A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Torque and Power in Rotating Machinery — mark scheme explained
The short answer
In the context of engineering physics, understanding the relationship between torque (T), power (P), work (W), angular displacement (θ), and angular velocity (ω) is crucial for analyzing rotating machinery. This spec point focuses on these relationships and the importance of accounting for frictional torque in such systems.
The question
A rotating machine has a torque of 50 N·m and an angular velocity of 10 rad/s. Calculate the power output of the machine. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Identify the given values: T = 50 N·m, ω = 10 rad/s.
- S2
Use the formula for power in rotational systems: P = T × ω.
- S3
Substitute the given values into the formula: P = 50 N·m × 10 rad/s.
- S4
Calculate the result: P = 500 W.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Identify the given values: T = 50 N·m, ω = 10 rad/s.
- S2
Use the formula for power in rotational systems: P = T × ω.
- S3
Substitute the given values into the formula: P = 50 N·m × 10 rad/s.
- S4
Calculate the result: P = 500 W.
Final answer: 500 W
Common mistakes
- Forgetting to convert revolutions to radians when calculating angular velocity. — Always convert revolutions to radians before using them in calculations. 1 revolution = 2π radians.
- Using inconsistent units when performing calculations involving torque and power. — Ensure all units are consistent. For example, use SI units (newtons, meters, seconds) throughout the calculation.
- Overlooking frictional torque when analyzing rotating machinery. — Always consider the impact of frictional torque in real-world applications. Account for it in your calculations to get a more accurate picture of system performance.
- Confusing angular displacement (θ) with angular velocity (ω). — Understand that angular displacement is the angle through which an object has turned, while angular velocity is the rate of change of this angle. Use θ for displacement and ω for velocity.
- Incorrectly applying the formula for work in rotational systems. — Always use the correct formula W = T × θ and ensure that torque (T) and angular displacement (θ) are correctly identified and substituted.
- Failing to check the units of the final answer. — Always double-check the units of your final answer to ensure they match the expected unit for the quantity being calculated.
Where the marks go
- Full worked solution (all marking points)4 marks