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Physics 2

Physics 2. W Richards The Weald School. Distance, Speed and Time. Speed = distance (in metres) time (in seconds). D. S. T. Seb walks 200 metres in 40 seconds. What is his speed? Lucy covers 2km in 1,000 seconds. What is her speed?

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Physics 2

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  1. Physics 2 W Richards The Weald School

  2. Distance, Speed and Time Speed = distance (in metres) time (in seconds) D S T • Seb walks 200 metres in 40 seconds. What is his speed? • Lucy covers 2km in 1,000 seconds. What is her speed? • How long would it take Freddie to run 100 metres if he runs at 10m/s? • Sue travels at 50m/s for 20s. How far does he go? • Hannah drives her car at 85mph (about 40m/s). How long does it take her to drive 20km?

  3. Distance-time graphs 2) Horizontal line = 4) Diagonal line downwards = • Diagonal line = 3) Steeper diagonal line = 40 30 20 10 0 Distance (metres) Time/s 20 40 60 80 100

  4. 40 30 20 10 0 Distance (metres) Time/s 20 40 60 80 100 • What is the speed during the first 20 seconds? • How far is the object from the start after 60 seconds? • What is the speed during the last 40 seconds? • When was the object travelling the fastest?

  5. Speed vs. Velocity This car is travelling at a speed of 20m/s This car is travelling at a velocity of 20m/s east Speed is simply how fast you are travelling… Velocity is “speed in a given direction”…

  6. Acceleration V-U Acceleration = change in velocity (in m/s) (in m/s2) time taken (in s) A T • A cyclist accelerates from 0 to 10m/s in 5 seconds. What is her acceleration? • A ball is dropped and accelerates downwards at a rate of 10m/s2 for 12 seconds. How much will the ball’s velocity increase by? • A car accelerates from 10 to 20m/s with an acceleration of 2m/s2. How long did this take? • A rocket accelerates from 1,000m/s to 5,000m/s in 2 seconds. What is its acceleration?

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

  8. 80 60 40 20 0 Velocity m/s T/s 10 20 30 40 50 • How fast was the object going after 10 seconds? • What is the acceleration from 20 to 30 seconds? • What was the deceleration from 30 to 50s? • How far did the object travel altogether?

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

  10. Balanced and unbalanced forces Reaction What would happen if we took the road away? Weight

  11. Balanced and unbalanced forces 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

  12. Balanced and unbalanced forces What would happen if we took the road away? The camel’s weight is no longer balanced by anything, so the camel falls downwards…

  13. 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”).

  14. Balanced and unbalanced forces

  15. Balanced and unbalanced forces 1) This animal is either ________ or moving with _____ _____… 2) This animal is getting _________… 3) This animal is getting _______…. 4) This animal is…

  16. Resultant Force Calculate the resultant force of the following: 500N 100N 700N 600N 50N 700N 700N 800N 800N 200N 100N

  17. Force and acceleration F M A 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/s2)

  18. Force, mass and acceleration F M A • A force of 1000N is applied to push a mass of 500kg. How quickly does it accelerate? • A force of 3000N acts on a car to make it accelerate by 1.5m/s2. How heavy is the car? • A car accelerates at a rate of 5m/s2. If it weighs 500kg how much driving force is the engine applying? • A force of 10N is applied by a boy while lifting a 20kg mass. How much does it accelerate by?

  19. Terminal Velocity Consider a skydiver: • 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

  20. 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

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

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

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

  24. Work done W F D 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=Forcexdistance moved in J in N in m

  25. Example questions • 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? • Alicia lifts a laptop 2m into the air with a force of 10N. How much work does she do? • Martin does 200J of work by pushing a wheelbarrow with a force of 50N. How far did he push it? • Chris cuddles his cat and lifts it 1.5m in the air. If he did 75J of work how much force did he use? • Carl drives his car 1000m. If the engine was producing a driving force of 2000N how much work did the car do?

  26. Elastic Potential Energy Elastic potential energy is the energy stored in a system when work is done to change its shape, e.g:

  27. 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 mv2

  28. Example questions • 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? • 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? • Dan is running and has a kinetic energy of 750J. If his mass is 60kg how fast is he running? • 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?

  29. Random questions… • Sophie tries to run 100m in 12 seconds and succeeds. How fast did she run? • Tommy accelerates at a rate of 2m/s2 for 3 seconds. If he started at 10m/s what was his final speed? • Charlie decides to lift his book up into the air. His book has a mass of 100g and he lifts it 50cm. Calculate the work done. • Lewis accelerates from 0 to 10m/s in 5 seconds. If his mass is 70kg how much force did his legs apply? • Rachel rides 1km at a speed of 20m/s. How long did the journey take? • Claire thinks it’s funny to push James with a force of 120N. If James has a mass of 60kg calculate his acceleration. • Lauren slams on the brakes on her bike and her brakes do 20,000J of work. If the combined mass is 100kg what speed was she travelling at? • Tom has a mass of 75kg. If he accelerates from 10 to 20m/s in 2s how much force did he apply?

  30. Random questions… • Georgina amuses herself by throwing things at Sarah. If she throws a ball with a speed of 20m/s and the distance between her and Sarah is 5m how long will it take to reach her? • Mr Richards throws calculators around the room with a force of 20N. If each calculator has a mass of 200g calculate the acceleration. • Sam has a mass of 70kg. What is his weight on Earth, where the gravitational field strength is 10N/kg? • Zak does some work by pushing a box around with a force of 1N. He does 5J of work and decides to call it a day. How far did he push it? • On the moon Matt might weigh 112N. If the gravitational field strength on the moon is 1.6N/kg what is his mass? What will he weigh on Earth? • Dan likes bird watching. He sees a bird fly 100m in 20s. How fast was it flying? • How much kinetic energy would Richard have if he travelled at a speed of 5m/s and has a mass of 70kg?

  31. Momentum Momentum = Mass x Velocity P (in kgms-1)(in kg)(in ms-1) M V Any object that has both mass and velocity has MOMENTUM. Momentum (symbol “p”) is simply given by the formula: What is the momentum of the following? • A 1kg football travelling at 10ms-1 • A 1000kg Ford Capri travelling at 30ms-1 • A 20g pen being thrown across the room at 5ms-1 • A 70kg bungi-jumper falling at 40ms-1

  32. Conservation of Momentum Speed = 50ms-1 Speed = 20ms-1 Speed = ??ms-1 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 M25. Car A collides with the back of car B and the cars stick together. What speed do they move at after the collision? Mass = 1000kg Mass = 800kg Mass = 1800kg Momentum before = momentum after… …so 1000 x 50 + 800 x 20 = 1800 x V… …V = 36.7ms-1

  33. Momentum in different directions Speed = 20ms-1 Mass = 1000kg Mass = 800kg Speed = 50ms-1 What happens if the bodies are moving in opposite directions? Momentum is a VECTOR quantity, so the momentum of the second car is negative… Total momentum = 1000 x 50 – 800 x 20 = 34000 kgms-1 Speed after collision = 34000 kgms-1 / 1800 = 18.9ms-1

  34. Another example α 241 237 Am Np 4 95 93 2 Consider the nuclear decay of Americium-241: If the new neptunium atom moves away at a speed of 5x105 ms-1 what was the speed of the alpha particle?

  35. More questions… • A white snooker ball moving at 5m/s strikes a red ball and pots it. Both balls have a mass of 1kg. If the white ball continued in the same direction at 2m/s what was the velocity of the red ball? • A car of mass 1000kg heading up the M1 at 50m/s collides with a stationary truck of mass 8000kg and sticks to it. What velocity does the wreckage move forward at? • A defender running away from a goalkeeper at 5m/s is hit in the back of his head by the goal kick. The ball stops dead and the player’s speed increases to 5.5m/s. If the ball had a mass of 500g and the player had a mass of 70kg how fast was the ball moving? • A gun has a recoil speed of 2m/s when firing. If the gun has a mass of 2kg and the bullet has a mass of 10g what speed does the bullet come out at?

  36. Force and momentum mv Force = Change in momentum (in kgm/s) (in N) Time (in s) F T Also called “impulse” 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… For example, David Beckham takes a free kick by kicking a stationary football with a force of 40N. If the ball has a mass of 0.5kg and his foot is in contact with the ball for 0.1s calculate: • The change in momentum of the ball (its impulse), • The speed the ball moves away with

  37. Example questions • Ben likes playing golf. He strikes a golf ball with a force of 80N. If the ball has a mass of 200g and the club is in contact with it for 0.2s calculate a) the change in momentum of the golf ball, b) its speed. • Nick thinks it’s funny to hit tennis balls at Tom. He strikes a serve with a force of 30N. If the ball has a mass of 250g and the racket is in contact with it for 0.15s calculate the ball’s change in momentum and its speed. • Dan takes a dropkick by kicking a 0.4kg rugby ball away at 10m/s. If his foot was in contact with the ball for 0.1 seconds calculate the force he applied to the ball. • Simon strikes a 200g golf ball away at 50m/s. If he applied a force of 50N calculate how long his club was in contact with the ball for.

  38. Safety features mv F T Let’s use Newton’s Second Law to explain how airbags work: Basically: • The change in momentum is the same with or without an airbag • But having an airbag increases the time of the collision • Therefore the force is reduced

  39. Static Electricity + - - + Static electricity is when charge “builds up” on an object and then stays “static”. How the charge builds up depends on what materials are used: + + - - + + + - - - + + - + - -

  40. Static Electricity - - + - - + + - - - - -

  41. Van de Graaf generators

  42. Uses of Static – Smoke Precipitators Chimney Negatively charged plates - - - - - - Positively charged grid + + +

  43. Uses and dangers of Static Electricity Find out how static electricity is used in the following: 1) Photocopiers 2) Paint sprayer Find out how static electricity is dangerous in the following situations: 1) Fuel pipes 2) Hospitals

  44. Circuit Symbols Variable resistor A V Diode Switch Bulb Ammeter Voltmeter LDR Resistor Cell Fuse Thermistor Battery

  45. Electric Current Note that electrons go from negative to positive + - e- e- Electric current is a flow of negatively charged particles (i.e. electrons). By definition, current is “the rate of flow of charge”

  46. Basic ideas… Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____. Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______, a unit named after Volta. Resistance is anything that resists an electric current. It is measured in _____. Words: volts, amps, ohms, voltage, ammeter, voltmeter

  47. More basic ideas… If a battery is added the current will ________ because there is a greater _____ on the electrons If a bulb is added the current will _______ because there is greater ________ in the circuit

  48. Current in a series circuit If the current here is 2 amps… The current here will be… The current here will be… And the current here will be… In other words, the current in a series circuit is THE SAME at any point

  49. Current in a parallel circuit Here comes the current… Half of the current will go down here (assuming the bulbs are the same)… And the rest will go down here… A PARALLEL circuit is one where the current has a “choice of routes”

  50. Current in a parallel circuit And the current here will be… The current here will be… The current here will be… The current here will be… If the current here is 6 amps

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