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
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
Ensure that all steps are clearly shown, including unit conversions and final units. Use appropriate significant figures and round off correctly. Label diagrams and equations to show your working.
What the command words demand
- Calculate
- Perform a numerical calculation to find the required value.
- Determine
- Identify or calculate the specified quantity using given data.
- Explain
- Provide a clear and concise explanation of the concept or process.
- Derive
- Show the steps involved in deriving a formula or equation.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 2-3 minutes per mark for questions on torque and rotational dynamics. This allows time for careful calculation and checking of answers.
- Identify the given values: F = 10 N, r = 2 m0 marks
- Use the formula T = F × r1 mark
- Substitute the values into the formula: T = 10 N × 2 m1 mark
- Calculate the result: T = 20 N·m1 mark
Final answer: 20 N·m
Work through every step correctly and you earn all 3 marks.
Another worked example
A solid cylinder with a mass of 5 kg and radius of 0.5 m is rotating about its central axis. Calculate the moment of inertia.
- Identify the formula for the moment of inertia of a solid cylinder: I = (1/2)mr 21 mark
- Substitute the given values into the formula: I = (1/2)(5 kg)(0.5 m) 21 mark
- Calculate the result: I = (1/2)(5 kg)(0.25 m 2 )1 mark
- Simplify the calculation: I = 0.625 kg·m 21 mark
Final answer: 0.625 kg·m 2
Work through every step correctly and you earn all 4 marks.
Common mistakes
Treating torque as a vector and trying to assign a direction with the right-hand rule for this spec point.
Why it happens: Students may confuse the directions when using the right-hand rule, leading to incorrect answers.
Fix: 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.
Why it happens: Students may mix up the units, leading to incorrect calculations.
Fix: 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.
Why it happens: Students may not remember the correct formulas for different shapes, leading to incorrect calculations.
Fix: 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).
Why it happens: Students may forget to include angular acceleration in their calculations, leading to incorrect results.
Fix: 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.
Why it happens: Students may confuse the radius with other distances, leading to incorrect torque calculations.
Fix: 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.
Why it happens: Students may not understand the context in which each equation is used, leading to incorrect application of formulas.
Fix: 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
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Calculate Torque | Apply the torque equation to a force acting at a distance from the axis. | 3 |
| Calculate Moment Of Inertia | Use the solid cylinder formula to find its moment of inertia about the central axis. | 4 |
| Calculate Angular Acceleration | Rearrange the rotational equation of motion to find angular acceleration from torque and inertia. | 4 |
| Calculate Angular Acceleration | Find the torque from the tangential force, then use it to calculate angular acceleration. | 5 |
| Total across these question types | 16 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.