# Distance-time graphs 40 30 20 10 0 20 40 60 80100 4) Diagonal line downwards = 3) Steeper diagonal line = 1)Diagonal line = 2) Horizontal line = Distance.

## Presentation on theme: "Distance-time graphs 40 30 20 10 0 20 40 60 80100 4) Diagonal line downwards = 3) Steeper diagonal line = 1)Diagonal line = 2) Horizontal line = Distance."— Presentation transcript:

Distance-time graphs 40 30 20 10 0 20 40 60 80100 4) Diagonal line downwards = 3) Steeper diagonal line = 1)Diagonal line = 2) Horizontal line = Distance (metres) Time/s © George Spencer School 2010

40 30 20 10 0 20 40 60 80100 1)What is the speed during the first 20 seconds? 2)How far is the object from the start after 60 seconds? 3)What is the speed during the last 40 seconds? 4)When was the object travelling the fastest? Distance (metres) Time/s © George Spencer School 2010

Distance-time graph for non-uniform motion 40 30 20 10 0 20 40 60 80100 Distance (metres) Time/s Object is accelerating up to here Object is now decelerating © George Spencer School 2010

Acceleration V-U TA Acceleration = change in speed (in m/s) (in m/s 2 ) time taken (in s) 1)A cyclist accelerates from 0 to 10m/s in 5 seconds. What is her acceleration? 2)A ball is dropped and accelerates downwards at a rate of 10m/s 2 for 12 seconds. How much will the ball’s speed increase by? 3)A car accelerates from 10 to 20m/s with an acceleration of 2m/s 2. How long did this take? 4)A rocket accelerates from 1,000m/s to 5,000m/s in 2 seconds. What is its acceleration? © George Spencer School 2010

Speed-time graphs 80 60 40 20 0 10 20 30 4050 Velocity m/s T/s 1) Upwards line = 2) Horizontal line = 3) Upwards line = 4) Downward line = © George Spencer School 2010

80 60 40 20 0 1)How fast was the object going after 10 seconds? 2)What is the acceleration from 20 to 30 seconds? 3)What was the deceleration from 30 to 50s? 4)How far did the object travel altogether? 10 20 30 4050 Velocity m/s T/s © George Spencer School 2010

Speed-time graph for non-uniform motion 40 30 20 10 0 20 40 60 80100 Distance (metres) Time/s Object’s acceleration is increasing Object’s acceleration is decreasing © George Spencer School 2010

Introduction to Forces A force is a “push” or a “pull”. Some common examples: Weight (mg) – pulls things towards the centre of the Earth Air resistance/drag – a contact force that acts against anything moving through air or liquid Upthrust – keeps things afloat _____ – a contact force that acts against anything moving © George Spencer School 2010

Balanced and unbalanced forces Consider a camel standing on a road. What forces are acting on it? Weight Reaction These two forces would be equal – we say that they are BALANCED. The camel doesn’t move anywhere. © George Spencer School 2010

Balanced and unbalanced forces What would happen if we took the road away? Weight Reaction The camel’s weight is no longer balanced by anything, so the camel falls downwards… © George Spencer School 2010

Air Resistance Air resistance is a force that opposes motion through air. The quicker you travel, the bigger the air resistance: The same applies to a body falling through a liquid (called “drag” or “upthrust”). © George Spencer School 2010

Examples of Air Resistance © George Spencer School 2010

Balanced and unbalanced forces © George Spencer School 2010

Balanced and unbalanced forces 1) This animal is either ________ or moving with _____ _____… 4) This animal is… 2) This animal is getting _________… 3) This animal is getting _______…. © George Spencer School 2010

Force and acceleration If the forces acting on an object are unbalanced then the object will accelerate, like these wrestlers: Force (in N) = Mass (in kg) x Acceleration (in m/s 2 ) F AM © George Spencer School 2010

Force, mass and acceleration 1)A force of 1000N is applied to push a mass of 500kg. How quickly does it accelerate? 2)A force of 3000N acts on a car to make it accelerate by 1.5m/s 2. How heavy is the car? 3)A car accelerates at a rate of 5m/s 2. If it weighs 500kg how much driving force is the engine applying? 4)A force of 10N is applied by a boy while lifting a 20kg mass. How much does it accelerate by? F AM © George Spencer School 2010

Stopping a car… Braking distance Too much alcohol Thinking distance (reaction time) Tiredness Too many drugs Wet roads Driving too fast Tyres/brakes worn out Icy roads Poor visibility © George Spencer School 2010

Car Safety Features © George Spencer School 2010

Work done When any object is moved around work will need to be done on it to get it to move (obviously). We can work out the amount of work done in moving an object using the formula: Work done = Force x distance moved in J in N in m W DF © George Spencer School 2010

Example questions 1.Bori pushes a book 5m along the table with a force of 5N. He gets tired and decides to call it a day. How much work did he do? 2.Alicia lifts a laptop 2m into the air with a force of 10N. How much work does she do? 3.Martin does 200J of work by pushing a wheelbarrow with a force of 50N. How far did he push it? 4.Chris cuddles his cat and lifts it 1.5m in the air. If he did 75J of work how much force did he use? 5.Carl drives his car 1000m. If the engine was producing a driving force of 2000N how much work did the car do? © George Spencer School 2010

Work and Power The POWER RATING of an appliance is simply how much work it does (i.e. how much energy it transfers) every second. In other words, 1 Watt = 1 Joule per second W TP W = Work done (in joules) P = Power (in watts) T = Time (in seconds) © George Spencer School 2010

Some example questions 1)What is the power rating of a light bulb that transfers 120 joules of energy in 2 seconds? 2)What is the power of an electric fire that transfers 10,000J of energy in 5 seconds? 3)Isobel runs up the stairs in 5 seconds. If she transfers 1,000,000J of energy in this time what is his power rating? 4)How much energy does a 150W light bulb transfer in a) one second, b) one minute? 5)Simon’s brain needs energy supplied to it at a rate of 40W. How much energy does it need during a physics lesson? 6)Ollie’s brain, being more intelligent, only needs energy at a rate of about 20W. How much energy would his brain use in a normal day? © George Spencer School 2010

Kinetic energy Any object that moves will have kinetic energy. The amount of kinetic energy an object has can be found using the formula: Kinetic energy = ½ x mass x velocity squared in J in kg in m/s KE = ½ mv 2 © George Spencer School 2010

Example questions 1)Nicole drives her car at a speed of 30m/s. If the combined mass of her and the car is 1000kg what is her kinetic energy? 2)Shanie rides her bike at a speed of 10m/s. If the combined mass of Shanie and her bike is 80kg what is her kinetic energy? 3)Dan is running and has a kinetic energy of 750J. If his mass is 60kg how fast is he running? 4)George is walking to town. If he has a kinetic energy of 150J and he’s walking at a pace of 2m/s what is his mass? © George Spencer School 2010

Terminal Speed Consider a skydiver: 1)At the start of his jump the air resistance is _______ so he _______ downwards. 2) As his speed increases his air resistance will _______ 3) Eventually the air resistance will be big enough to _______ the skydiver’s weight. At this point the forces are balanced so his speed becomes ________ - this is called TERMINAL SPEED Words – increase, small, constant, balance, accelerates © George Spencer School 2010

Terminal Speed Consider a skydiver: 4) When he opens his parachute the air resistance suddenly ________, causing him to start _____ ____. 5) Because he is slowing down his air resistance will _______ again until it balances his _________. The skydiver has now reached a new, lower ________ _______. Words – slowing down, decrease, increases, terminal speed, weight © George Spencer School 2010

Velocity-time graph for terminal velocity… Velocity Time Speed increases… Terminal velocity reached… Parachute opens – diver slows down New, lower terminal velocity reached Diver hits the ground On the Moon © George Spencer School 2010

Gravitational Potential Energy To work out how much gravitational potential energy (GPE) an object gains when it is lifted up we would use the simple equation… GPE = Mass x Acceleration of free-fall x Change in height (Joules) (newtons) (=10N/kg) (metres) © George Spencer School 2010

Some example questions… How much gravitational potential energy have the following objects gained?: 1.A brick that has a mass of 1kg lifted to the top of a house (10m), 2.A 1,000kg car lifted by a ramp up to a height of 2m, 3.A 70kg person lifted up 50m by a ski lift. How much GPE have the following objects lost?: 1.A 200g football dropping out of the air after being kicked up 30m, 2.A 500gN egg falling 10m out of a bird nest, 3.A 1,000kg car falling off its 2m ramp. 4.Nathan when falling 1.5m to the ground after being hit by a van (Nathan’s mass is around 80kg). © George Spencer School 2010

Roller Coasters 1) Electrical energy is transferred into gravitational potential energy 2) Gravitational potential energy is transferred into kinetic energy 3) Kinetic energy is transferred back into gravitational potential energy © George Spencer School 2010

Weight vs. Mass Earth’s Gravitational Field Strength is 10N/kg. In other words, a 1kg mass is pulled downwards by a force of 10N. W gM Weight = Mass x Gravitational Field Strength (in N) (in kg) (in N/kg) 1)What is the weight on Earth of a book with mass 2kg? 2)What is the weight on Earth of an apple with mass 100g? 3)Dave weighs 700N. What is his mass? 4)On the moon the gravitational field strength is 1.6N/kg. What will Dave weigh if he stands on the moon? © George Spencer School 2010

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