P6

• Reaction time


  1. You see a runner passing the finish line

  2. Light reflects from the runner and hits your eye

  3. A signal travels from your eye to your brain

  4. A signal travels from your brain to your finger to press the button
• This takes a certain time, called reaction time
• Your reaction time is probably about 0.2 seconds
• Factors that affect a driver's reaction time


  • Distracted by other people in the car
  • Eating or drinking
  • Using a radio or satellite navigation system
  • Being older in age
• Speed


  The faster you're going, the longer the reaction time
• Thinking distance


  The distance the car travels before the driver's foot starts to apply a force on the brake pedal
• Factors that affect thinking distance

  • Drinking alcohol
  • Using drugs, as well as some medicines
  • Being tired
• Braking distance


  The distance taken to stop once the brakes have been applied
• Factors that affect braking distance

  • Your speed - the faster you're going, the further it takes to stop
  • Mass of the car - a car full of people and luggage won't stop as quickly as an empty car
  • Condition of the brakes - worn or faulty brakes won't be able to brake with as much force
  • How good the grip of your tyres is - more likely to skid when the road is dirty, icy or wet, and if the tires are bald
• Stopping distance


  The total distance that the car travels from the moment the driver sees the problem until the car is stationary
• Stopping distance = thinking distance + braking distance
• Large decelerations are dangerous
• Acceleration


  Change in speed/time
• If the car slows down suddenly

  The belt exerts a force on you
• Large deceleration requires a large force
• Without the force, you would continue to move with your original speed, and could collide with the dashboard or the seat in front of you, causing injury to yourself or others
• Seatbelts have to be replaced after an accident because they stretch
• If your deceleration is very large, the seatbelt can cause compression injuries
• You can injure your internal organs because they continue to move inside your body even if your ribs are stopped by the seatbelt
• The force on an object can be lowered by

  Slowing the object down over a longer time, i.e. decreasing its deceleration
• You can also think of this in terms of momentum - the longer it takes for a change in momentum, the smaller the forces acting
• Safety features in cars

  • They are designed to increase collision times, which reduces the deceleration and forces, and so reduces the risk of injury
• Crumple zones


  Areas at the front and back of a car which crumple up easily in a collision, increasing the time taken to stop
• Bike helmets


  They contain a crushable layer of foam which increases the time taken for your head to stop, reducing the deceleration and forces acting
• Shin pads


  The pad deforms if hit, reducing deceleration and the force on your leg
• Some safety features also work by stopping you hitting hard surfaces like the dashboard or the road
• Ruler experiment


  1. Stand with your hand open

  2. Get someone else to hold a ruler so it hangs between your thumb and forefinger, and your forefinger's lined up with zero

  3. The person holding the ruler drops it without warning

  4. Try to catch the ruler as quickly as possible

  5. The measurement on the ruler at the point where it was caught is how far the ruler dropped in the time it took you to react
• The longer the distance, the longer the reaction time
• You can calculate how long the ruler was falling for (the reaction time) because its acceleration is constant
• For accuracy, keep on repeating and find the mean distance that the ruler fell
• Make sure it's a fair test, e.g. use the same ruler and have the same person drop it
• You can investigate factors that affect reaction time, e.g. introduce distractions
• Different units of speed
• How fast do everyday objects move?
• Main energy sources on Earth

  • Renewable
  • Non-renewable
• Renewable energy source


  One that will never run out
• Renewable energy sources


  • Most of them do less damage to the environment than non-renewable
  • They provide less energy than non-renewables and the weather-dependent ones can be unreliable
• Renewable energy sources


  • Bio-fuels
  • Wind power
  • The Sun
  • Hydro-electricity
  • The tides
• Biofuels


  Can be made from anything from farm waste, animal droppings and landfill rubbish to specially grown crops (e.g. sugar cane, vegetable oils or trees)
• Biofuels are renewable because we can just grow more