phot

on

the

o ry of l ight

1905). Boh r

188

1962)

to p rig ht

quantized the ato m 1913). Sch rodinqer

1887-

1961) bottom le ft) an d

He is

enb

erg 190 1- 1976) b o tt o m rig

ht

pr

op

osed alt erna t ive

bu t

eq uivale

nt

ma th

ema

t ic a l th eories

1925).

Man y

oth er twentieth-century physic ists

made

ma jo r con t r i b utions .

Newton

fo r everyday l i fe

Special relativity

di v erg e s fro m

N

ewton

ian phy sics w h en things m ove

w it h v eloc it ies co m para ble to the

sp eed of light , fo r exa m ple electrons

in a particle

accelerato

r.

General

relativity di sag rees w it h Euclidean

geom etry an d Newtonian gravitation

ove

r e

xt r

em el y la rge d istances. o r in

ex t re

mely

str ong gr avita tional f ields .

Ouantum theory dive

rges

f ro m

Newtoni

an ph y sic s o n a suba

tom

ic

scale. Howe v er . ordi nary ex perience is

in co

mp

le t e agreement

with

N ewton s

pi cture o f th e wo rld .

8 3


2/20

O

JE

T I V E S

all motion is relative J

the first

l aw o f

motion

o

resultant forces change

motion

-

----

Who

is moving

a real world

car freewheels

where a car might

stop in practice

b idealworld - nofriction

car continues until it

reaches its starting

height ...

if the track n v rr s s back up,

the car continues for ever

Galileo s thought experiment

a real world

NEWTON S

FIRST

LAW OF

MOTION

A natural state of motion

Imagine yourself

l os t in

deep

space. All

you can

see

ar

e the

distant stars,

bu t

they

are

too far away to tell if they

are moving

.

Then

something

comes into

view.

It

is

small

and

far away

bu t

it is

getting

closer. As it

d ri ft s p ast, you

recognise your

t wi n, a ls o l os t in s pa ce ,

and

yo u wave

sadly

to

each other

.

Some time later

a

thought occurs

to you : is it you

who

is

moving or

is it yo u r t wi n,

or are

you both moving? Is

there any

way

of telling)

A

similar thought

occurred

to Gal il eo. He

imagined

being

locked

below

decks

in a

windowless room

on a

ship which mayor may not

be

sailing smoothly across the

s ea . Is

there any experiment tha t could

be

carried out

insi e the room to

determine whether the ship

is

moving or

not?

He

considered

jumping

up

and down, pouring water

into

a

jug

and

throwing

a ball,

bu t

a

moment s

reflection

shows that

none

of

these

would show the

slightest sign

of the

ships

motion. If

you doubt this

and

think tha t

jumping

v er ti call y in a

moving

boat

would cause

you to

land

behind your

take-off

point, transfer

the

argument

to a jet

aircraft

travelling at

hundreds

of

metres

pe r

second

,

or

try it on a trainl)

Galileo

concluded that uniform

motion in a

straight

l in e is

just

as

natural

as

being

at r es t, and so r est itse lf is

no t

special. All it means is at

rest wit li respect to your surroun ings

We

are

used

to

using the

Earth

as

our

frame

of reference.

bu t

t hi s is

simply

a

small planet

orbiting

a

minor

star

some

way from the

centre

of a

rotating

spiral

galaxy

which

is itself

movin

g in the

gravitational

fields

of other

galaxies . In

other

words

,

our

Earth

is not so spec ial

after

all

or

, as

Einstein might

have said:

Th

ere

is no

such thing

as

absolute

rest or

absolute

motion

; all

motion

is relative.

The

l aw s of

mechanics

are the same

in a

laboratory

at res t

or

in

uniform motion.

This

conclusion was

revolutionary. It

meant that all

motion

is relative.

For

two

thousand years people

had

argued that

all

bodies tend

to

come

to

rest

naturally. Galileo

disagreed.

A roller coaster ride to infinity

Galil eo

described

a

simple

experiment

in

which

a ball rol ls

down

a

Ll-shaped

track and then

r ises to verv

nearly the same height on the

far

side

of

the track

. Galileo

assumed that the only reason

it failed to

reach

the original height was because

of frictional forces

f rom the t rack and

air ,

I f

these

could

be

removed

it really

would reach

the

same

height. He

then reduced

the s lope

on

the far s ide so

that

the ball

had

to travel

urther

to r is e

through

the

same

vertical

height.

It did.

What

if

the

far

side never

rises

up

again? The

ball will

continue

r ol li ng fo re ve r at a

constant speed

in a

natural

state

of

motion

e ve ry bit as

significant

as

being

at

rest on Earth

Most

of

the

movement

we see

around

us is a ff ec te d by f ri ct io na l f orc es

that stop things movin

g

past thei

r

surroundings.

Howev er , it is

possible

to

simulate

an ideal friction-free environment on

an air track or air

table,

and

then the r iders

do

continue with almost

constant velocity

between

collisions.

Images from

inside

orbiting spacecraft

also

confirm that

if

something

is

dropped or thrown

it

continues

moving

in a

straight

l in e at

constant

speed

until

it

hits

something.

One

of

the most

important

effects of Galilee s

work was

the

realization

that 110 re

sult nt

force is required to

m int in uniiorm motion

since

none

is

required

to ke ep something at re st) .

Newton

developed Galilee s

ideas

into

the l aw s of

motion.

m _ .

_ , >fYi ] Cf tf

E ttXSJ S

in

uniform motion.


3/20

MECHANICS

N e w t o n s f i rs t

la w

of motion

Objects continue to

move

at constant velocity which

ma y

be zero unti l they are

acted

upon

by a resultant force .

h t does the first l w tell us

The consequences

of the fir st Jaw

can

often

seem

s urprising.

An object moving at

constant

velocity re me mbe r thi s

means constant

speed and direction is in equilibrium. All the forces that ac t upon it

i f an y) cancel

out.

Sometimes

people

find this a bit hard to take.

Surely, they argue , if a ca r is t ravel li ng f or wa rd s at a ste ady 70 mph

the

thrust

f ro m the engine

must

be

it igger than

the to tal of

drag

and other frictional forces? Not true - thrust and drag must balance.

Well, what

about

lifting something,

surel

y you

need

to pull up it

h rder

than

gr avity pulls it

down?

Not t rue

- to lift something a t a

steady rate you must apply a lifting force exactlyequal to the weight

of th e thing you are lifting. Of course,

to strt it

movin you need a

bigg

er

force, but

that

too agrees with Newton s law: the object is

ch

n

ing

its velocit y as it

starts

to rise .

Anything that is changing its velocitv must have a resultant for ce

act

ing on it. That could mean accelerating, decelerating, or

changing

its direction.

For

example,

when

you

drop somethin

g it falls

with

an

increasing velocity. The re mu st be a resultant force acting on it. We

call this force g ra vi ty. The

Moon

is changing its velocity. Its speed is

constant but

its direction of motion is continually turning toward the

Earth. There

must be a force acting on it in the

direction

of the Earth

- gravity again.

The first la w is so me t ime s c al le d the la w of inertia , since it seems to

encapsulate

the tendency of matt

er

to keep moving in the way it is

alread y moving.

There is no resultant force on this

cruising

Concorde.

Voyager 1 was

launched

in 1977 and

Voyager 2 in 1979. They have now

passed and photographed the outer

planets and

are

headed away from

the

solar

system. They

both carry specially

coded

audio recordings

entitled

Sounds

of

Earth

for the

entertainment

of any aliens

who happen

to

find them

.

Shut your eyes on board a

cruising

plane and you can almost

imagine

it is

motionless There is no way to

distinguish

uniform motion from rest

so there is no meaning to the idea of

absolute

rest

; all

motion

is relative.

PR TI E

1 A stone is dropped from the top of a

sailing

boat s

mast

to the

deck

below. Will it land in

front,

behind

or

level

with

the

foot

of

the

mast

if:

a The boat is at rest on the water?

b

The boa t

is mov in g forwards at a steady

speed?

c

The boa t

is moving

backwards

at a steady

speed?

d

The

boat is accelerating in the forward

direction?

2 What two forces act on a person in a stationary

lift? How do these forces compare?

3

How

do

the

forces described in

question

2

change when the

lift is:

a Moving up at

constant

speed;

b Moving down at

constant

speed;

c A cc el er at in g upwards;

d A cc el erat in g downwards;

ad

vanc

ed PHYSICS

e

Decelerating

downwards;

f

Decelerating upwards?

4 A friend argues that he knows how to s urvi ve if

a lift falls

dO\\ I

its

shaft.

You h av e to wait until

the

moment just before

the

crash and

then leap

into

the

air. The lift will crash and you can step

ou t of the wreckage. Is this possible?

5 A ca r is driving

straight

up a

steep

hill

into

a

headwind

at a steady speed. Draw a free-body

diagram of the forces acting on the car, and use

a

vector diagram

to show their

resultant.

6

uniform motion

requires no

resultant

force to

k ee p it g oi ng , wh r do we need to put petrol in

cars?

7

Copernicus revolutionized cosmol

ogy by

claiming that the

planets orbit the

Sun rather

than the Ear th . all motion is relative isn t

either

point

of v ie w equally acceptable?

85


4/20

A spacecraft dr if ti n g a t con s

tant

velocity in dee p sp ace h as no force s

ac ti ng upon it. A

ca r

travelling a long a

rnotorway

a t a co nsta nt

100 km h is a ffected by ma n y d if fer

ent

for ces, b u t b

oth

th e

car

an d the

s

pac

ec ra ft ha ve the

same

ki nd

of

mot

io n - co ns

tant

velocity in a strai

ght

lin e -

and

in b

oth cas

es

the resultant force

is zero .

The tota

l fric ti

ona

l

d rag o n the c

ar

is exa ctly eq ua l to its

forwar

d t

hru

st , a nd the we ight of

the ca r is ba la nc e d by an

equ

al up w a rd for ce fro m th e road on the tyr es .

H the

spacecra

ft fires its roc kets it will no lo n ger co n t

inue

at a co nstant

ra te; its velo citv will increase

and

it will accele

rate

in th e

di r

ection o f th e

re sultant for ce.

If

the

ca r

eng in e s

udden

lv stops

and

th e

thrust

d

isappears

there will be an unbalanced

or

resultant forc e on th e ca r wh ich op p os es its

mo tion. It will d ecel

erate

until it

stops

m oving. I n bo th cases the effect of a

resul tant force is an accel

era

t ion in the direc tion of the force in the cas e

of the car a fo

rwar

d decelerat ion is the same as a backward accelera tion).

The effec t

o

a resu ltn t force

is

to change the s ta te

o

motion

o

the

object

a l l \

vhic h it acts

FORCE

AND MOTION

esult nt force mass, nd acceleration

You co u ld in vestigate th e link be tween res ul

tan

t fo rc e a nd accele

ra t

ion

bv ap p ly ing kn

own

for ces to a ve h icle a nd m ea surin g its ac

celer

at ion

u

sin

g a mo t

ion

se nsor: eith e r ligh t ga tes

or

tick er t ap e. You should ge t th e

foll

owin

g results.

Accel erati o n is d irectly p

ropo

rtio na l to re

sult

a n t for ce rid er

of

co ns

tan

t

mas

s).

a

ex

F cons tant mass

Acce l er at io n is in versely proportiona l to

rider mass

co

nstan

t re su

ltan

t

force).

a

oc

l cons

tant resu

l

tant

force)

Th is is best

summar

ized by:

F

a c

newton's first law allday and adams

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