Distance-time graphs George Spencer School 2010 2

Distance-time graphs © George Spencer School 2010 2) Horizontal line = 40 4) Diagonal line downwards = 30 Distance (metres) 20 10 0 Time/s 20 1) Diagonal line = 40 60 80 100 3) Steeper diagonal line =

© George Spencer School 2010 40 Distance (metres) 30 20 10 0 Time/s 20 40 60 80 100 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? © George Spencer School 2010 4) When was the object travelling the fastest?

© George Spencer School 2010 Distance-time graph for non-uniform motion 40 Distance (metres) Object is accelerating up to here 30 Object is now decelerating 20 10 0 Time/s 20 40 60 © George Spencer School 2010 80 100

© George Spencer School 2010 Acceleration V-U Acceleration = change in speed (in m/s) (in m/s 2) time taken (in s) A T 1) A cyclist accelerates from 0 to 10 m/s in 5 seconds. What is her acceleration? 2) A ball is dropped and accelerates downwards at a rate of 10 m/s 2 for 12 seconds. How much will the ball’s speed increase by? 3) A car accelerates from 10 to 20 m/s with an acceleration of 2 m/s 2. How long did this take? 4) A rocket accelerates from 1, 000 m/s to 5, 000 m/s in 2 © George Spencer School 2010 seconds. What is its acceleration?

Speed-time graphs © George Spencer School 2010 1) Upwards line = 80 Velocity m/s 4) Downward line = 60 40 20 0 10 20 2) Horizontal line = 30 40 50 3) Upwards line = © George Spencer School 2010 T/s

© George Spencer School 2010 80 60 Velocity m/s 40 20 0 T/s 10 20 30 40 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 50 s? 4) How far did the object travel altogether? © George Spencer School 2010 50

© George Spencer School 2010 Speed-time graph for non-uniform motion 40 Distance (metres) Object’s acceleration is increasing 30 Object’s acceleration is decreasing 20 10 0 Time/s 20 40 60 © George Spencer School 2010 80 100

Introduction to Forces © George Spencer School 2010 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 _____ – a contact force that acts against anything moving School 2010 © George Spencer Upthrust – keeps things afloat

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

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

Air Resistance © George Spencer School 2010 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 © George Spencer School 2010 1) This animal is either ____ or moving with _____… 2) This animal is getting _____… 3) This animal is getting _______…. 4) This animal is… © George Spencer School 2010

Force and acceleration © George Spencer School 2010 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 M A © George Spencer School 2010

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

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

Car Safety Features © George Spencer School 2010

Work done © George Spencer School 2010 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 W in m F © George Spencer School 2010 D

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

Work and Power © George Spencer School 2010 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 W = Work done (in joules) P = Power (in watts) T = Time (in seconds) P T © George Spencer School 2010

Some example questions © George Spencer School 2010 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, 000 J of energy in 5 seconds? 3) Isobel runs up the stairs in 5 seconds. If she transfers 1, 000 J of energy in this time what is his power rating? 4) How much energy does a 150 W light bulb transfer in a) one second, b) one minute? 5) Simon’s brain needs energy supplied to it at a rate of 40 W. 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 20 W. How much energy would his brain use in a normal day? © George Spencer School 2010

Kinetic energy © George Spencer School 2010 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 KE = ½ in m/s mv 2 © George Spencer School 2010

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

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

Gravitational Potential Energy © George Spencer School 2010 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) (=10 N/kg) © George Spencer School 2010 (metres)

Some example questions… © George Spencer School 2010 How much gravitational potential energy have the following objects gained? : 1. A brick that has a mass of 1 kg lifted to the top of a house (10 m), 2. A 1, 000 kg car lifted by a ramp up to a height of 2 m, 3. A 70 kg person lifted up 50 m by a ski lift. How much GPE have the following objects lost? : 1. A 200 g football dropping out of the air after being kicked up 30 m, 2. A 500 g. N egg falling 10 m out of a bird nest, 3. A 1, 000 kg car falling off its 2 m ramp. 4. Nathan when falling 1. 5 m to the ground after being hit by a van (Nathan’s mass is around 80 kg). School 2010 © George Spencer

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

Weight vs. Mass © George Spencer School 2010 Earth’s Gravitational Field Strength is 10 N/kg. In other words, a 1 kg mass is pulled downwards by a force of 10 N. W Weight = Mass x Gravitational Field Strength (in N) (in kg) (in N/kg) M g 1) What is the weight on Earth of a book with mass 2 kg? 2) What is the weight on Earth of an apple with mass 100 g? 3) Dave weighs 700 N. What is his mass? 4) On the moon the gravitational field strength is 1. 6 N/kg. What will © George Spencer School 2010 Dave weigh if he stands on the moon?