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Tuesday, November 17, 2009

[Physics SPM] Exam Pointers V

Source: Maths & Science Guide

Explain how upthrust is produced during the flight of an aeroplane. [ 5 marks ]

Frequent Mistakes
Student's Answer:
Upthrust is produced based on the Bernoulli's principle, which states that a low pressure is generated at the point where a fluid travels at high speed. Hence, when the air flows at high speed, a low pressure is generated and subsequently produces a net forces that acts upwards on the aeroplane. This net force constitutes the upthrust.

The command word is "explain", and the objective of the question refers to how the upthrust is produced. Since the allocation of marks is five, the answer should consist of five main points. The student has identified the command word as well as the objective of the question. However, the lengthy statements cause the answer to stray away from the objective. Also, even though the answer mentions that air flows at high speed, it does not state where - above or below the wings or body of the aeroplane. Hence, the explanation is inaccurate.

Correct Answers
  • The shape of an aeroplane is that of an aerofoil.
  • This shape ensures the speed of air is higher above the wings and body of the plane than that under them.
  • Based on the Bernoulli's principle, the air pressure under the plane is higher that the above it.
  • Hence, a net force that acts upwards is generated on the aeroplane.
  • This provides the upthrust that lifts the plane during its flight.

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