LONGMAN 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 LONGMAN LONGMAN GROUP LIMITED London Associatedcompanies. branchesandrepresentativesthroughout the world ©LongmanGroupLtd1970 Allrights reserved.Nopart ofthispublication may bereproduced.storedinaretrievalsystem ortransmittedinanyform orbyanymeans~ electronic.mechanical.photocopying. recording orotherwise - without thepriorpermissionof thecopyright owner. Firstpublished1970 ISBN 0 582 32202 2 Printed inGreatBritain by Butlerand Tanner Ltd. Frome andLondon The author and publisher are grateful to the following for per I ACKNOWLEDGEMENTS I mission to reproduce copyright photographs: front cover Ford MotorCompanyLimited;backcoverPicturepointLimited;page 4 Teltron Limited; page 5 (left) Dr. Harold E. 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We are particularlygratefultoHeinemannEducationalBooks Limited and to Kodansha Limited for their cooperation. 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