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Changing acceleration at maximum engine power
Scientists are developing a hypersonic aeroplane that will travel much faster than normal aeroplanes.
An aeroplane accelerates from a low speed to a high speed with the engines at maximum power. Explain why the acceleration is not constant.
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Marking points
- 1 State that at maximum power the forward force from the engines is constant.
- 2 State that air resistance increases as the aeroplane accelerates.
- 3 Use the relationship that resultant force is the engine force minus air resistance.
- 4 Conclude that the resultant force decreases.
- 5 Link the decreasing resultant force to decreasing acceleration because acceleration is directly proportional to resultant force.
Full-mark answer
At maximum power the forward force from the engines is constant. As the aeroplane gets faster, air resistance increases. The resultant force is the engine force minus the air resistance, so the resultant force decreases as speed increases. Since acceleration is directly proportional to resultant force, the acceleration decreases and is therefore not constant.
Why this answer loses marks
Air resistance increases so it slows down.
This mentions drag but does not explain the constant engine force, resultant force, or proportionality to acceleration.