Friction V-Speed and Safety

Friction and the motor car

In order that a car can change its speed or direction, there must be friction between its tyres and the road surface. A new tyre with lots of tread has a rough surface and will provide lots of grip. A worn tyre with a smoother surface will provide less grip and the cart will be much more difficult to control and stop.
Having tyres which are in food condition is important if you need to stop quickly, but there are several other important factors which will affect how quickly a car can stop. These are:
• The reaction time of the driver,
• The efficiency of the breaking system of the car,
• The speed of the car,
• The weather/road conditions,
Figure illustrates how the speed of a car affects the total distance it will travel before it stops. The total stopping distance is equal to the thinking distance + the breaking distance.
Braking force
When a car driver brakes, the braking force between the brakes and the wheels slows the wheels down.
Explain why you must brake harder to stop in the same direction when you are traveling at higher speed.
Tyres must grip
Friction between the tyres and the road makes the tyres grip the road. When you brake, the wheels slow down. The friction with the road must then increase to slow the whole car. The friction force depends on the tyre design and the road surface.
• Why does the car skid if you brake too hard?
• Why must the driver brake carefully when the road is wet?
• Why must the driver brake very carefully when there is ice on the roads?
• Why is it safer to have a rough road surface before a pedestrian crossing?
Why do tyres have tread?
You need good tyres to stop quickly. Tyres can grip the road only if they are touching it. They lose their gripwhen the road is wet. The tread on a tyre is designed to push away the water. In dry conditions, the tread doesn’t help. In dry weather, racing cars use tyres with no tread.
Why do racing drivers stop to change their tyres when it starts raining?
Why do worn tyres increase the chance of a skid?
You can’t stop instantly
If someone steps out in front of a car, it takes time for the driver to react. This is called the reaction time. The distance the car travels during the reaction time is called the thinking distance.
Look at the diagram. Why does it take time to react?
Look at the table.
• What is the thinking distance when traveling at 30 miles per hour?
• What happens to the thinking distance if the speed is doubled?
When the driver presses the brake pedal, it takes time for the brakes to slow the car down. During this time, the car travels a distance called the braking distance.
Look at the table.
• What is the braking distance for a speed of 30 miles per hour?
• What happens to the braking distance if the speed is increased?
• Copy and complete the formula.
• Stopping distance = thinking distance + _________ distance
After drinking alcohol, people may feel perfectly normal but their reactions are actually much slower.
• Why is it a bad idea for people to drive after drinking alcohol?
• What other factors could affect the reaction time?