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Motion

Motion. 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?

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Motion

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  1. Motion

  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? • Farley 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. Vector vs Scalar • A quantity with a magnitude only is called a scalar. Eg distance, speed, mass • A quantity with a magnitude and a direction is called a vector. Eg velocity, weight, acceleration • What would the following quantities be? Force, Time, Momentum, Energy

  4. Displacement • Displacement is defined as the change in position of a body. • s = final position – initial position • “As the crow flies” • It is a vector quantity with a magnitude (m) and a direction

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

  6. 40 30 20 10 0 Displacement (metres) Time/s 20 40 60 80 100 • What is the velocity during the first 20 seconds? • How far is the object from the start after 60 seconds? • What is the velocity during the last 40 seconds? • When was the object travelling the fastest?

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

  8. Mohit “The Jogger” Punjabi Mohit runs 100m north in a time of 20 s, then turns and walks 50m south to go home and study in a further 25 s. a) Calculate Mohit’s average speed whilst running b) What is his average velocity whilst running? c) What is his average speed for the 150m? d) What is his average velocity for the whole activity? e) What is Mohit’s average velocity in kmh-1?

  9. Answers • 5.0 ms-1 • 5.0 ms-1 north • 3.3 ms-1 • 1.1 ms-1 north • 4.0 kmh-1

  10. Murdoch College “Olympics” record holders Calculate the following record breaking speeds in ms-1

  11. Murdoch College “Olympics” record holders Calculate the following record breaking speeds in ms-1

  12. Acceleration V-U Acceleration = change in velocity (in m/s) (in m/s2) time taken (in s) a t • Clarissa runs away from Oliver and accelerates from 0 to 10m/s in 5 seconds. What is her acceleration? • A ball is dropped by Laurence on Ampere’s head from the window and accelerates downwards at a rate of 10m/s2 for 4 seconds. What velocity will the ball hit Ampere at? • Mohit’s scooter accelerates from 10 to 20m/s with an acceleration of 2m/s2. How long did this take? • Zhilin joins NASA and travels on a rocket which accelerates from 1,000m/s to 5,000m/s in 20 seconds. What is his acceleration?

  13. 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

  14. 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?

  15. The 4 Acceleration equations • V = u + at • s = t( u + v/2) [average velocity by time] • s = ut + ½ at2 [substitute 1 into 2] • v2 = u2 + 2as [square both sides of 1]

  16. Acceleration Shaun decides he is da man and takes off from the traffic lights at the acceleration shown below. What is his acceleration after 1s, 2s and 3s? Has he accelerated at constant acceleration? 0 ms-1 10 ms-1 20 ms-1 30 ms-1 t = 0s t = 1s t = 2s t = 3s

  17. Acceleration Next traffic lights Shaun accelerates at the same rate but does not look so cool as Nicholas in his hotted up race car does not need to stop and races through the lights just as they turn green traveling at 30 ms-1. How long before Nicholas catches Shaun? 0 ms-1 10 ms-1 20 ms-1 30 ms-1 t = 0s t = 1s t = 2s t = 3s Go Sandeep

  18. Shaun’s junk heap vs Nicholas’s Speedster

  19. Parachutin’ Stefan • Stefan decides that he has had enough of Physics and he jumps out of a plane. • Thankfully Rhys was looking after him and put a parachute on his back. If Stefan jumps out at 1000m how long would you expect him to take to hit the ground if their was no resistance due to the wind? s = ut + ½ at2 1000 = ½ 9.8 x t2 t = 14.28 s

  20. Parachutin’ Stefan • What Velocity will Stefan theoretically hit at if he did not reach terminal velocity and forgot to open his chute? v = u + at v = 0 + 9.8 ms-2 x 14.28 s v = 139.9 ms-1

  21. Parachutin’ Stefan • If Stefan strikes the ground and rebounds at 5.0 ms-1. • What is his change in speed? • What is his change in velocity? • Assume he was travelling at v = 140 ms-1 down. 1. Change in speed = 140 -5 = 135 ms-1 2. Change in velocity = 140 down – 5 up = 140 + 5 = 145 ms-1 Up

  22. Part II Forces

  23. 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

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

  25. 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

  26. 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…

  27. 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…

  28. Balanced and unbalanced forces

  29. 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)

  30. 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?

  31. 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

  32. 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 ________ _______.

  33. 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

  34. 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?

  35. Friction • What is friction? • Give 3 examples where it is annoying: • Give 3 examples where it is useful: • What effect does friction have on the surfaces?

  36. Stopping a car… Thinking distance Braking distance Tiredness Too much alcohol Too many drugs Poor visibility Looking at girls / boys Wet roads Icy roads Tyres/brakes worn out Driving too fast

  37. 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

  38. Example questions • Mark 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? • Annielyn 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? • Joseph cuddles his cat and lifts it 1.5m in the air. If he did 75J of work how much force did he use? • Nicholas drives his car 1000m. If the engine was producing a driving force of 2000N how much work did the car do?

  39. 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

  40. 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?

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