Forces - 5.6.3.2 Reaction Time (GCSE Physics AQA)

Reaction Time

Normal Reaction Times

As previously discussed, different people will have different reaction times. A typical reaction time could be anywhere from 0.2 – 0.9 seconds. In this section, we will explore factors affecting reaction times in more detail.

Factors Affecting Reaction Times

The following factors can all affect the ability of a driver to react to hazards on the road.

1. Tiredness – the more tired you are, the more difficult it is for you to react. This increases reaction times, therefore leading to an increase in stopping distance.
2. Drugs – drugs can affect your ability to react when driving. For example, taking some medications can make you feel drowsy, resulting in an increased reaction time.
3. Alcohol – drinking alcohol can impair a driver’s ability to react. It increases reaction times, leading to an increase in stopping distance.
4. Distractions – distractions, such as talking on the phone, can lead to a driver losing concentration on the road. This leads to an increase in reaction times and once again, an increase in stopping distance.

Evaluating Effects of These Factors

In AQA exams, you may be given some data relating to the factors that we have just discussed. Using this data, you should be able to compare these and evaluate each of their effects.

Measuring Reaction Times

We can measure reaction times in the following way.

1. Gather the equipment. You will need a metre ruler.
2. Choose a subject. Ask someone to sit at a desk, with their hand out.
3. Line up the ruler. You should hold the ruler up, but make sure that the ruler is not touching the person’s hand. The ‘zero’ of the ruler should be in line with the person’s thumb and forefinger, as if they are getting ready to catch it.
4. Drop the ruler. Without telling the person, drop the ruler.
5. Measure the distance. At the point where the person catches the ruler, read off the distance. This will give you a distance.
6. Use acceleration due to gravity. We know that the acceleration due to gravity is 9.8 m/s2. Using this value, we can find the value for time, since we know the equation ‘acceleration = change in velocity / time taken’. However, firstly we need to find the change in velocity.
7. Find the change in velocity. We can use an equation that we learnt previously to find the change in velocity : v2 − u2 = 2 a s. Rearranging this equation, we can make v the subject of the formula and therefore we can find the final velocity. Knowing the initial velocity, we can find the change in velocity.
8. Find the time. Using the change in velocity and acceleration, we can find the time using the following equation: a = ∆v / t. We can rearrange the formula to make ’t’ the formula of the subject, which is the reaction time of the person.
9. Compare to typical results. Typically, human reaction. times are anywhere between 0.2 and 0.9 seconds. You need to be able to remember these values for AQA exams.

Interpreting Measurements

• Measurements may not be accurate. When performing this experiment, results may not always be entirely accurate. In order to improve the accuracy, you should carry out plenty of repeat readings. This will help you to remove any anomalies that you come across.
• People may be distracted. If the experiment room is noisy, or the person in the experiment is not feeling well, they may be distracted. This could have an impact on the reaction times and often increase them to larger than expected values.
• Measurements must be made precisely. Since we are using a ruler to take measurements, we must make sure that they are precise. The divisions on the ruler must be at least in millimetres (the smaller the measuring unit, the more precise the results).