Speed vs Velocity 1 Is this car travelling

Speed vs. Velocity 1) Is this car travelling at constant speed? 2) Is this car travelling at constant velocity? 17/03/2018

Vector vs. scalar 17/03/2018 Scalar quantities have size (“magnitude”) only and no direction. Vector quantities have both size and direction. Scalar or vector? ? ? Scalar Vector 2. Distance Acceleration 10. 1. Mass 6. Energy 7. Time 3. Displacement 4. Speed 9. Force 8. Current 5. Velocity

17/03/2018 Distance, Speed and Time D Speed = distance (in metres) time (in seconds) S T 1) Freddie walks 200 metres in 40 seconds. What is his speed? 5 m/s 2) Hayley covers 2 km in 1, 000 seconds. What is her speed? 2 m/s 3) How long would it take Lauren to run 100 metres if she runs at 10 m/s? 4) Jake travels at 50 m/s for 20 s. How far does he go? 5) Izzy drives her car at 85 mph (about 40 m/s). How long does it take her to drive 20 km? 10 s 1000 m 500 s

17/03/2018 Distance, Speed and Time D Speed = distance (in metres) time (in seconds) S T 1) Sarah walks 2000 m in 50 minutes. What is her speed in m/s? 0. 67 m/s 2) Jack tries to walk the same distance at a speed of 5 m/s. How long does he take? 400 s 3) James drives at 60 mph (about 100 km/h) for 3 hours. How far has he gone? 4) The speed of sound in air is 330 m/s. Molly shouts at a mountain and hears the echo 3 seconds later. How far away is the mountain? (Careful!) 300 km 495 m

17/03/2018 Distance-time graphs 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 =

17/03/2018 40 Distance (metres) 30 20 10 0 20 40 60 80 1) What is the speed during the first 20 seconds? 100 Time/s 0. 5 m/s 2) How far is the object from the start after 60 seconds? 40 m 3) What is the speed during the last 40 seconds? 1 m/s 4) When was the object travelling the fastest? 40 -60 s

17/03/2018 Acceleration V-U Acceleration = change in velocity (in m/s) (in m/s 2) time taken (in s) A 1) A cyclist accelerates from 0 to 10 m/s in 5 seconds. What is her acceleration? T 2 m/s 2 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 velocity increase by? 120 m/s 3) A car accelerates from 10 to 20 m/s with an acceleration of 2 m/s 2. How long did this take? 5 s 4) A rocket accelerates from 1, 000 m/s to 5, 000 m/s in 2 seconds. What is its acceleration? 2000 m/s 2

17/03/2018 Acceleration V-U Acceleration = change in velocity (in m/s) (in m/s 2) time taken (in s) A T 1) Will accelerates from standstill to 50 m/s in 25 seconds. What is his acceleration? 2 m/s 2 2) Pierre accelerates at 5 m/s 2 for 5 seconds. He started at 10 m/s. What is his new speed? 35 m/s 3) Elliott is in trouble with the police. He is driving up the A 29 and sees a police car and brakes from 50 m/s to a standstill. His deceleration was 10 m/s 2. How long did he brake for? 5 s 4) Another boy racer brakes at the same deceleration but only for 3 seconds. What speed did he slow down to? 20 m/s

17/03/2018 Velocity-time graphs 1) Upwards line = 80 Velocity m/s 4) Downward line = 60 40 20 0 10 2) Horizontal line = 20 30 40 50 3) Upwards line = T/s

17/03/2018 80 60 Velocity m/s 40 20 0 T/s 10 20 30 40 50 1) How fast was the object going after 10 seconds? 40 m/s 2) What is the acceleration from 20 to 30 seconds? 2 m/s 2 3) What was the deceleration from 30 to 50 s? 3 m/s 2 4) How far did the object travel altogether? 1700 m

17/03/2018 80 60 Velocity m/s 40 20 0 T/s 10 20 30 40 50 1) How fast was the object going after 10 seconds? 10 m/s 2) What is the acceleration from 20 to 30 seconds? 4 m/s 2 3) What was the deceleration from 40 to 50 s? 6 m/s 2 4) How far did the object travel altogether? 1500 m

17/03/2018 80 60 Velocity m/s 40 20 0 T/s 10 20 30 40 50 This velocity-time graph shows Coryn’s journey to school. How far away does she live? 2500 m

Introduction to Forces 17/03/2018 A force is a “push” or a “pull”. Some common examples: Weight (mg) – pulls things towards the centre of the Earth Friction – a contact force that acts against anything moving Air resistance/drag – a contact force that acts against anything moving through air or liquid Upthrust – keeps things afloat

Free body force diagrams 17/03/2018 The Earth pulls Newton down with a gravitational force of 700 N. direction what type size Newton pulls the Earth up with a gravitational force of 700 N. Action and reaction are equal and opposite!!

17/03/2018 Balanced and unbalanced forces 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. Weight

17/03/2018 Balanced and unbalanced forces Reaction What would happen if we took the road away? Weight

Air Resistance 17/03/2018 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”).

Balanced and unbalanced forces 17/03/2018

17/03/2018 Balanced and unbalanced forces 1) This animal is either ____ or moving with _______… 3) This animal is getting _______…. 2) This animal is getting ____… 4) This animal is also either _______ or moving with ______. . Words - Stationary, faster, slower or constant speed?

Summary Complete these sentences… 17/03/2018 If an object is stationary and has NO resultant force on it the object will… If an object is stationary and a resultant force acts on it the object will… If an object is already moving and NO resultant force acts on it the object will… If an object is already moving and a resultant force acts on it the object will… …accelerate in the direction of the resultant force …continue to move at the same speed and the same direction …continue to stay stationary …accelerate in the direction of the resultant force

Resultant Force 17/03/2018 Calculate the resultant force of the following: 500 N 100 N 700 N 600 N 50 N 700 N 200 N 800 N 100 N

Force and acceleration 17/03/2018 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

17/03/2018 Force, mass and acceleration 1) A force of 1000 N is applied to push a mass of 500 kg. How quickly does it accelerate? 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? F M A 2 m/s 2 2000 kg 2500 N 0. 5 m/s 2

17/03/2018 Weight vs. Mass 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 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? g 20 N 1 N 3) James weighs 700 N on the Earth. What is his mass? 70 kg 4) On the moon the gravitational field strength is 1. 6 N/kg. What will James weigh if he stands on the moon? 112 N

Terminal Velocity 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 VELOCITY Words – increase, small, constant, balance, accelerates 17/03/2018

Terminal Velocity 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 velocity, weight 17/03/2018

17/03/2018 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 Diver hits the ground

Topic 4 – Momentum, Energy, Work and Power 17/03/2018

Stopping a car… 17/03/2018 What two things must the driver of the car do in order to stop in time?

Tiredness Too many drugs Heavy vehicle Tyres/brakes worn out Stopping a car… Thinking distance (reaction time) Braking distance 17/03/2018 Too much alcohol Poor visibility Wet/icy roads Driving too fast Total Stopping Distance = Thinking Distance + Braking Distance

17/03/2018 Momentum Any object that has both mass and velocity has MOMENTUM. Momentum (symbol “p”) is simply given by the formula: P Momentum = Mass x Velocity (in kgm/s) (in kg) (in m/s) M V What is the momentum of the following? 1) A 1 kg football travelling at 10 m/s 2) A 1000 kg Ford Capri travelling at 30 m/s 3) A 20 g pen being thrown across the room at 5 m/s 4) A 70 kg bungi-jumper falling at 40 m/s 10 kgm/s 30, 000 kgm/s 0. 1 kgm/s 2800 kgm/s

Conservation of Momentum 17/03/2018 In any collision or explosion momentum is conserved (provided that there are no external forces have an effect). Example question: Two cars are racing around the M 25. Car A collides with the back of car B and the cars stick together. What speed do they move at after the collision? Speed = 50 m/s Mass = 1000 kg Speed = 20 m/s Mass = 800 kg Mass = 1800 kg Speed = ? ? m/s Momentum before = momentum after… …so 1000 x 50 + 800 x 20 = 1800 x V… …V = 36. 7 m/s

17/03/2018 Momentum in different directions What happens if the bodies are moving in opposite directions? Speed = 50 m/s Mass = 1000 kg Speed = 20 m/s Mass = 800 kg Momentum is a VECTOR quantity, so the momentum of the second car is negative… Total momentum = 1000 x 50 – 800 x 20 = 34000 kgm/s Speed after collision = 34000 kgm/s / 1800 = 18. 9 m/s

Another example 17/03/2018 Consider the nuclear decay of Americium-241: 237 93 Np 241 95 Am If the new neptunium atom moves away at a speed of 5 x 105 m/s what was the speed of the alpha particle? 2. 96 x 107 m/s 4 2 α

More questions… 1. A car of mass 1000 kg heading up the M 1 at 50 m/s collides with a stationary truck of mass 8000 kg and sticks to it. What velocity does the wreckage move forward at? 2. A defender running away from a goalkeeper at 5 m/s is hit in the back of his head by the goal kick. The ball stops dead and the player’s speed increases to 5. 5 m/s. If the ball had a mass of 500 g and the player had a mass of 70 kg how fast was the ball moving? 3. A white snooker ball moving at 5 m/s strikes a red ball and pots it. Both balls have a mass of 1 kg. If the white ball continued in the same direction at 2 m/s what was the velocity of the red ball? 4. A gun has a recoil speed of 2 m/s when firing. If the gun has a mass of 2 kg and the bullet has a mass of 10 g what speed does the bullet come out at? 17/03/2018 5. 6 m/s 70 m/s 3 m/s 400 m/s

17/03/2018 Recap question on momentum 1. Matt and Dan are racing against each other over 400 m at Sports Day. Matt is running at 8 m/s and catches up with Dan who is running at 6 m/s. After the collision Matt stops and Dan moves slightly faster. If Matt’s mass is 60 kg and Dan’s is 70 kg calculate how fast Dan moves after the collision. 12. 9 m/s 2. Bobbie is driving her 5 kg toy car around. It is travelling at 10 m/s when it hits the back of Heather’s (stationary) leg and sticks to it. Assuming Heather’s leg can move freely and has a mass of 10 kg calculate how fast it will move after the collision. 3. 3 m/s

Safety features 17/03/2018 How do air bags and crumple zones work? Basically: 1) The change in momentum is the same with or without an airbag 2) But having an airbag increases the time of the collision and therefore reduces the “rate of change of momentum” 3) Therefore the force is reduced

Force and momentum 17/03/2018 Newton’s second law of motion says that the force acting on an object is that object’s rate of change of momentum. In other words… Force = Change in momentum (in kgm/s) (in N) mv-mu Time (in s) Also called “impulse” F T For example, Ronaldo takes a free kick by kicking a stationary football with a force of 40 N. If the ball has a mass of 0. 5 kg and his foot is in contact with the ball for 0. 1 s calculate: 1) The change in momentum of the ball (its impulse), 2) The speed the ball moves away with

Example questions 1) Paddy likes playing golf. He strikes a golf ball with a force of 80 N. If the ball has a mass of 200 g and the club is in contact with it for 0. 2 s calculate a) the change in momentum of the golf ball, b) its speed. 17/03/2018 16 Kgm/s, 80 m/s 2) Courtney thinks it’s funny to hit tennis balls at Kit. She strikes a serve with a force of 30 N. If the ball has a mass 4. 5 Kgm/s, 18 m/s of 250 g and the racket is in contact with it for 0. 15 s calculate the ball’s change in momentum and its speed. 3) Tom takes a dropkick by kicking a 0. 4 kg rugby ball away at 10 m/s. If his foot was in contact with the ball for 0. 1 seconds calculate the force he applied to the ball. 40 N 4) Jenny strikes a 200 g golf ball away at 50 m/s. If she applied a force of 50 N calculate how long her club was in contact with the ball for. 0. 2 s

17/03/2018 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 E in m F D

Example questions 1. Hannah pushes a book 5 m along the table with a force of 5 N. She gets tired and decides to call it a day. How much work did he do? 2. Courtney lifts a laptop 2 m into the air with a force of 10 N. How much work does she do? What type of energy did the book gain? 17/03/2018 25 J 20 J, GPE 3. Tom does 200 J of work by pushing a wheelbarrow with a force of 50 N. How far did he push it? What type of energy did the wheelbarrow gain? 4 m, KE 4. Dan 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? 50 N 5. Simon drives his car 1000 m. If the engine was producing a driving force of 2000 N how much work did the car do? 2 MJ

Stopping a car… 17/03/2018 Recall our earlier situation regarding stopping distances…

Energy and Power 17/03/2018 The POWER RATING of an appliance is simply how much energy it uses every second. In other words, 1 Watt = 1 Joule per second E E = Energy (in joules) P = Power (in watts) T = Time (in seconds) P T

Some example questions 17/03/2018 1) What is the power rating of a light bulb that transfers 120 joules of energy in 2 seconds? 60 W 2) What is the power of an electric fire that transfers 10, 000 J of energy in 5 seconds? 2 KW 3) Rob runs up the stairs in 5 seconds. If he transfers 1, 000 J of energy in this time what is his power rating? 0. 2 MW 4) How much energy does a 150 W light bulb transfer in a) one second, b) one minute? 150 J, 9 KJ 5) Jonny’s brain needs energy supplied to it at a rate of 40 W. How much energy does it need during a 50 minute physics lesson? 120 KJ 6) Lloyd’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? 1. 73 MJ

Gravitational Potential Energy 17/03/2018 To work out how much gravitational potential energy (GPE) an object gains when it is lifted up we would use the simple equation… GPE (Joules) = Weight (newtons) x Change in height (metres) GPE (Remember - W=mg) mg H

Some example questions… 17/03/2018 How much gravitational potential energy have the following objects gained? : 1. A brick that weighs 10 N lifted to the top of a house (10 m), 100 J 2. A 1, 000 kg car lifted by a ramp up to a height of 2 m, 20 KJ 3. A 70 kg person lifted up 50 cm by a friend. 350 J How much GPE have the following objects lost? : 1. A 2 N football dropping out of the air after being kicked up 30 m, 60 J 2. A 0. 5 N egg falling 10 m out of a bird nest, 5 J 3. A 1, 000 kg car falling off its 200 cm ramp. 20 KJ

Kinetic energy 17/03/2018 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

Example questions 1) Bex 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) Emma rides her bike at a speed of 10 m/s. If the combined mass of Emma and her bike is 80 kg what is her kinetic energy? 3) Rob is running and has a kinetic energy of 750 J. If his mass is 60 kg how fast is he running? 4) Josh 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? 17/03/2018 450, 000 J 4000 J 5 m/s 75 kg

Stopping a car… What happens inside the car when it stops? In order to stop this car the brakes must “do work”. This work is used to reduce the kinetic energy of the vehicle and the brakes will warm up. 17/03/2018