What mistakes do students make with Practical skills?

Using a stopwatch for very short intervals, leading to large percentage uncertainty. Use light gates or time multiple events. Misaligning light gates so the object does not interrupt the beam properly. Adjust the gates so the object passes cleanly through. Forgetting to reset the timer between measurements. Reset or zero the timer before each reading.

What are the exam tips for Practical skills in Physics OCR A-Level?

State the timing method and explain why it is suitable for the interval measured. For light gates, describe the setup: two gates, a timer, and the object interrupting the beam. Include the resolution of the timer when discussing uncertainty.

OCR · A-Level · Physics

Course version: H556 · Version from Sept 2015

Practical skills: (d) use of a stopwatch or light gates for timing

This skill covers selecting and using a stopwatch or light gates to measure time intervals.

Checked against the OCR H556 specification (opens in a new tab) on 10 November 2026.

Point 4 of 11

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Select a stopwatch for timing longer intervals where reaction time is a small fraction of the total time.
Use light gates to measure short time intervals with reduced human reaction error.
Set up light gates so that the object interrupts the beam and the timer records the correct interval.
Read and record times with appropriate precision and units.
Explain the advantages and limitations of each timing method.

Common mistakes and how to put them right

Using a stopwatch for very short intervals, leading to large percentage uncertainty.
Put it right: Use light gates or time multiple events.
Misaligning light gates so the object does not interrupt the beam properly.
Put it right: Adjust the gates so the object passes cleanly through.
Forgetting to reset the timer between measurements.
Put it right: Reset or zero the timer before each reading.
Assuming light gates measure speed directly without entering the distance.
Put it right: Check the mode and input the distance if required.

Exam tips

State the timing method and explain why it is suitable for the interval measured.
For light gates, describe the setup: two gates, a timer, and the object interrupting the beam.
Include the resolution of the timer when discussing uncertainty.

Try the lesson

The start of the Practical skills: (d) use of a stopwatch or light gates for timing lesson, made from checked teaching notes for this specification point.

Listen

This skill covers selecting and using a stopwatch or light gates to measure time intervals. A stopwatch is suitable for longer intervals, such as the period of a pendulum, but has reaction-time error. Light gates measure the time an object takes to pass between two points, often using a timer connected to two gates; they are more precise for short intervals and reduce human reaction error. When using light gates, ensure the object interrupts the beam cleanly and that the gate is set to the correct mode (e.g. time, speed). For example, to measure the speed of a falling card, use two light gates a known distance apart and record the time between interruptions. The skill is assessed in practical contexts where you must choose the appropriate timing method and use it correctly. A strong answer earns marks for these points. Select a stopwatch for timing longer intervals where reaction time is a small fraction of the total time. Use light gates to measure short time intervals with reduced human reaction error. Set up light gates so that the object interrupts the beam and the timer records the correct interval. Read and record times with appropriate precision and units. A common slip: Using a stopwatch for very short intervals, leading to large percentage uncertainty. Instead, use light gates or time multiple events. Exam tip: State the timing method and explain why it is suitable for the interval measured.

See it

A top-down flowchart. It starts at a node labelled Timing task, which leads to a decision node labelled Longer interval?. The Yes branch leads to Use a stopwatch, which leads to Reaction-time error. The No branch leads to Use light gates, which leads to Two gates and a timer. From Two gates and a timer, two arrows lead to Object interrupts the beam cleanly and to Set the correct mode; both of these lead to Reduced human reaction error. — Shows how to choose between a stopwatch and light gates, and the key setup points for light gates.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. It starts at a node labelled Timing task, which leads to a decision node labelled Longer interval?. The Yes branch leads to Use a stopwatch, which leads to Reaction-time error. The No branch leads to Use light gates, which leads to Two gates and a timer. From Two gates and a timer, two arrows lead to Object interrupts the beam cleanly and to Set the correct mode; both of these lead to Reduced human reaction error.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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