Stopping Motion

Measuring Reaction Time

A reaction time is the length of time it takes for a person to react to a stimulus. __The typical reaction time for a person is in the range from __0.2s to 0.9s.

Reaction times are affected by the following:

  • Tiredness
  • Drugs
  • Alcohol

All of the above increase a person’s reaction time. This means that it takes them longer to react and they are more likely to have an accident.

You need to be able to explain a method to measure human reaction times:Stopping Motion, figure 1Notice that in this experiment we are required to use two equations. Sounds like a potential five mark question to me! Make sure you can see how they are used together.

Stopping Distance

When a driver presses hard on the brakes the car does not stop instantly. It is the same when you press the brakes on your bike - you don’t just stop suddenly.

Stopping distance is made up of two elements:

  • Thinking distance
  • Braking distanceStopping Motion, figure 1


We already discussed factors that affect the thinking distance. These factors affect braking distance only:

  • Mass of the vehicle
  • Speed of the vehicle
  • State of the brakes
  • State of the__ road__
  • The amount of__ friction __between the road and the tyre

Large Deceleration

Large decelerations are dangerous because there is only so much force that the body can cope with. Too much force and a person’s insides will be torn apart causing severe injury and death.

Wearing a seatbelt compared to not wearing a seatbelt in a collision can make a difference of over 1 tonne of force!