A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Angular Motion and Uniform Angular Acceleration — mark scheme explained
The short answer
Understanding angular motion is crucial in engineering physics, particularly when dealing with rotating systems such as wheels, gears, and turbines. This section covers key concepts like angular displacement, angular speed, angular velocity, and angular acceleration, along with their mathematical representations and graphical methods.
The question
A wheel starts from rest and accelerates uniformly at a rate of 2 rad/s 2 . Calculate the angular velocity after 5 seconds. [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: initial angular velocity (ω 0 ) = 0 rad/s, angular acceleration (α) = 2 rad/s 2 , time (t) = 5 s.
- S2
Use the equation for uniform angular acceleration: ω = ω 0 + αt.
- S3
Substitute the values into the equation: ω = 0 + 2 × 5.
- S4
Calculate the result: ω = 10 rad/s.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Identify the given values: initial angular velocity (ω 0 ) = 0 rad/s, angular acceleration (α) = 2 rad/s 2 , time (t) = 5 s.
- S2
Use the equation for uniform angular acceleration: ω = ω 0 + αt.
- S3
Substitute the values into the equation: ω = 0 + 2 × 5.
- S4
Calculate the result: ω = 10 rad/s.
Final answer: 10 rad/s
Common mistakes
- Using degrees instead of radians for angular displacement and related quantities. — Always use radians when dealing with angular motion problems. Convert any given angles from degrees to radians if necessary.
- Forgetting to use the right-hand rule to determine the direction of angular velocity and acceleration. — Practice using the right-hand rule regularly. Point your right thumb along the axis of rotation, and the direction in which your fingers curl represents the positive direction of angular velocity and acceleration.
- Misapplying the equations for uniform angular acceleration in non-uniform cases. — Always check if the problem states that the angular acceleration is constant before applying these equations. If it's not, use other methods or derive the necessary equations from first principles.
- Confusing angular speed and angular velocity. — Always specify whether you are dealing with angular speed or angular velocity. Use the right-hand rule to determine the direction of angular velocity when necessary.
- Failing to check units and dimensions in calculations. — Always ensure that all quantities have consistent units before performing calculations. Convert units as needed and double-check the dimensions of your final answer.
- Not using the correct initial conditions when applying the equations for uniform angular acceleration. — Always identify and write down the initial conditions given in the problem. Use these values when applying the equations for uniform angular acceleration.
Where the marks go
- Full worked solution (all marking points)4 marks