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24/05/2017 Space for Reflection W Richards The Weald School 24/05/2017 P5a Satellites, Gravity and Circular Motion The Earth’s Orbit Ellipse 24/05/2017 Gravity 24/05/2017 Gravity is an attractive force that affects anything with mass: Note that this force goes both ways – the Earth is attracted to us. Centripetal force and The Earth 24/05/2017 Gravity (and the fact that the Earth is moving at high speeds) keeps the Earth in orbit. Notice that the orbit path is slightly elliptical Orbit times 24/05/2017 Mercury = 88 days Mercury Venus Earth Mars Mars = 687 days Pluto = Jupiter 90,500 days Saturn Uranus Neptune Pluto Satellites 24/05/2017 The word “satellite” can be used to describe anything that _____ something else. The moon is a _______ satellite of the Earth and we also have many ________ satellites. All of these satellites are continually ________ towards the Earth but their “tangential” ________ keeps them moving in a circular orbit. Words – velocity, orbits, accelerating, natural, artificial Comets 24/05/2017 Comets are balls of dust and frozen gas. They have very elliptical orbits: What happens to the speed of the comet when it approaches the sun and why? More information on gravity 24/05/2017 The amount of gravity attracting an object decreases the further out the object is… F If you double the distance the gravitational force divides by 4… F/4 If you triple the distance the force divides by 9… F/9 Artificial Satellites Geostationary orbits: 24/05/2017 Low polar orbits: Artificial Satellites 24/05/2017 Artificial satellites have been around for 50 years and have 3 main uses: 1) Observation (e.g. Hubble Space Telescope) – these are in orbit high above the Earth and can observe the universe without interference by the ____________ 2) Communications (e.g. ___, phone, car “SatNav” systems) – these satellites are in “geostationary” orbits. This means that the satellite always stays above ____ ____ point on the Earth and takes a ______ to complete an orbit 3) Monitoring (e.g. weather, spy satellites) – these satellites have a “___ _____” orbit and may scan around the Earth several times a day and travel _____. Words – the same, atmosphere, low polar, TV, day, faster 24/05/2017 P5b Vectors and Equations of Motion Speed vs. Velocity 24/05/2017 Speed is simply how fast you are travelling… This car is travelling at a speed of 20m/s Velocity is “speed in a given direction”… This car is travelling at a velocity of 20m/s east Relative Speed 24/05/2017 Consider two cars driving past each other: 20mph 50mph What is the relative speed of the cars compared to each other? Circular Motion 1) Is this car travelling at constant speed? 2) Is this car travelling at constant velocity? 24/05/2017 Speed vs. Velocity Speed = Speed = Velocity = Velocity = 24/05/2017 Start -1 metre 1 metre Speed Speed = = Velocity Velocity = = “Speed” is how fast you go. “Velocity” is how fast in a given direction. Vector vs. scalar 24/05/2017 Scalar quantities have size (“magnitude”) only and no direction. Vector quantities have both size and direction. Scalar or vector??? Scalar Vector 8. Power 2. Distance12. Acceleration 1. Mass 6. Energy 7. Time 3. Displacement 4. Speed 11. Force 10. Current 5. Velocity 9. Momentum Adding Vectors 24/05/2017 Calculate the “resultant vector” for these pairs of vectors: 10N 5N 10km 100ms-1 5ms-1 10km 14.1km 100.1ms-1 24/05/2017 Distance, Speed and Time revision Speed = distance (in metres) time (in seconds) S V T 1) Simon walks 200 metres in 40 seconds. What is his speed? 2) Howard covers 2km in 1,000 seconds. What is his speed? 3) How long would it take Ryan to run 100 metres if he could run at 12m/s? 4) Ben throws a book at Dan and it travels at 50m/s for 0.2s. How far away was Dan? 5) Chris is learning to drive. He drives his car at 85mph (about 40m/s). How long does it take him to drive 20km? Equations of Motion u+v s= 2 t v = u + at s = ut + ½at2 v2 = u2 + 2as 24/05/2017 Some hard questions 24/05/2017 1) Ben drops a ball on Dan’s foot. How long does the ball take to fall 1m? 2m? Why is the second answer not twice the first? 2) Ryan flies to Belgium. His aeroplane has a maximum acceleration on the ground of 3.4ms-2. What is the minimum length of runway needed to reach its take off speed of 110ms-1 and how long will this take? 3) Luke likes watching kangaroos. A kangaroo jumps to a vertical height of 2.8m. For how long was it in the air? 4) Tom likes baseball. A baseball pitcher can release a ball at 40ms-1 after accelerating through a distance of 2.5m. Calculate the average acceleration of the ball. P5c Projectile Motion 24/05/2017 Vertical Projection 24/05/2017 If I throw this ball upwards with a speed of 40ms-1 how high will it go? Use v2 = u2 + 2as 0 = 402 + (2 x -9.81 x s) 0 = 1600 – 19.62s 1600 = 19.62s s = 1600/19.62 s = 81.5m Practice Questions 24/05/2017 1) How far will a cricket ball go if it is thrown upwards with an initial velocity of 10ms-1? 2) How far will a table tennis ball go if it is thrown upwards with an initial velocity of 5ms-1? 3) A human cannonball is projected vertically upwards and she reaches a vertical height of 20m before coming back down. How fast was she going when she left the cannon? 4) A test tube falls off the table. If the table is 1m high how fast was the test tube going when it hit the floor? Projectile Motion Aha! If I let go of the branch when he fires his gun I’ll be safe because the bullet will go above me… This curved path is called a “trajectory” and its shape is “parabolic”. 24/05/2017 Analysing Projectile Motion 24/05/2017 Throughout this motion the horizontal velocity stays the same: The vertical velocity changes due to the effect of gravity: Projectile Motion 24/05/2017 Question – how long did this take and how fast was the bullet? 1.5m 50m 1) Use x = ut + ½at2 vertically to find the time 2) Then use speed = distance / time horizontally to get the speed Example questions 24/05/2017 1) Ben throws a bowling ball at Tom and it lands on his foot. If the ball started 1.2m above Tom’s foot and the distance between them was 2m calculate both the time taken and the initial velocity of the ball. 2) Rob fires a gun and the bullet leaves the barrel at a speed of 200ms-1. If it landed on the ground 500m away calculate how long the journey took and how high up Rob was holding the gun from ground level. 3) Andrew likes knocking test tubes off the table. If he hits one with an initial velocity of 2ms-1 and the table is 1m high calculate the time taken for the test tube to hit the floor and how far away from the table it landed. Recap questions 24/05/2017 1) Andrew Murray hits a tennis ball and it passes horizontally over the net and lands just inside the baseline of the court. The net has a height of 1.07m and is 11.9m from the baseline. Find the horizontal speed of the ball. 2) Ronaldo takes a free kick and it flies into the top corner horizontally. If the corner is 2.4m above the ground and the goal is 18m away calculate the time taken for the ball to reach the goal. Projectile Motion in Sport 24/05/2017 When playing golf, football, throwing a javelin etc the range clearly depends on the angle. What is the best angle for the longest range? P5d Action and Reaction 24/05/2017 Action and reaction 24/05/2017 When body A exerts a force on body B, body B exerts an equal and opposite force on body A. My third law of motion! My third law says that if I push to the right I will move backwards as well. Newton 1642-1727 Action and reaction What will happen if I push this satellite away from me? 24/05/2017 Action and reaction Consider a man standing on the Earth: 24/05/2017 Momentum 24/05/2017 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 kgms-1) (in kg) (in ms-1) M What is the momentum of the following? 1) A 1kg football travelling at 10ms-1 2) A 1000kg Ford Capri travelling at 30ms-1 3) A 20g pen being thrown across the room at 5ms-1 4) A 70kg bungi-jumper falling at 40ms-1 V The Conservation of Momentum 24/05/2017 In any collision or explosion, momentum is conserved. Here is an example: Before I fired this gun I had no momentum. This means that after I fired the gun I must also have no momentum. Therefore, if the bullet goes forwards, I will have to move backwards to balance the bullet’s momentum. Conservation of Momentum 24/05/2017 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? Speed = 50ms-1 Mass = 1000kg Speed = 20ms-1 Mass = 800kg Mass = 1800kg Speed = ??ms-1 Momentum before = momentum after… …so 1000 x 50 + 800 x 20 = 1800 x V… …V = 36.7ms-1 24/05/2017 Momentum in different directions What happens if the bodies are moving in opposite directions? Speed = 50ms-1 Mass = 1000kg Speed = 20ms-1 Mass = 800kg 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 More questions… 24/05/2017 1) A white snooker ball moving at 5ms-1 strikes a red ball and pots it. Both balls have a mass of 1kg. If the white ball continued in the same direction at 2ms-1 what was the velocity of the red ball? 2) A car of mass 1000kg heading up the M1 at 50ms-1 collides with a stationary truck of mass 8000kg and sticks to it. What velocity does the wreckage move forward at? 3) A defender running away from a goalkeeper at 5ms-1 is hit in the back of his head by the goal kick. The ball stops dead and the player’s speed increases to 5.5ms-1. If the ball had a mass of 500g and the player had a mass of 70kg how fast was the ball moving? 4) A gun has a recoil speed of 2ms-1 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? Another example 24/05/2017 Consider the nuclear decay of Americium-241: 237 93 Np 241 95 Am The momentum before the decay was zero, so the Neptunium atom and alpha particle must have equal and opposite momentum. 4 2 α Particle Motion in Gases 24/05/2017 Gas pressure is caused by particles hitting the side of a container. Anything we do that increases those collisions will increase the pressure: These collisions mean that the particles are changing in momentum every time they hit the sides of the container. Therefore the side of the container is exerting a force back on the particles. Particle Motion in Gases 24/05/2017 Consider decreasing the volume: The particles should collide with the sides of the container _____ often, therefore the pressure is ________. Particle Motion in Gases 24/05/2017 Now consider increasing the temperature as well: The particles should collide with the sides of the container _____ often, therefore the pressure is ________. This could cause the container to ______. Another example 24/05/2017 Consider a rocket: Rockets use the ideas of ______ and Newton’s third law. The gas is under high _______ which means the particles are pushed out with a large ______. The particles therefore exert an equal and opposite reaction on the rocket, pushing it upwards. To launch a satellite into space, a ______ force is needed so you’ll need lots of ____ and it should be under high pressure. Words – fuel, momentum, large, force, pressure P5e Satellite Communication 24/05/2017 Diffraction 24/05/2017 Diffraction is an effect seen when a wave travels around a corner or through a narrow gap: More diffraction if the size of the gap is similar to the wavelength More diffraction if wavelength is increased (or frequency decreased) Artificial Satellites 24/05/2017 Satellites use digital microwaves to send us communications: Geostationary satellites use higher frequency waves The satellite dish needs to be pointed exactly at the satellite as the microwaves are not diffracted much due to the size of the transmitter on the satellite being much bigger than the wavelength of the microwaves. Low polar satellites use lower frequency waves Analogue vs. Digital Signals 24/05/2017 Analogue signals (like talking or music) continually vary in amplitude and/or frequency 1 0 + Digital signals, however, are either off or on, and the information is sent in a series of pulses There are two main advantages of digital: 1) More information can be sent down the same cable 2) Better quality, because a digital signal can be amplified without amplifying the extra noise: Radio Waves 24/05/2017 Some radio waves (frequencies between 30Mhz and 30GHz) pass through the Earth’s atmosphere – therefore microwaves are used to communicate with satellites. Some radio waves (above 30GHz) are absorbed or scattered by the rain and dust in the atmosphere. Some radio waves are reflected by the atmosphere (they have a frequency below 30MHz). Diffraction of Radio Waves 24/05/2017 A high frequency (short wavelength) wave doesn’t get diffracted much – the house won’t be able to receive it… Diffraction depends on frequency… 24/05/2017 A low frequency (long wavelength) wave will get diffracted more, so the house can receive it… P5f Nature of Waves 24/05/2017 Interference 24/05/2017 Interference is seen when two waves of the same type cross: “Reinforcement” “Cancellation” Interference Patterns in Water 24/05/2017 Interference of Sound Waves 24/05/2017 Path Difference Constructive interference Destructive interference 24/05/2017 1st Max Min Max Min 1st Max 2nd Max Phase Difference 24/05/2017 Phase difference means when waves have the same frequency but oscillate differently to each other. For example: These two waves have different amplitudes but the same frequency and hit their peaks at the same time – they are “in phase” These two waves start opposite to each other – they are “in antiphase” Coherence 24/05/2017 Two waves are said to be “coherent” if they have the same frequency and the same constant phase difference. For example: These waves have a different frequency, so phase is irrelevant. Coherence 24/05/2017 These waves have the same frequency, the same amplitude and they are in phase so they are “coherent” How Light Travels Light travels in transverse waves in straight lines: Laser Shadows are evidence of this: However, light can “bend” when it goes through a more dense medium: 24/05/2017 24/05/2017 Interference Patterns from 1 slit Intensity Distance 24/05/2017 Interference Patterns from 2 slits Intensity If white light is used you see this: Distance 24/05/2017 Interference Patterns from 2 slits Consider red light (monochromatic, so it’s coherent) being shone through a “double slit”: The fact that light is diffracted in the first place is evidence that light travels as waves. Constructive interference (reinforcement) For the best effect, the size of the gaps must be comparable to the wavelength Destructive interference (cancellation) Polarisation 24/05/2017 All electromagnetic waves are transverse. Consider one wave: If you looked at it “end on” it might look like this: And lots of them might look like this: Polarisation 24/05/2017 Polaroid sunglasses use this effect to “tint” light. Light reflected from water is also polarised. What would happen if you looked at light reflected from water using polaroid sunglasses? 24/05/2017 Particle Theory and Wave Theory Light can be reflected, refracted and diffracted. These three things are called “wave behaviour” so light must travel as a wave. It makes sense! Isaac Newton, 1643-1727 Ah yes, but light also demonstrates some “particle behaviour” so many scientists say that it travels as a set of particles called photons. However, not everyone agrees with this! Max Planck, 1858-1947 P5g Refraction of Waves 24/05/2017 Refraction through a glass block: 24/05/2017 Wave slows down and bends towards the normal due to entering a more dense medium Wave slows down but is not bent, due to entering along the normal Wave speeds up and bends away from the normal due to entering a less dense medium Refraction 24/05/2017 Refraction is when waves ____ __ or slow down due to travelling in a different _________. A medium is something that waves will travel through. When a pen is placed in water it looks like this: In this case the light rays are slowed down by the water and are _____, causing the pen to look odd. The two mediums in this example are ______ and _______. The amount of refraction that a medium will do can be measured by its “refractive index” – the higher the index, the more refractive the medium is. Words – speed up, water, air, bent, medium Calculating the Refractive Index 24/05/2017 The refractive index can be calculated by comparing the speed of light in the medium to the speed of light in air: Refractive index = Speed of light in vacuum Speed of light in medium 1) If the speed of light is 3x108m/s and a medium slows this down to 2x108m/s what is the medium’s refractive index? 2) Another ray of light enters a medium with refractive index 1.4. If its speed in a vacuum was 3x108m/s what will its speed in this medium be? Dispersion of Light 24/05/2017 Different colours of light have different refractive indices (and a different speed) when travelling through glass. Therefore this happens: RED LIGHT is refracted THE LEAST PURPLE LIGHT is refracted THE MOST Finding the Critical Angle… 24/05/2017 1) Ray gets refracted 3) Ray still gets refracted (just!) 2) Ray still gets refracted 4) Ray gets totally internally reflected THE CRITICAL ANGLE The critical angle is ______ for a medium with high refractive index Calculating the Critical Angle 24/05/2017 Consider when the light was refracted at 90O: THE CRITICAL ANGLE The critical angle can be calculated if you know the refractive index of the two mediums: Sin c = nr ni 1) The refractive index of water is 1.3 and the refractive index for air is 1. Calculate the critical angle for a ray of light emerging from water into air. 2) Determine the critical angle for glass or plastic using the equipment provided. 24/05/2017 Uses of Total Internal Reflection Optical fibres: An optical fibre is a long, thin, _______ rod made of glass or plastic. Light is _______ reflected from one end to the other, making it possible to send ____ chunks of information Optical fibres can be used for _________ by sending electrical signals through the cable. The main advantage of this is a reduced ______ loss. TIR is also used in Cat’s Eyes. Words – communications, internally, large, transparent, signal P5h Optics 24/05/2017 Converging and diverging lenses 24/05/2017 CONVERGING (Convex) Thickest at the centre A thinner convex lens Ray diagrams for convex lenses 24/05/2017 A “distant object” Focal length The rays of light are refracted INWARDS and meet at the focus, F. F The image formed is REAL – in other words, it can be seen on a screen Ray diagrams 24/05/2017 To draw ray diagrams follow these three rules: 1) Draw a ray from the TOP of the object PARALLEL to the axis and then going through F: F F 2) Draw a ray from the TOP of the object going through the CENTRE of the lens (which will be undeviated) 3) Draw a ray from the top of the object through F to the left of the lens and parallel to the axis This image is REAL, INVERTED and DIMINISHED Ray diagrams 2 24/05/2017 If the object is below the axis follow this step: 4) Draw a ray from the bottom of the object parallel to the axis and then up through the focal point: F F 24/05/2017 F F 24/05/2017 F F 24/05/2017 F F 24/05/2017 F F More about lenses 24/05/2017 Compare thin and thick lenses: Notice that these glasses have got a large curvature. How would you make strong glasses but also make them thinner and with less curvature? Practical uses of lenses 24/05/2017 Cameras use the lens arrangement where the object is beyond twice the focal length – this is why they can’t be used at very short range. Magnifying glasses use the arrangement where the object is between the focal point and the lens – this is why they don’t work when you move them away from the object. Magnification 24/05/2017 Basically, magnification means “how much bigger the object looks”: Magnification = Image size Object size Example questions: 1) What is the magnification of a magnifying glass that enlarges a 4mm ant to 20mm? 2) A microscope has a 100x magnification. If it is used to look at a speck of dust that is 0.01mm big how big will it look through the microscope?