Interpret and evaluate measurements from simple methods to measure the…

The idea

Reaction time is the interval between detecting a stimulus and making a response. A simple classroom method is the ruler-drop test: one student holds a ruler vertically while another places a thumb and finger either side of the zero mark. The ruler is released without warning and the catcher closes the thumb and finger as quickly as possible. The distance the ruler falls is read from the top of the thumb. A larger distance means a longer reaction time, because fall distance is proportional to the square of the time taken. Measurements are interpreted by comparing distances between students or between repeats, and evaluated by considering sources of error such as anticipation, inconsistent finger position and the effect of practice.

What earns the marks

• State that reaction time is the time between a stimulus being detected and a response being made, and that the ruler-drop test measures this indirectly through the distance the ruler falls.
• Interpret a larger fall distance as a longer reaction time, using the relationship that distance fallen is proportional to time squared for an object falling freely.
• Compare repeat measurements for one student and identify anomalous values or a wide spread, which indicates unreliable timing or anticipation.
• Evaluate the method by identifying variables that must be controlled, such as the starting position of the thumb and finger, the release height and whether the catcher has practised the task.
• Explain that reaction time varies between individuals and within one individual, so a single measurement is not sufficient and a mean of repeats gives a more reliable value.
• Suggest an improvement such as using a computer-based timer or a random delay before release to reduce the chance of the catcher predicting the release.

Common mistakes and how to put them right

• Treating the fall distance as directly proportional to reaction time.

  Put it right: For free fall the distance is proportional to the square of the time, so doubling the distance does not simply double the reaction time.

• Recording the distance from the bottom of the ruler or from an arbitrary mark rather than from the same reference point each time.

  Put it right: Read the scale at the top of the thumb and keep the reference point consistent across repeats.

• Concluding that one student has a faster reaction time from a single trial.

  Put it right: Repeat measurements are needed and a mean should be compared, with anomalous results excluded or investigated.

Exam tips

• When asked to evaluate a method, name the specific source of error and say how it would affect the measured distance or calculated time.
• Use the word 'reliable' for repeatable results and 'accurate' for closeness to the true value, and apply each term to the correct context.
• If a question gives a table of distances, compare values for the same student before comparing different students, and comment on the spread of repeats.