Table Of ContentLONGMAN PHYSICS TOPICS
LONGMAN PHYSICS TOPICS
General Editor J. L. Lewis. Malvern College; formerly
Associate Organiser.Nuffield O-Ievel Physics Project
This series provides background material for modern
courses in physics. The authors were closely associated
with the Nuffield Foundation Physics Project. and thus
have an intimate knowledge of itsspirit.Thesebooks are
not textbooksin the conventional sense.nordo they give
the answersto investigationsthat pupils will be carrying
out inthelaboratory.Insteadthey showtherelevanceand
application in the outside world of the principles studied
in school.
This series provides background material for modern
courses in physics. The authors were closely associated
with the Nuffield Foundation Physics Project. and thus
have an intimate knowledge of its spirit.Thesebooks are
not textbooksin the conventional sense. nordo they give
LONGMAN PHYSICS TOPICS General Editor: John L. Lewis
MASS IN MOTION
I
I
Jim Jardine
Headofthe PhysicsDepartment
George Watson'sCollege,Edinburgh
formerlyScottish Team,Nuffield PhysicsProject
Illustrated by Geoffrey Salter
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Firstpublished1970
ISBN 0 582 32202 2
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This book is one intheseriesofPhysicsbackgroundbooks
NOTE
intendedprimarilyfor use withtheNuffieldO-levelPhysics
TO THE
Project. Most of the team of writers who havecontributed
,TEACHER to the series were associated with that project. It was
always intended that the Nuffield teachers' materials
should be accompanied by background books for pupils
to read, and a number of such books are beingproduced
under the Foundation's auspices. This series is intended
as a supplement to the Nuffield materials - not books
giving the answers to all the investigations pupils will be
doing in the laboratory, certainlynottextbooksinthecon
ventional sense, but books, easy to read and copiously
illustrated, which show how the principles studied in
school are applied in the outside world.
The books are such that they can be used with con
ventional courses as well as with the new programmes.
Whatever coursethe pupils are following, they often need
straightforward books to help clarify their knowledge,
sometimes to help them catch up on any topicthey missed
in their school course. It ishoped that this serieswillmeet
that need.
This background series will provide suitable material
for reading in homework. This volume is divided into
sections, and a teacher may feel that one section at atime
is suitable for each homework session.
Forces 4
ICONTENTS
I Measuring motion 14
Newton's first and second laws 22
Inertia 30
Projectiles 34
Explosions and collisions 44
Summary 59
Answers to questions in the text 61
To start a ball rolling you throw it or kick it. Ineachcase
FORCES
I I you are exerting a force on the ball. To make it change
direction you can head the ball.
Again you are exerting a force on the ball. When you
catch a cricket ball you stop its motion by exerting a
force on it.
In all the above cases forces are being used to change
the motion of a ball. Does the ball exert aforce onyou in
each case?' (NB: the answer to this and to the other
numbered questions in this book willbefound onpp. 61-4.)
In the tube illustrated here, electrons are given off by a
hot filament andspeededup byan electricforce. Thebeam
is then bent by a magnetic force produced by two large
coils.
Of course it is possible to exert a force on something
without moving it. You can lean against a wall, squeeze a
rubber ball or twist a piece of plasticine, but even then
part of the object moves with respect to the rest.
The next photograph shows atennisball whichhas been
squashed as it strikes a tennis racket. Whatisthe important
difference in the behaviour ofrubber and ofplasticine after
they have been squashedr?
4
This photograph shows the titanium boom on the
America's Cup winner Intrepid bent under the action of
tremendous forces. Titaniumwaschosensince itcombines
strength with flexibility.
5
When a body is stationary, either no forces act on it- an
unlikely state of affairs - or the forces are balanced.
In the diagram left, two balanced forces keep the
television set at rest. If we consider the forces acting
on the linesman's feet in the photograph as a single force,
wecan saythatheisinequilibriumundertheactionofthree
balanced forces. These forces are represented by three
arrows in the diagram. Do you notice anythingspecialabout
the directions ofthese threeforcesr' .
lableexe,tsanupwa'dforceontelevis,ooset
FRICTION
When a car runs out of petrol on a perfectly level road, it
eventually stops. Its motion has been altered. A force, or
forces, must have been acting on the car. What aresomeof
theseforces?'
The following pictures show how motion can be arrest
ed by solid to solid friction in a disc brake (left) or by air
resistance in a parachute.
6
Something to do
Examine the frictional forces between two flat pieces of wood, metal, glass
etc. Can you findmore thanone wayofreducingthe friction?
If you can find an old dry wheelbearing(for example, in abicycle orroller
skate), put adrop ofoilon itand seethe effect produced.
Ifforces are necessary to change the shapeor motionof
a body, it might be interesting to see what happens ifone
of these forces - friction - is reduced.
Something to do
Hereisasimple balloon puck you can build at home.
Glue a cork in the centre of the rough side ofapiece ofhardboard, and then
drill a 3-mm hole through the centre of the cork and board. Fit an inflated
balloon on the cork so that the air escapes through the hole, and place the
puck on a smooth level surface such as a polished table. How does the
puck movewhen yougiveit apush?
7
The photograph shows a hovercraft moving on a
I
FORCES I
cushion of air. As air friction is very much less than the
friction between solid and solid, or solid and liquid, the
hovercraft's driving engine will not need to exert a large
force to keep it going.
A huge oilstoragetankwasrecentlyfloated onacushion
of air and then moved 350metres by a small tractor.
The French Aerotrain is supported and guided by air
bearing pads, and is capable of speeds greater than
300 km/h,
Ifit werepossibleto reducethefrictionalforcescompletely,
how much force would be needed to keep the Aerotrain
movingr' Canyou give anexample ofabody moving without
friction?"
8
