Safety In Public Transport
Visão Geral
A bus pulling away from a stop, a train running between stations and a car on an open road all obey the same short list of rules, and those rules decide something very practical: the length of road a vehicle needs before it can come to rest. That length is not a property of the brakes alone. It is set by the speed, by the mass, by how long the driver takes to respond, and by how much grip the tyres can find on the surface underneath them.
This lesson turns that everyday fact into physics you can calculate. You will see why a vehicle at a steady speed has no resultant force acting on it at all, split the distance needed to stop into two parts that behave in completely different ways, and use the kinetic energy of a moving vehicle to work out how far the brakes must act. By the end you will be able to explain, in numbers rather than opinions, why raising the speed by half can add far more than half to the distance a vehicle needs.
Objetivos
- When a vehicle travels at a steady speed in a straight line the resistive forces are balancing the driving force.
- The greater the speed of a vehicle the greater the braking force needed to stop it in a certain distance. The greater the braking force the greater the deceleration of the vehicle. Large decelerations may lead to brakes overheating and/or loss of control. Students should understand for a given braking force, the greater the speed, the greater the stopping distance.
- The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking distance). A driver’s reaction time can be affected by tiredness, distractions, drugs and alcohol.
- When the brakes of a vehicle are applied, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increases.
- A vehicle’s braking distance can be affected by adverse road and weather conditions and poor condition of the vehicle. Students should understand that ‘adverse road conditions’ includes wet or icy conditions. Poor condition of the car is limited to the car’s brakes or tyres.
Mapa mental
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Nota de Aula
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Avaliação da Lição
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Irá encontrar uma mistura de tipos de perguntas, incluindo perguntas de escolha múltipla, perguntas de resposta curta e perguntas de redação. Cada pergunta é cuidadosamente elaborada para avaliar diferentes aspetos do seu conhecimento e competências de pensamento crítico.
Use esta secção de avaliação como uma oportunidade para reforçar a tua compreensão do tema e identificar quaisquer áreas onde possas precisar de estudo adicional. Não te deixes desencorajar pelos desafios que encontrares; em vez disso, vê-os como oportunidades de crescimento e melhoria.
- A bus travels along a straight, level road at a steady speed. Which statement about the forces on the bus is correct? A. The driving force is greater than the resistive forces. B. The resistive forces are equal to the driving force. C. The resistive forces are greater than the driving force. D. There are no forces acting on the bus. Answer: B
- Which statement correctly describes the stopping distance of a vehicle? A. It is the distance travelled while the brakes are acting. B. It is the distance travelled during the driver's reaction time. C. It is the thinking distance added to the braking distance. D. It is the braking distance subtracted from the thinking distance. Answer: C
- Which of these increases the thinking distance of a vehicle but has no effect on its braking distance? A. An icy road surface B. Badly worn tyres C. A tired driver D. Poorly maintained brakes Answer: C
- A car of mass 1000 kg travelling at 20 m/s is brought to rest by a constant braking force of 5000 N. What is the braking distance? A. 20 m B. 40 m C. 80 m D. 100 m Answer: B
- When the brakes of a moving vehicle are applied, which statement is correct? A. The kinetic energy of the vehicle increases and the brakes cool down. B. The kinetic energy of the vehicle decreases and the temperature of the brakes increases. C. The kinetic energy of the vehicle is destroyed by the friction force. D. The kinetic energy of the vehicle is unchanged and only its speed falls. Answer: B
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