WJEC Physics for AS: Study and Rev Guide

How to use this book As examiners and former examiners we have written this new study guide to help you be aware of what is required for – and structured the content to guide you towards – success in the WJEC AS examination, or your year 12 studies towards an A level in Physics. There are two main sections to the book: Knowledge and Understanding This first section covers the key knowledge required for the examination. You’ll find notes on the content of both examination units: – Unit 1 Motion, Energy and Matter − Unit 2 Electricity and Light including the practical and data-handling skills which you will need to develop. In addition there are a number of features throughout this section that will give you additional help and advice as you develop your work: Unit introduction The key sub-sections are listed with their page references and their corresponding exam questions. Each then has a short summary giving you an essential overview of the topic, plus a revision checklist as you work through your revision process. ▪▪ Key terms : many of the terms in the WJEC specification can be used as the basis of a question, so we have highlighted those terms and offered definitions. ▪▪ Quickfire questions : are designed to test your knowledge and understanding of the material. QUICKFIRE QUICKFIRE QUIC KFIRE q Describe theN3 ‘equal andopposite forces’ to those shown in the diagram. QUICKFIRE QUICKFIRE QUICKFIRE w Identify the forces acting on the skydiverand the N3 equal and opposite forces: QUICKFIRE QUICKFIRE QUICKFIRE e InQF2,assuming that the skydiver isat terminal velocity,which forces are equal andoppositebut arenotN3pairs?Explain. QUICKFIRE QUICKFIRE QUICKFIRE r Identify theN3pair forces in theEarth/Sun system inFig. 1.3.2. air resistance weight Pointer Avoid referring to the normal contact force as the normal reaction. This sounds like a poor statement of N3. 1.3.2 Newton’s 3rd law of motion Inall the situationsabove, thereare twoobjects involved; stickandball; SunandEarth;ballandEarth.This isalways the case: thehockey stick exertsa forceon theball ( F 1 )and the ballexertsa force ( F 2 )on thehockey stick.AndN3 tellsus that these forcesareequalandopposite. i.e. F 2 = - F 1 . In caseyoudon’tbelieve that this force, F 2 ,exists, just lookat thedents in thisusedhockey stick. Ifyou’d likeanotherquickdemo ofN3, justkickabrickwall.Your footwillexerta forceon thewall (it mighteven causeabitofdamage) and thewallwillexerta forceon your foot (hence thepain). Example Abox sitsona table.Give theN3equalandopposite force toeachof thenamed forces (weightandnormal contact force) inFig.1.3.4. Answer Theweight is the gravitational force that theEarthexertson the box.So the ‘equaland opposite’ force to this is the gravitational force that theboxexertson theEarth, i.e if theboxweighs10N, itpulls theearthupwardswitha forceof10N! Thenormal contact force is theupwards forceexertedby the table on the box, so theequalandopposite force to this is thedownwards forceexerted by thebox (in fact thesearebothelectromagnetic forcesarisingbecause the atoms in theboxand table surfacesarebeing slightly squashed). Warning :Some forcesareequalandopposite,but theyarenotNewton’s3rd lawpairs.So don’t fall into the following trap: Theweight isequalandopposite to thenormal contact force so theyarea pairofN3equalandopposite forces. Thismight seem sensibleatfirstglancebut it iswrong!Thereare three rules thatmustbeobeyed: Fig.1.3.3 TheeffectofN3onahockey stick! Normalcontact force weight Fig.1.3.4N3 forcesonabox F 2 F 1 30 ASPhysics:Study andRevisionGuide 4 AS Physics: Study and Revision Guide