Presentation on theme: "National 4/5 Physics In addition to set homework you will be expected to finish off class notes and regularly review work against the learning outcomes."— Presentation transcript:
1National 4/5 PhysicsIn addition to set homework you will be expected tofinish off class notes and regularly review work againstthe learning outcomes.You will be expected to take responsibility for your ownlearning and for seeking help when you need it. At theend of each section, you must ensure all notes arecompleted and examples attempted.
2In unit 1 we will learn about the physics of motion. We will focus on the language,principles and laws whichdescribe and explain themotion of an object. Kinematics, also known as Mechanics is the science of describing the motion of objects using words,diagrams, numbers, graphsand equations.Print for lab booksThe goal is to develop mental modelswhich describe and explain the motion ofreal-world objects.
3Key words: vectors, scalars, distance, displacement, speed, velocity. By the end of this section you will be able to:Describe what is meant by vector and scalar quantitiesState the difference between distance anddisplacementState the difference between speed and velocityState that force is a vector quantityUse a scale diagram to find the magnitude and directionof the resultant of two forces acting at right angles toeach other.Tuesday 28th AugustInitial lesson only 25 minutes with class moves etc
4Key words: average speed By the end of this section you will be able to:Describe how to measure an average speedCarry out calculations involving distance, timeand average speed.
5Which of these are units of speed? metresgallonsmiles per hoursecondsminutesamperesmileskilometres per secondmiles per minutewattsmetres per secondNewtons
6Speeds in….In Physics we normally use unitsm/s for velocity.
7Average speed (m/s) High speed train Snail Sound 270 m/s 13.4 m/s UK townFast jet747 jumbo jet10.3 m/s29790 m/s97 m/sAir moleculeEarth in orbitFalcon60 m/s648 m/s7500 m/sOlympic sprinterUK motorwayEarth satellite500 m/sCan you match the correct speedsm/s340 m/sWalking speedLight speedConcorde1.7 m/s31 m/s833 m/s
8Average speed ( m/s ) Light speed 300000000 m/s Earth in orbit Earth satellite7500 m/sFast jet833 m/sConcorde648 m/sAir molecule500 m/sSound340 m/s747 jumbo jet270 m/sFalcon97 m/sHigh speed train60 m/sUK motorway31 m/sUK town13.4 m/sFamiliarisation with realistic speeds in m/s to aid calculations.Olympic sprinter10.3 m/sWalking speed1.7 m/sSnail0.006 m/s
9What is speed? When we talk about speed we mean… the distance covered by an object in agiven time.
10What is speed? If Hamish (the dog) runs 10 metres in 2 seconds, what is his speed?
11What is speed?His speed is 5 metres per second.So speed is
12What is speed?If you forget the formula think of cars travelling at 30 kilometres per hourkmPerHour=
13Key words: average speed By the end of this section you will be able to:Describe how to measure an average speedCarry out calculations involving distance, timeand average speed.
15Speed CalculationsA cyclist travels 100 m in12 s. What is her speed?
16Step 1: write down what you know. d = 100 m t = 12 s speed = ?
17Step 2: write down your formula. You can use the triangle to help you but remember you get no marks for this!d = 100 m t = 12 s speed = ?d = speed x t
18d = speed x t Step 3: substitute in your values. d = 100 m t = 12 s
19d = speed x t 100 = speed x 12 Step 4: rearrange d = 100 m t = 12 s
20d = speed x t 100 = speed x 12 speed = = 8.33 100 12 Step 5: calculate d = 100 mt = 12 sspeed = ?10012
21d = speed x t 100 = speed x 12 Speed = = 8.33 m/s 100 12 Step 6: units!!!!d = speed x t100 = speed x 12Speed = = 8.33 m/sd = 100 mt = 12 sspeed = ?10012
22Key words: average speed, instantaneous By the end of this section you will be able to:Describe how to measure instantaneous speed.Identify situations where average speed andinstantaneous speed are different.
23Instantaneous and average speed Are instantaneous and average speed the same?
24Instantaneous or average? A car’s speed between North Berwick andEdinburghAverage
25Instantaneous or average? The speed read from a car’s speedometerInstantaneous
26Instantaneous or average? A tennis ball’s speed as it crosses the netInstantaneous
27Instantaneous or average? A racing car’s speed over a lap of the trackAverage
28Instantaneous or average? A parachutist’s speed as he/she landsInstantaneous
29Scalars and Vectors Imagine a boat making a distress call to the coastguard.The boat tells thecoastguard he is 60 kmfrom Aberdeen.
30Scalars and Vectors Is this enough information for the coastguard to findthe boat?
32distance (size) direction Scalars and Vectors The coastguard needs bothdistance (size)anddirectionto find the boat.
33Scalars and Vectors - Definition A scalar is a quantity which has onlymagnitude (size). It is defined by anumber and a unit.A vector is a quantity which hasmagnitude (size) and direction. It isdefined by a number, a unit and adirection.Print for lab books
34Distance and Displacement A pupil walks from her house to her school.Her brother makes the same journey, but via a shop.How far has the girl walked?How far has her brother walked?500 m300 m400 m
35Distance and Displacement The girl has walked 500 m.Her brother has walked 700 m.Distance is a scalar quantity – it can be defined simply by a number and unit.500 m300 m400 m
36Distance and Displacement Distance is simply a measure of how much ground an object has covered.500 m300 m400 m
37Distance and Displacement But how far out of place is the girl? And her brother?Displacement is a vector which requires number, unit and direction.500 m300 m400 m
38Distance and Displacement The girl has a displacement of 500 m at a bearing of 117° East of North.500 m300 m400 m
39Distance and Displacement What is her brother’s displacement?500 m300 m400 m
40Distance and Displacement Her brother has a displacement of 500 m at a bearing of 117° (117° East of North).500 m300 m400 m
41Distance and Displacement Their displacement (how far out of place they each are) is the same.500 m300 m400 m
42Speed and VelocitySpeed is a scalar quantity requiring only magnitude (number and unit).Velocity is a vector, requiring magnitude and direction.Print for lab books
43Speed and Velocity Speed tells us how fast an object is moving. Velocity tells us the rate at which an object changes position.Print for lab booksPhysics animations?
44Speed and Velocity Imagine a person stepping one step forward, then one step back at a speed of0.5 ms-1.What is the person’s velocity? Remembervelocity keeps track of direction. Thedirection of the velocity is the same asthe direction of displacement.
46Key words: vectors, scalars, distance, displacement, speed, velocity. By the end of this section you will be able to:Describe what is meant by vector and scalar quantitiesState the difference between distance anddisplacementState the difference between speed and velocityState that force is a vector quantityUse a scale diagram to find the magnitude and directionof the resultant of two forces acting at right angles toeach other.Wednesday 29th August
49A physics teacher walks 4 meters East, 2 meters South, 4 meters West, and finally 2 meters North. The entire motion lasted for 24 seconds. Determine the average speed and the average velocity.
50 The physics teacher walked a distance of 12 meters in 24 The physics teacher walked a distance of 12 meters in 24seconds; thus, her average speed was 0.50 m/s.However, since her displacement is 0 meters, her averagevelocity is 0 m/s. Remember that the displacement refers tothe change in position and the velocity is based upon thisposition change. In this case of the teacher's motion, there isa position change of 0 meters and thus an average velocity of0 m/s.
52Key words: vectors, scalars, resultant, scale diagram By the end of this lesson you will be able to:Describe what is meant by vector and scalar quantitiesState the difference between distance anddisplacementState the difference between speed and velocityState that force is a vector quantityUse a scale diagram to find the magnitude and directionof the resultant of two forces acting at right angles toeach other.Thursday 30th August.
53Vectors Vectors can be represented by a line drawn in a particular direction.The length of the line represents themagnitude of the vector.The direction of the line represents thedirection of the vector.
54Addition of Vectors When two or more scalars are added together, the result is simply a numericalsum.For example a mass of 3kg and a mass of5 kg, when added, make a mass of 8kg.
55Addition of Vectors 8 N When two or more vectors are added together, providing they act in the samedirection, the addition is straightforward.5 N3 N8 N
56Addition of Vectors 2 N If they are acting in opposite directions 5 N
57Key words: vectors, resultant By the end of this section you will be ableto:Use Pythagoras and Trigonometry to findthe magnitude and direction of theresultant of two forces acting at rightangles to each other.Thursday 30th August
58Addition of Vectors The resultant of two or more vectors which act at angle to each other can befound either using a scale diagram, or byPythagoras and trigonometry.
59To find the resultant of a set of vectors using a scale diagram 1. Decide on a suitable scale and write thisdown at the start2 Take the direction to the top of the page asNorth. Draw a small compass to show this.3 Draw the first vector ensuring it is thecorrect length to represent the magnitudeof the vector, and it is the correctdirection.
60To find the resultant of a set of vectors using a scale diagram Draw an arrow to represent the secondvector starting at the head of the first.Vectors are always added head to tail.5 The resultant vector can now be determinedby drawing it on the diagram from the tailof the first to the head of the last vector.The magnitude and direction of this vectoris the required answer.6 The final answer must have magnitude and direction – either a bearing from North or an angle marked clearly on the diagram
61Scale DiagramsScale: remember if the question is in ms-1 then your scale should be a conversion from cm to ms-1.Direction: draw compass on page1st vector: length and direction2nd vector: tail of 2nd starts at tip of firstResultant vector: tail of 1st to tip of lastAnswer must include magnitude (including units) and directionPrint for lab books
62Scale Diagrams Direction should be given as a three figure bearing from Northe.g. 045° or 175° or 035°If you give any other angle, you mustclearly mark it on the scale diagram.Print for lab books
63A car travels 100 km South, then 140 km East. The time taken for the wholejourney is 3 hours.Using a scale diagram (and the six stepprocess) findthe car’s total distance travelledits average speedits overall displacementits average velocityReview on board – pupils to come up and draw / write
64Scale Diagrams Scale diagrams are used to find the magnitude and direction of the resultantof a number of a set of vectors.
74We ADD vectors HEAD to TAIL [tip to toe] 6N6N North, 8N East - what is the resultant force R ?We ADD vectors HEAD to TAIL [tip to toe]8N6NR8N
75Key words: acceleration, velocity By the end of this section you will be able to:Explain the term “acceleration”State that acceleration is the change invelocity per unit timeCarry out calculations involving the relationshipbetween initial velocity, final velocity, time anduniform acceleration.
76Measuring Acceleration Activity What do you expect to happen to the value of acceleration as the light gate is moved further up the slope?Position of light gate from bottom of slopeAcceleration (m/s2)1st attempt2nd attempt3rd attemptPosition 1mPosition 2Position 3Position 4Average acceleration (m/s2)Print for lab books
77What is acceleration?Acceleration is the change in velocity of an object per second (in one second).Is acceleration a vector or scalar quantity?
78Acceleration What is the definition of acceleration? Is it a vector or a scalar?Acceleration is the rate of change of velocity per unit time OR change in velocity per unit time.Vector – since velocity is a vector.
79What is acceleration? The rocket starts off at 0 m/s and 1 second later is travelling at 10 m/s.What is its acceleration?10 metres per second per second10 m/s2change in speedin one second
80Calculating acceleration We need to know…the change in velocity so…initial velocity (u)final velocity (v)and…time (t)
83Acceleration a = acceleration measured in m/s2 u = initial velocity measured in m/sv = final velocity measured in m/st = time measured in s
84Units of acceleration a = acceleration is measured in m/s2 final velocity – initial velocitya =timeacceleration is measured in m/s2If the speed is measured in kilometres per hour, acceleration can be measured inkilometres per hour per second.
85Acceleration An object accelerates at a rate of 4 m/s2. What does this mean?The object goes 4 m/s faster eachsecond.
86Acceleration The object goes 4 m/s faster each second. If the object is initially at rest, whatis its velocity after:1s? 4 m/s2s? 8 m/s3s? 12 m/s4s? 16 m/s
87Acceleration What does it mean if an object has a negative value of acceleration?It means that it is slowing down.For example: an object which has anacceleration of -2 m/s2 is becoming 2 m/sslower each second.
88Acceleration Calculations A car, starting from rest, reaches avelocity of 18 m/s in 4 seconds. Find theacceleration of the car.What do I know?Initial velocity u = 0 m/sFinal velocity v = 18 m/stime t = 4 s
89Acceleration Calculations What do I know?Initial velocity u = 0 m/sFinal velocity v = 18 m/stime t = 4 sFormula?Example on p5 to do themselves
90Acceleration Calculations A cheetah starting from rest acceleratesuniformly and can reach a velocity of 24m/s in 3 seconds. What is theacceleration?Use technique and show all working!Units!!
91Acceleration Calculations A student on a scooter is travelling at6 m/s. 4 seconds later, she is travelling at2 m/s. Calculate her acceleration.Use technique and show all working!Units!!What do you notice about her change invelocity?
92Rearranging the acceleration equation v-ua tPupils found rearranging the equation difficult!
94Key words: acceleration, velocity By the end of this section you will be able to:Explain the term “acceleration”State that acceleration is the change invelocity per unit timeCarry out calculations involving the relationshipbetween initial velocity, final velocity, time anduniform acceleration.Graph results
95Acceleration using two light gates The length of the mask is 5 cm. Calculatethe acceleration.Remember calculate u (initial velocity) andv (final velocity) and use
96Acceleration using a double mask The length of each section mask is 4 cm. The gap is also 4 cm. Calculate the acceleration.Remember calculate u (initial velocity) andv (final velocity) and use
97Key words: acceleration, velocity, displacement By the end of this seection you will be able to:Draw velocity-time graphs of more than oneconstant motion.Describe the motions represented by avelocity-time graph.Calculate displacement and acceleration, fromvelocity-time graphs, for more than one constantacceleration.Tuesday 11th September
98Graphing Motion Information about the motion of an object can be obtained from velocity-timegraphs.Similarly, we can graph motion based ondescriptions of the motion of an object.
99Velocity-time graph The motion of a moving object can be represented on a velocity – time graph.Virtual Int 2 Physics – can pupils predict – remember area under speed time graph = distance, so area under velocity time graph = displacement.
100Vectors and Direction direction. When dealing with vector quantities wemust have both magnitude anddirection.When dealing with one-dimensionalkinematics (motion in straight lines) weuse + and – to indicate travel in oppositedirections. We use + to indicate accelerationand – to indicate deceleration.
101Velocity-Time Graphs Describe the motion of this object. Constant velocity – does not change with time
102Velocity-Time Graphs Describe the motion of this object. Increasing with time – constant acceleration
103Velocity-Time Graphs Describe the motion of this object. Decreases with time – constant deceleration
104Velocity-Time GraphsDescribe the motion of this object.
105Speed-Time Graphs Calculate the distance covered by the object in the first 10 s of its journey.The area under the graph tells us the distancetravelled.
106Speed-Time Graphs Calculate the distance covered by the object in the first 10 s of its journey.The area under the graph tells us the distancetravelled.
107Describe the effects of forces in terms of their ability to Key words: forces, newton balance, weight, mass, gravitational field strength.By the end of this section you will be able to:Describe the effects of forces in terms of their ability tochange the shape, speed and direction of travel of an object.Describe the use of a newton balance to measure force.State that weight is a force and is the Earth’s pull on anobject.Distinguish between mass and weight.State that weight per unit mass is called the gravitationalfield strength.Carry out calculations involving the relationship between weight, mass andgravitational field strength including situations where g is not equal to 10N/kg.
108What effect can a force have? Force is simply a push or a pull.Some forces (e.g. magnetic repulsion, orattraction of electrically chargedobjects) act at a distance.
109What is force? A force can change the shape of an object change the velocity of an objectchange the direction of travel of an object
110Units of Force?Force (F) ismeasured innewtons (N).
111Measuring Forces A Newton (or spring) balance can be used to measure Show newton balances – hang masses on and read off force.
112Mass and Weight We often use the words mass and weight as though they mean the same…but do they?
113Mass and Weight An object’s mass is a measure of how much “stuff” makes upthat object – how much matter, or howmany particles are in it.Mass is measured ingrams or kilograms.
114Mass and Weight An object’s weight is the force exerted by gravity on a mass.Since it is a force, weight must bemeasured innewtons.
115Investigating the relationship between mass and weight How can we find the relationship betweenmass and weight?A newton balance can be used to find theweight of known masses.
116ResultsMassWeight in N100g200g300g400g500g1kg2kg5kg
117Relationship between mass and weight From this we can see a relationshipbetween mass and weight100g = 0.1 kg -> 1 N1kg -> 10 NTo convert kg -> N multiply by 10To convert N -> kg divide by 10
118Gravitational Field Strength (g) Gravitational field strength on Earth is10 N / kg
119What is gravitational field strength? This is the pull of gravity on eachkilogram of mass.So on Earth, the pull of gravity on a 1kgmass is10 N
120What is gravitational field strength? and the pull of gravity on a 2 kg mass is20 N
121A planet’s gravitational field strength is the pull of gravity on DefinitionA planet’s gravitationalfield strength is thepull of gravity ona 1 kg mass.
122Gravity in the universe Is gravitational field strength always thesame?No! It varies on different planets.
123Your weight on different planets Use the website to find your weight ondifferent planets for a mass of 60 kg (aweight of 600 N on Earth).From this calculate the gravitational fieldstrength for each planet.
124Mass on Earth = 60 kgWeight on Earth = 600 NGravitational field strength =Weight on Mercury = N g =Weight on Venus = N g =Weight on the Moon = 99.6 N g =Weight on Mars = N g =Weight on Jupiter = N g =Weight on Saturn = N g =
12510 N / kg Units for g We found g by dividing weight in newtons by mass in kilograms.What are the units for g?10 N / kg
126Which of the planets has the greatest gravitational field strength?Why do you think this is the case?
127Weight, mass and gravity We have seen that there is a link betweenweight, mass and gravity.On Earth1 kg acted on by 10 N / kg weighs 10 Nm x g = WmassGravitational field strength gweight
128Weight, mass and gravity W = mgWhy is weight measured in newtons?Gravitational field strength measured in N / kgMass measured in kgWeight measured in newtons
129Key words: friction, force By the end of this section you will be able to:State that the force of friction can opposethe motion of an object.Describe and explain situations in whichattempts are made to increase or decreasethe force of friction.18th September 2007
130Frictional Forces Moving vehicles such as cars can slow down due to forces acting on them.These forces can be due to…road surface and the tyresthe brakesair resistance.
131Frictional Forces The force which tries to oppose motion is called the force of friction.A frictional force always acts to slow anobject down.
132Increasing Friction In some cases, we want to increase friction. Some examples of this are:Car brakes – we need friction betweenthe brake shoes and the drum to slowthe car downBicycle tyres – we need friction to give“grip” on the surface
133Increasing Friction On the approach to traffic lights and roundabouts, different road surfaces areused to increase friction compared withnormal roads.
134Decreasing Friction In some cases, we want to decrease friction. Some examples of this are:Ice skatingSkiingAircraft design
135Reducing Friction Friction can be reduced by: Lubricating the surfaces – this generallymeans using oil between two metalsurfaces. This is done in car engines toreduce wear on the engine – metal partsaren’t in contact because of a thin layerof oil between them.
136Reducing Friction Friction can be reduced by: Separating surfaces with air (e.g. ahovercraft).Making surfaces roll (e.g. by using ballbearings).
137Reducing Friction Friction can be reduced by: Streamlining. Modern cars are designedto offer as little resistance (or drag) tothe air as possible, reducing friction onthe car.
138StreamliningCars, aeroplanes and rockets are streamlined (that is, have theirdrag coefficient reduced) by:Reducing the front areaHaving a smooth body shape
139Key words: force, vector, balanced forces By the end of this section you will be ableto:State that force is a vector quantity.State that forces which are equal in size butact in opposite directions on an object arecalled balanced forces and are equivalent tono force at all.Explain the movement of objects in terms ofNewton’s first law.
140Force and direction. Force is a vector quantity. What do we mean by this?To describe it fully we must have sizeand direction.
141Balanced ForcesFFBalanced forces are EQUAL FORCES which act in OPPOSITE DIRECTIONS. They CANCEL EACH OTHER OUT.
142If balanced forces act on a STATIONARY OBJECT, it REMAINS STATIONARY. If balanced forces act on a MOVING OBJECT, it continues moving in the same direction with CONSTANT VELOCITY.F
143This is summarised by NEWTON’S FIRST LAW which states: An object remains at rest, or moves in a straight line with constant velocity unless an UNBALANCED FORCE acts on it.
144To understand NEWTON’S FIRST LAW remember: An object tends to want to keep doing what it is doing (so if it is sitting still it wants to stay that way, and if it is moving with constant velocity it wants to keep going).
145This reluctance to change motion is known as inertia. The greater the mass, the greater the reluctance.Think! Is it easier to stop a tennis ball travelling towards you at 10 m/s or to stop a car travelling towards you at 10 m/s?
146Forces and Supported Bodies A stationary mass mhangs from a rope.What is the weight ofthe mass? In whatdirection doesthis act?W = mg downwardsm
147Forces and Supported Bodies The mass is stationary.Newton’s law tells usthat the forces mustbebalanced forces.The weight iscounterbalanced by aforce of the same sizeacting upwards due tothe tension in thestring.m
148Forces and Supported Bodies A book of mass mrests on a shelf.What is the weight ofthe book? In whatdirection doesthis act?W = mg downwardsm
149Forces and Supported Bodies The mass is stationary.Newton’s law tells usthat the forces must bebalanced forces.The weight iscounterbalanced by aforce of the same sizeacting upwards due tothe shelf.m
150What forces are acting on this stationary hovering helicopter? lift =W = mgW = mg
151Newton’s First Law Newton’s first law tells us that when the forces on an object are balanced, astationary object will remain stationary.But it also says that if when forces arebalanced, an object moving at constantvelocity will continue in the same directionwith the same velocity.
152The ENGINE FORCE and the FRICTION FORCE must be equal. A moving carIf a car moves with constant velocity, then what forces are acting on it?The ENGINE FORCE and the FRICTION FORCE must be equal.Engine forceFriction force
153Newton’s Law & Car Seat Belts If a car stops suddenly, someone inside the car appears to be “thrown forwards”.In fact, they simply carry on moving with the car’s previous speed.A seat belt prevents this happening by applying an unbalanced force to the person, in the direction opposite to motion. This causes rapid deceleration.
154No seatbelt – what’s going to happen when the car hits the wall? Explain this in terms of Newton’s 1st law.
155What’s going to happen when the motorbike hits the wall? Explain this in terms of Newton’s 1st law.
156Air bagsAir bags produce a similar effect to seatbelts. They apply a force which opposes the motion, causing rapid deceleration.The large surface area also spreads the force of impact, reducing the pressure and reducing injury.
157Forces in a Fluid Terminal velocity Any free-falling object in a fluid (liquid or gas) reaches a top speed, called ‘terminal velocity’.
158Terminal VelocityThe air resistance acting on a moving object increases as it gets faster.Terminal velocity is reached when the air-resistance (acting upwards) has increased to the same size as the person’s weight (acting downwards)
159a = -10 m/s2 time = 0s, velocity = 0 m/s, friction = 0 N Friction Ff(air resistance) = 0 Na = -10 m/s2W = weight
162Velocity – Time Graphvelocity(m/s)Terminal velocitytime (s)
163air resistanceTerminal velocity is reached when the air resistance balances the weight.weight
164Terminal Velocity What effect does opening a parachute have on the terminal velocity?When the parachute is opened, air resistanceincreases a lot. There is now an unbalanced forceupwards, which causes deceleration. The velocitydecreases, and the air resistance decreases untilthe forces are balanced again. The parachutistfalls to the ground with a lower terminal velocity.
165Key words: Newton’s second law, unbalanced forces, mass, force, accelerationBy the end of this section you will be able to:Describe the qualitative effects of the change ofmass or of force on the acceleration of an objectDefine the newtonCarry out calculations using the relationshipbetween a, F and m and involving more thanone force but in one dimension only
166The example of the parachutist accelerating until the forces are balanced helps us to understand NEWTON’S SECOND LAW which states:When an object is acted on by a constant UNBALANCED FORCE the body moves with constant acceleration in the direction of the unbalanced force.
167Force, mass and acceleration Acceleration (m/s2)F = maForce (N)mass (kg)
168Force, mass and acceleration One newton (1N) is the force required toaccelerate 1 kg at 1 m/s2
169F = maFind the unbalanced force required to accelerate a 4 kg mass at 5 m/s2What do I know?m = 4kga = 5m/s2F = maF= 4 x 5F = 20 N
170Key words: free body diagrams, resultant force By the end of this section you will be ableto:Use free body diagrams to analyse the forceson an objectState what is meant by the resultant of anumber of forcesUse a scale diagram, or otherwise, to find themagnitude and direction of the resultant oftwo forces acting at right angles to eachother.
171Newton’s First Law A body remains at rest, or continues at constant velocity, unless acted upon by anexternal unbalanced force.(that is objects have a tendency to keepdoing what they are doing)
172Newton’s Second Law Newton’s Second Law is about the behaviour of objects when forces are notbalanced.The acceleration produced in a body isdirectly proportional to the unbalancedforce applied and inversely proportional tothe mass of the body.
173Newton’s Second Law In practice this means that the acceleration produced increases asthe unbalanced force increasesthe acceleration decreases as the mass ofthe body increases
174Which forces? An object may be acted upon by a number of forces but only an overall unbalanced forcewill lead to acceleration in the directionof that force.
175Forces are measured in…? Newton’s Second Law can be written asor more commonly
176Forces are measured in…? which gives us the definition of the Newton:1N is the resultant (or unbalanced)force which causes a mass of 1kg toaccelerate at 1m/ s2
177Quick Quiz 5 10 5 10 1 Unbalanced force (N) Mass (kg) Acceleration (m/ s2)10220455105101
178Direction of force To the right To the right Consider the oil drop trail left by the carin motion.In which direction is the acceleration?In which direction is the unbalancedforce?To the rightTo the right
179Direction of force Consider the oil drop trail left by the car in motion.In which direction is the unbalancedforce?To the left – the car is moving to the right and slowing down.
180Newton’s First and Second Laws RememberForces do not cause motionForces cause acceleration
181Free-Body Diagrams Free body diagrams are special examples of a vector diagram.They show the relative magnitudeand direction of all forces actingon an object.They are used to help you identifythe magnitude and direction of anunbalanced Force acting on anobject.Weight
182Using Newton’s Second Law In the simplest casemFun
183Using Newton’s Second Law Direction of acceleration?Direction of unbalanced force?Formula for calculating acceleration?mF1F2Fu
184Solving Problems Always draw a diagram showing all known quantities (forces – magnitude anddirection, resultant acceleration anddirection, mass of object(s) )Remember that Fun=ma can be applied tothe whole systemWhen working in the vertical directionalways include the weight
185Key words: acceleration, gravitational field strength, projectiles By the end of this section you will be ableto:Explain the equivalence of acceleration due togravity and gravitational field strengthExplain the curved path of a projectile interms of the force of gravityExplain how projectile motion can be treatedas two separate motionsSolve numerical problems using the above methodfor an object projected horizontally.
186Acceleration due to Gravity Definition:A planet’s gravitational field strength equals the force of gravity PER UNIT MASS.Units?N/kgTo calculate an object’s weight, use this equation -
187Acceleration due to Gravity Near a planet’s surface all objects experience the same gravitational acceleration.This acceleration is numerically equal to the planet’s gravitational field strength.
188Acceleration due to Gravity For example, on Earth –g = 10 N/kgA free-falling object will experience acceleration of a = -10 m/ s2What does the –ve sign tell you?
189Gravitational field strength Is the gravitational field strength the same on eachplanet?How does distance affect gravitational field strength?It decreases the further away you are from the planet’ssurface.What will happen to the weight of an object as it getsfurther from the surface? Explain your answer.It will decrease.
190The force of gravity near the Earth’s surface givesall objects the sameacceleration.So why doesn’t thefeather reach the groundat the same time as theelephant?
192An object is released from rest close to the Earth’s surface. Which formula can be used to find its velocityat a given time?v = u + atwhere v = ? u = a = t =What is its velocity:At the time of release?After 1 second?After 2 seconds?After 3 seconds?After 4 seconds?
194Forces acting on projectiles What would happen to a ball kicked off acliff, in the absence of gravity?
195Forces acting on projectiles There would be no vertical motiontherefore the ball would continue atconstant speed in a straightline (remember Newton’s first law)
196Objects projected horizontally Think about… What is the initial vertical speed of aprojectile fired horizontally?How will the horizontal speed vary duringthe object’s flight?0 m/sIt will remain the same as the initial horizontal speed.
197Objects projected horizontally Think about… Describe the vertical motion of an objectprojected horizontally:It will accelerate downwards due to gravity.
198Objects projected horizontally Think about… What formula can be used to find thehorizontal displacement of an objectfired horizontally if horizontal velocityand time of flight are known?time of flight (s)horizontal displacement (m)sh = vhthorizontal velocity (m/s)
200Summary Horizontal motion Vertical motion Forces Are there forces present? If so, inwhat direction arethey acting?NoYesThe force of gravityacts downwardAccelerationIs there acceleration?If so, in whatdirection? What isthe value of theacceleration?Acceleration = "g" downwardat 10 m/s2VelocityConstant or changing?ConstantChangingby 10 m/s each second
201Solving Numerical Problems Always write down what you know – many questions have a lot of text surrounding the Physics so pick out the information from the questionWrite down other relevant information you have e.g. acceleration due to gravitySelect formula – this isn’t a test of memory so while you should learn your formulae, don’t be afraid to check against the data book or text bookSubstitute values and rearrange formulaWrite answer clearly remembering magnitude and direction, and units.
202Example A flare is fired horizontally out to sea from a cliff top, at a horizontal speed of 40 m/s. Theflare takes 4 s to reach the sea.What is the horizontal speed of the flare after 4 s?There are no forces acting in the horizontal. Thehorizontal speed remains the same = 40 m/s.
203Example (b) Calculate the vertical speed of the flare after 4s final speed v = ?initial vertical speed u = 0 m/s Initial vertical speed is always 0 m/s!acceleration a = 10 m/s2time t = 4 sv = u + atv = x 4v = 40 m/s
204Example (c) Draw a graph to show how vertical speed varies with time. Initial vertical speed = 0 m/sFinal vertical speed = 40 m/s
205Example (d) Use this graph to calculate the height of the cliff. Displacement = area under velocity-time graph½ bh = ½ x 4 x 40= 80 mHeight of cliff = 80 m
206Key words: Newton’s third law, newton pairs By the end of this section you will be able to:State Newton’s third lawIdentify “Newton pairs” in situations involvingseveral forcesState that momentum is the product of massand velocity.State that momentum is a vector quantity.
207Forces acting between objects Newton realised thatWhen a body is acted upon by a force there must be another body which also has a force acting on it. The forces are equal in size but act in opposite directions.
208Newton’s Third LawIf object A exerts a force on object B, then B exerts an equal and opposite force on AForces always occur in equal and opposite pairsFor every action there is an equal and opposite reaction
209Firing a gunForce of GUN on BULLETForce of BULLET on GUN
210Force of RUNNER on BLOCKS Starting a sprintForce of RUNNER on BLOCKSForce of BLOCKS on RUNNER
211A falling appleForce of EARTH on APPLEForce of APPLE on EARTH
212A RocketForce of GAS on ROCKETForce of ROCKET on GAS
213Key words: work done, energy, force, distance, power, time By the end of this section you will be ableto:State that work done is a measure of theenergy transferred.Carry out calculations involving therelationship between work done, force anddistance.relationship between work done, power andtime.
214Work done? What is meant by work done in Physics? When a force acts upon an object tocause a displacement of the object, it issaid that work was done upon the object.
215Work done? There are three key ingredients to work – force, displacement, and cause.In order for a force to qualify as having donework on an object, there must be adisplacement and the force must cause thedisplacement.Note; at this level, we can use distance instead of displacement.
216Work done? Formula linking work done, force and displacement? Examples of work done?a horse pulling a plough through the fielda shopper pushing a grocery cart down the aisle of a supermarketa pupil lifting a backpack full of books upon her shouldera weightlifter lifting a barbell above his headan Olympian launching the shot-put, etc.In each case described here there is a force exerted upon anobject to cause that object to be displaced.
217Work done A dog pulls a 4 kg sledge for a distance on 15 m using a force of 30 N. How muchwork does he do?What do I know?F = 30Nd = 15m
219Power Power is the rate of doing work i.e. if work is done then the work done persecond is the power.Energy in joulestime in secondsPower in watts (joules per seconds)
220Power A dog pulls a 4 kg sledge for a distance on 15 m using a force of 30 N in 20 s.Calculate the power of the dog.What do I know?F = 30Nd = 15mt = 20s
221PowerWhat do I know?F = 30Nd = 15mt = 20sFormula?
222PowerWhat do I know?F = 30Nd = 15mt = 20sEw = 450JFormula?
223By the end of this section you will be able to: Key words: gravitational potential energy,mass, gravitational field strength, kineticenergyBy the end of this section you will be ableto:Carry out calculations involving the relationshipbetween change in gravitational potential energy,mass, gravitational field strength and change inheight.between kinetic energy, mass and velocity.
224Gravitational Potential Energy …is the potential energygained by an object whenwe do work to lift itvertically in a gravitationalfield.
225Gravitational Potential Energy The work done in lifting anobject verticallyWhat force is required?
226Gravitational Potential Energy To lift the object we must overcome the weight W=mg
227Gravitational Potential Energy Vertical distance – we call this height h