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Kinetic energy and stopping distances (P3e)

Kinetic energy and stopping distances (P3e). Be able to calculate KE Be able to list factors that can effect the braking distance. Stopping Distance. Cars rely on friction at the brakes and tyres in order to stop.

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Kinetic energy and stopping distances (P3e)

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  1. Kinetic energy and stopping distances (P3e) Be able to calculate KE Be able to list factors that can effect the braking distance

  2. Stopping Distance • Cars rely on friction at the brakes and tyres in order to stop. • The total distance a car requires to stop is called the total stopping distance. This is the sum of the distance covered in the time it takes for the driver to react known as the thinking distance and the distance the car travels before coming to rest after the brakes are pressed called the braking distance.

  3. Total stopping distance • Total stopping distance = thinking distance + braking distance Total stopping distance = thinking distance + braking distance

  4. The diagram below gives the shortest stopping distances for a car on a dry road with good brakes and tyres.

  5. From the diagram a clear pattern can be seen between the velocity of the car and the total stopping distance. The average reaction time of a driver is about 0.7s as the velocity of the car increases so will the thinking distance from the relationship; Distance travelled = velocity x reaction time The braking distances increases because at a faster velocity the car possesses more kinetic energy. This energy has to be transferred to the brakes; this is transferred as heat and is the reason why brakes become hot. The relationship between velocity and kinetic energy is, Kinetic Energy (Joules) = ½ x mass x velocity2 Therefore if the car travels three times the velocity it has 9 times the kinetic energy which means the braking distance will be nine times longer.

  6. Practice these

  7. Factors affecting the total stopping distance • Make a list of all the factors you thin k can effect the stopping distance. • Now divide them into two groups – • Those that effect the thinking distance and those that effect the braking distance

  8. The driver’s reaction time The thinking distance depends on the driver’s reaction time. Reactions are strongly influenced by the state of the mind. Thus, a driver under the influence of a drug such as alcohol will have a much reduced reaction time which will increase the thinking distance. Tiredness and fatigue also influence reaction time. A tired driver will react a lot slower than an alert driver.

  9. Test your reaction time wit any of these reaction test timers • http://getyourwebsitehere.com/jswb/rttest01.html (traffic lights) • http://www.topendsports.com/testing/reaction-timer.htm (colour change screen) • http://www.bbc.co.uk/science/humanbody/sleep/sheep/reaction_version5.swf (sheep crossing)

  10. Factors effecting…… Velocity As illustrated in the chart above the braking distance increases with velocity. This is explained by the relationship between kinetic energy and velocity. The greater the velocity the greater the kinetic energy the brakes have to transfer. Mass The mass of the vehicle is also related to kinetic energy in the relationship; Kinetic Energy (Joules) = ½ x mass x velocity2 The greater the mass the greater the kinetic energy, thus a heavier car will require a longer braking distance.

  11. Road surface Friction between the car tyres and the road surface stop it from skidding and sliding. On a wet or icy road surface the contact between the tyres and the road is considerably reduced. If the driver brakes hard the car will skid therefore the driver needs to apply a reduced force on the brakes increasing the braking distance.

  12. Tyre condition Tyres are designed with grooves which channel the water away on wet roads in order to ensure contact of the tyre is made with the road. If the tyres are worn the friction between the road and tyres is reduced increasing braking distance.

  13. Worn tyre effect stopping distance

  14. Brake conditions • Worn brakes will take longer to transfer the kinetic energy of the car increasing the braking distance.

  15. Fuel consumption • Research and make a list of factors that can effect the fuel consumption of a vehicle • Try to answer these questions that follow:- • Print sheet for students to take home.

  16. Fuel consumption Explain why you should not over load your car? List three things that can cause an increase in fuel consumption? Explain impact of a lorry transporting a very heavy load, what problems do they face? Sara wants to buy a new car she see two identical cars except one runs on petrol the other on diesel. Some people say diesel is better you get more for your money, explain what this means and advise Sara on which car she should get? Find out what car you have in your house hold is it fuel efficient?

  17. Plenary: quick test

  18. Questions Look at what you have learnt today Now write a question to which you know the answer on a piece of card Starting with the teacher she will ask a question who ever gets it right gets to read out their question who ever gets that right they read out there's and so on……

  19. evaluation • Review • Refer back to objectives – have they been met • Suggest how this lesson can be improved add or remove slides • and share your resources please………… I just did!

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