9 k speeding up (boardworks)

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9 k speeding up (boardworks)

  1. 1. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 20051 of 41 KS3 Physics 9K Speeding Up
  2. 2. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 20052 of 41 9K Speeding Up Contents Distance, time and speed Balanced and unbalanced forces Friction Summary activities
  3. 3. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 20053 of 41 To work out the speed of an object you need to know: Distance, time and speed  the distance travelled;  how long it took to travel that distance.
  4. 4. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 20054 of 41 Average speed is calculated using this equation: Speed can be measured in different units, e.g. m/s, km/h, km/s, miles per hour. The units of distance and time used will give the units to be used for speed. d s x t formula triangle Calculating average speed total distance total time average speed =
  5. 5. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 20055 of 41 Speed formula triangle
  6. 6. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 20056 of 41 A boy takes 1 hour to travel from his home to the cinema, a distance of 10 km. Calculate his average speed in km/h. d s x t Cover the quantity to be calculated - s (speed) = 10 km/h Speed calculation example 10 km 1 h = d (distance in km)average speed (in km/h) t (time in h) =
  7. 7. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 20057 of 41 3600 s 10,000 m = = 2.8 m/s Sometimes the units have to be changed in a speed calculation. Here is the same problem but with different units: 1x60x60 Speed calculation example – units check A boy takes 1 hour to travel from his home to the cinema, a distance of 10 km. Calculate his average speed in m/s. d (distance in m)average speed (in m/s) t (time in s) = d s x t Cover the quantity to be calculated - s (speed)
  8. 8. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 20058 of 41 = 3.6 km/h x 2 h distance (km) = speed (km/h) x time (h) Question 1 A group set off from home and walk at an average speed of 3.6 km/h. How far would they travel in two hours? Give your answer in km. d s x t Speed calculation – question 1 = 7.2 km
  9. 9. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 20059 of 41 10 km 5.4 km/h = Question 1 How long would it take a woman to walk 10 km if her average speed is 5.4 km/h ? d s x t time = distance speed Speed calculation – question 1 = 1.85 hours
  10. 10. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200510 of 41 This graphing experiment shows an animation of a car travelling along a straight road. Car graphing activity – instructions 1. Copy the results table shown on the next slide and complete it as the movie is played. 2. Record the distance the car has travelled every five seconds. 3. Plot a graph of your results.
  11. 11. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200511 of 41 Results table for distance/time graph Time/seconds Distance/metres 0 5 10 15 20 25 30 35 40 45 50 55 Car graphing activity – results table layout
  12. 12. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200512 of 41 Car graphing activity – animation
  13. 13. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200513 of 41 Results table for distance/time graph Time/seconds Distance/metres 0 0 5 16 10 76 15 186 20 234 25 484 30 634 35 784 40 904 45 974 50 994 55 994 Car graphing activity – results table
  14. 14. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200514 of 41 Distance / Time graph for car 0 200 400 600 800 1000 1200 0 5 10 15 20 25 30 35 40 45 50 55 Time / seconds Distance/metres Car graphing activity – results graph
  15. 15. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200515 of 41 Distance / Time graph for car 0 200 400 600 800 1000 1200 0 5 10 15 20 25 30 35 40 45 50 55 Time / seconds Distance/metres Car graphing activity – results graph
  16. 16. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200516 of 41 Distance / Time graph for car 0 200 400 600 800 1000 1200 0 5 10 15 20 25 30 35 40 45 50 55 Time / seconds Distance/metres The car has stopped. The graph is flat – the distance of the car from the start point is not changing. The graph is straight – there is no change in speed. The speed of the car is changing – the graph is not flat. The slope of the graph is less steep as the car begins to slow down. The car is starting to move. The curve shows that the speed is changing. The curve is upwards as the car accelerates at the start of the journey. The car is going fast but at a constant speed. The graph is straight in this part of the journey. Car graphing activity – results graph analysis
  17. 17. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200517 of 41 The speed of the car can be calculated by looking at the gradient of the distance/time graph. Speed is “Distance Travelled divided by Time Taken” These values can be read off the distance/time graph at different points, and this is the same as the gradient of the graph. Gradient of a distance/time graph
  18. 18. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200518 of 41 Distance / Time graph for car 0 200 400 600 800 1000 1200 0 5 10 15 20 25 30 35 40 45 50 55 Time / seconds Distance/metresConsider the gradient of this graph at the point shown by the two arrows in a triangle: The car has travelled from 200m to 800m = 600m. It took from 16s to 36s to travel this distance = 20s. Gradient of a distance/time graph So the speed at this point = 600m/20s = 30m/s.
  19. 19. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200519 of 41 1. Time how long it takes you to run 100m. Speed experiment – instructions total distance total time average speed = 3. Repeat the experiment for each member of your group. 2. Then calculate your average speed for the run. 4. What was the fastest average speed for your group?
  20. 20. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200520 of 41 Name distance (m) time (s) average speed (m/s) 100 100 100 100 100 Conclusion The fastest member of the group with an average speed of ________ was __________. Speed experiment – results
  21. 21. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200521 of 41 9K Speeding Up Contents Distance, time and speed Balanced and unbalanced forces Friction Summary activities
  22. 22. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200522 of 41 A force is a push or a pull. A force cannot be seen but you can see how a force affects an object. What is a force?
  23. 23. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200523 of 41 500 N 500 N Think of a car travelling at a constant speed of 50 mph. 50 mph Balanced and unbalanced forces The engine provides sufficient force to just overcome all the frictional forces that are acting to decrease the speed.
  24. 24. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200524 of 41 cross- wind A crosswind acting on the car produces a sideways force. The crosswind causes the direction of the car to change – this happens because the sideways forces on the car are not balanced. 50 mph Balanced and unbalanced forces If the car turns right so that the wind is now behind the car, what will happen to the speed?
  25. 25. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200525 of 41 The air resistance will decrease because the car has a “tail wind” (it is being blown from behind). This means the forces acting on the car are no longer balanced. 500 N 400 N >50mph 500 N 500 N 60mph Balanced and unbalanced forces The car will increase in speed (accelerate) until the forces are balanced again.
  26. 26. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200526 of 41 If the forces on an object are balanced:  If the object is stopped, it will remain stopped.  If the object is moving, then it will continue to move at the same speed and in a straight line. In other words, the object will continue to do what it is already doing without any change. Balanced and unbalanced forces – summary If the forces are unbalanced two things can happen:  The speed will change.  The direction of motion will change. This is called acceleration.
  27. 27. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200527 of 41 The sum effect of more than one force is called the resultant force. 400 N500 N 100 N A resultant force of 100 N is accelerating the car. Resultant force The resultant force is calculated by working out the difference between opposing forces.
  28. 28. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200528 of 41 10N20N 5N5N 1. What is the resultant force on the block? Resultant force = 20N –10N = 10N down The block will accelerate downwards. Resultant force – question 1
  29. 29. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200529 of 41 5N 2. What is the resultant force on the block? 5N 5N 5N Resultant force = 5N – 0N = 5N right The vertical forces are equal in size and opposite in direction so there is no resultant force in the vertical direction. The block will accelerate to the right. Resultant force – question 2
  30. 30. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200530 of 41 10N 13N 3. What is the resultant force on the block? 10N 20N 3N 7N 17N Resultant force = (20N +10N) – 13N = 17N right The vertical forces are equal in size and opposite in direction so there is no resultant force in the vertical direction. The block will accelerate to the right. Resultant forces – question 3
  31. 31. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200531 of 41 Resultant force activity
  32. 32. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200532 of 41 9K Speeding Up Contents Distance, time and speed Balanced and unbalanced forces Friction Summary activities
  33. 33. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200533 of 41 Friction always tries to slow moving object down – it opposes motion. Friction is created whenever two touching objects or surfaces move past each other. Friction also occurs when things move through air. This is called air resistance or drag. (The size of the frictional force equals the applied force unless the applied force is bigger than the maximum value of the frictional force. If this is the case then the frictional force remains at the maximum possible value.) Friction
  34. 34. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200534 of 41 One more? Probably the most important… Label all sources of friction that can act on this bike. tyre and road brake pad and rim wheel bearing wheel bearing pedal bearing links in chain air resistance or “drag” What are the sources of friction?
  35. 35. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200535 of 41 Air resistance is a type of friction caused when objects move through the air. 400N 300N Air resistance and drag Cars are designed so that they are streamlined. The flow of air around the body is made as smooth as possible so that air resistance is minimized. Air resistance depends on:  the size of the car;  the shape of the car;  the speed of the car.
  36. 36. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200536 of 41 One of the most important sources of friction in cars is that between the tyres and the road. The friction between the tyres and the road is affected by the:  inflation pressure of the tyres;  road surface;  surface condition caused by the weather (rain, ice, etc). When the car brakes, the maximum possible amount of friction is desirable so that the car does not skid. Other sources of friction in cars
  37. 37. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200537 of 41 Effects of frictional forces
  38. 38. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200538 of 41 9K Speeding Up Contents Distance, time and speed Balanced and unbalanced forces Friction Summary activities
  39. 39. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200539 of 41 Glossary acceleration – A change in speed. air resistance – A frictional force that acts against an object moving through air. balanced forces – Forces acting on an object that do not change its speed or direction. drag – A frictional force, such as air resistance or water resistance, which slows down a moving object. friction – A force that occurs when two things rub against each other and so slows down a moving object. speed – How quickly an object is moving. It equals the distance moved divided by the time taken, often measured in ‘metres per second’ (m/s). streamlined – A smooth shape which reduces drag. unbalanced forces – Forces acting on an object that change its speed or direction.
  40. 40. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200540 of 41 Anagrams
  41. 41. © Boardworks Ltd 20041 of 20 © Boardworks Ltd 200541 of 41 Multiple-choice quiz

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