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P5 Space 4 Reflection Revision Seminar Centripetal force Consider a ball of Pleistocene attached to some string: The ball is kept in its path by the tension in the string – an example of a CENTRIPETAL FORCE. This force also produces the change in velocity due to the direction constantly changing. This force is INCREASED if you increase the mass of the object, its speed or decrease the radius of the circle. Other examples of centripetal forces: 05/05/2017 Orbits Electrons 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 What is the momentum of the following? 1) A 1kg football travelling at 10m/s 2) A 1000kg Ford Capri travelling at 30m/s 3) A 20g pen being thrown across the room at 5m/s 05/05/2017 4) A 70kg bungi-jumper falling at 40m/s V Conservation of Momentum 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 Teville Gate. Car A collides with the back of car B and the cars stick together. What speed do they move at after the collision? Speed = 50m/s Mass = 1000kg Speed = 20m/s Mass = 800kg Mass = 1800kg Speed = ??m/s Momentum before = momentum after… …so 1000 x 50 + 800 x 20 = 1800 x V… 05/05/2017 …V = 36.7m/s More questions… 1) A white snooker ball moving at 5m/s strikes a red ball and pots it. Both balls have the same mass. If the white ball continued in the same direction at 2m/s what was the velocity of the red ball? 2) 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? 3) 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? 4) 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? (Hint: total momentum before is zero and the gun has a negative velocity afterwards) 05/05/2017 A past exam question… June 2000 Two lorries are travelling in the same direction along a motorway. Lorry A Lorry B Mass = 20,000kg Mass = 30,000kg Speed = 14m/s Speed = 20m/s 1) Calculate the momentum of Lorry A as it travels along the motorway. 2) Calculate the momentum of Lorry B as it travels along the motorway. (3 marks) 3) Lorry B collides with Lorry A and they stick together. Calculate the common speed of the lorries immediately after the collision. 05/05/2017 (3 marks) Newton’s Laws of Motion These are my three laws of motion (summarised): 1) If an unbalanced force acts on an object that object will either accelerate or change direction: F 2) That force is given by F=ma 3) When a force acts on an object there is an equal force acting in the opposite direction (“Action and 05/05/2017 reaction are equal and opposite”) M A Newton’s nd 2 Law Instead of F=ma Newton actually said that the force acting on an object is that object’s rate of change of momentum. In other words… mv Force = Change in momentum (in kgm/s) (in N) Time (in s) F T 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: 1) The change in momentum of the ball, 05/05/2017 2) The speed the ball moves away with Example questions 1) A golfer 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. 2) A tennis player 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. 3) A footballer takes a goal kick by kicking a 0.4kg football 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. 4) A golfer strikes a 200g golf ball away at 50m/s. If he applied a force of 50N calculate how long his club was in 05/05/2017 contact with the ball for. Energy loss in collisions In the “Forces” module we looked at how to calculate an object’s kinetic energy: Kinetic energy = ½ x mass x velocity squared in J in kg in m/s We’ve also said that in a collision momentum is conserved (unless an external force acts). The same cannot usually be said for kinetic energy… For example, consider the following collision. How much kinetic energy is lost? Before Speed = 50m/s Speed = 20m/s Mass = 1000kg Mass = 800kg After 05/05/2017 Mass = 1000kg Speed = 20m/s Mass = 800kg Speed = 30m/s Energy loss in collisions Consider a head-on collision where the cars stick together. How much kinetic energy is lost in this example? Where does all the energy go? Before Speed = 50m/s Speed = 30m/s After Speed = 10m/s In this example more kinetic energy was lost. We say it was a “less elastic collision”. An “elastic collision” is 05/05/2017 one where the kinetic energy is conserved. Diffraction More diffraction if the size of the gap is similar to the wavelength More diffraction if wavelength is increased (or frequency decreased) Diffraction depends on frequency… A high frequency (short wavelength) wave doesn’t get diffracted much – the house won’t be able to receive it… Diffraction depends on frequency… A low frequency (long wavelength) wave will get diffracted more, so the house can receive it… Transmitting information As we said before, different types of electromagnetic radiation can be used to send different types of information, e.g. an optical fibre: Optical fibres have two main advantages: they can send more information compared to electrical cables of the same diameter and with less signal weakening. Another example is radio waves: Ionosphere The longer wavelength waves are transmitted by being reflected off the 05/05/2017 ionosphere (an electrically charged layer in the Earth’s atmosphere). Transmitting information Sounds can be transmitted long distances by converting them into electrical signals first: A microphone converts ______ waves into electrical signals with the same amplitude and ________. These waves are then transmitted through cables where they are regularly _________ to reduce signal loss. Radio waves can also be used as “carrier waves”: Signal ____ wave ______ wave Transmitter Wave is demodulated back into a ____ Words – signal, frequency, modulated, carrier, sound, amplified 05/05/2017 Refraction through a glass block: 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 Finding the Critical Angle… 1) Ray gets refracted 3) Ray still gets refracted (just!) THE CRITICAL ANGLE 2) Ray still gets refracted 4) Ray gets internally reflected 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 _________ or ENDOSCOPES (medical devices that are used to see inside the body) Words to use – communications, internally, large, transparent Dispersion of Light This happens because the light is refracted: RED LIGHT is refracted THE LEAST VIOLET LIGHT is refracted THE MOST Lenses Lenses use the idea of refraction: When light enters a MORE DENSE medium it slows down… A prism uses this idea to split light. This happens because purple light is refracted more than red light Another example: The lens in the eye is used to focus what we see: Converging and diverging lenses CONVERGING (Convex) Thickest at the centre DIVERGING (Concave) Thinnest at the centre Ray diagrams The rays of light are refracted INWARDS and meet at the focus, F. F F The image formed is REAL – in other words, it can be seen on a screen The rays of light are refracted OUTWARDS. A VIRTUAL image is formed – in other words, the image doesn’t actually exist Ray diagrams To draw ray diagrams follow these two rules: 1) Draw a ray from the TOP of the object PARALLEL to the axis and then going through F: F 2) Draw a ray from the TOP of the object going through the CENTRE of the lens (which will be undeviated) This image is REAL, UPSIDE DOWN and SMALLER than the object Practical uses of lenses Cameras use the lens arrangement where the object is beyond 2F – this is why they can’t be used at very short range. Magnifying glasses use the arrangement where the object is between F and the lens – this is why they don’t work when you move them away from the object.