What are the features of questions requiring Theoretical Explanation?

John is riding a bicycle along a straight road with uniform speed 10 m s-1. When he sees a warning signal, John applies the brakes for 2 s to bring the bicycle to rest with uniform deceleration. Assume John’s reaction time (i.e. the time lapse between seeing the signal and starting to apply to brake) is 0.2s. Using Newton’s laws of motion, explain why it takes more time to bring the bicycle to rest for John to carry an excessive amount of goods on the bicycle.

Suppose the highway in the figure is on a slope with traffic running downhill. Do you think the distance between two arrows should be greater than, equal or less than 80m? By considering the forces acting on a car, or otherwise, explain your answer. Assume that the speed limit of the highway and the braking force on the car remain unchanged.

The figure shows a horizontal straight highway with a speed limit of 100 km/h (i.e. 27.8 m/s). For safety reasons, drivers are advised to maintain a safety distance of 80 m from the car ahead. Large arrows (chevrons) are painted on the highway (each at 80 m apart) and road signs are set up to remind drivers of this safety distance.

A student uses the set-up as shown to study the relationship between the pressure and volume of a fixed mass of gas at constant temperature. The piston is pushed in or pulled out to vary the volume of gas and the corresponding pressure is measured by the Bourdon gauge.

Explain, in terms of the kinetic theory of gases, the increase in the pressure of the gas when the volume decreases at a constant temperature.

Figure 3 shows a water rocket. The rocket is filled with water and compressed air. Explain why the rocket rises when the trigger is pulled and name the law or principle involved.

(Source: HKEAA CE 2004 Q2)

Source: HKEAA DSE 2013 Q1

Sometimes it may not mention the word “theory”

Requirements: To explain why the phenomenon happens relating to physics theories, principles or laws. It puts special emphasis on the relationship between the observations and physics theories/principles/laws.

Common vocabulary you may find in questions

•Use …… theory to explain ……•In terms of ……, explain why……•Explain

Theoretical Explanation

Schematic Structure

Contents and Functions Language features

Statement of Theory

State clearly the theory, law, or principle relating to the event or phenomenon

According to…

Explanation Sequence

Relate the concepts to the events or phenomena. In addition, explanation of the various characteristics with causal relation may be used.

Relate the phenomena with “when”, “because”, “since” to the theory Causal conjunctions, “hence”, “thus”, “and”, “lead to” for the causal relations

Conclusion (optional)

Summarize the explanation

Conjunctions “Therefore”, “Hence” and “Thus” always bring out the conclusion

Theoretical Explanation WritingGenre Structure and language features