A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Torque and Rotational Dynamics — mark scheme explained
The short answer
In the realm of engineering physics, understanding torque and rotational dynamics is crucial.
The question
A force of 10 N is applied at a distance of 2 m from the axis of rotation. Calculate the torque. [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: F = 10 N, r = 2 m
- S2
Use the formula T = F × r
- S3
Substitute the values into the formula: T = 10 N × 2 m
- S4
Calculate the result: T = 20 N·m
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Identify the given values: F = 10 N, r = 2 m
- S2
Use the formula T = F × r
- S3
Substitute the values into the formula: T = 10 N × 2 m
- S4
Calculate the result: T = 20 N·m
Final answer: 20 N·m
Common mistakes
- Treating torque as a vector and trying to assign a direction with the right-hand rule for this spec point. — For spec point 3.11.1.4, torque is treated as a scalar magnitude: T = Fr (force perpendicular to the radius) and T = Iα. The right-hand rule / vector direction is not required here.
- Confusing units for force, radius, and torque. — Always check and convert units to ensure consistency in calculations. Use N for force, m for radius, and N·m for torque.
- Using the wrong formula or value for moment of inertia based on the object's shape. — Memorize and practice using the correct formulas for common shapes such as solid cylinders, hollow spheres, and rods.
- Overlooking angular acceleration when using the second equation (T = I × α). — Always identify and use all relevant variables in the given equations. Double-check that you have included angular acceleration if it is required.
- Incorrectly identifying the radius (r) as the distance from the center of mass instead of the point of application of force. — Clearly identify and label all distances in the problem. Ensure that r is the distance from the point where the force is applied to the axis of rotation.
- Failing to recognize when to use T = F × r versus T = I × α. — Practice identifying the type of problem and the given variables. Use T = F × r when force and radius are provided, and use T = I × α when moment of inertia and angular acceleration are involved.
Where the marks go
- Full worked solution (all marking points)3 marks