A-level Physics · Physics
Mechanics: a clear study guide
Mechanics problems reward a clear model. Draw the situation, choose a positive direction, list the forces and check that the equation matches the motion described.
Describe motion with vectors
Displacement, velocity and acceleration have direction as well as size. Choose a sign convention and keep it throughout the calculation. SUVAT equations apply to constant acceleration, so state or check that assumption before using them.
A velocity-time graph gives displacement from the area under the graph and acceleration from its gradient.
- Distance and speed are scalars.
- Displacement, velocity and acceleration are vectors.
- Use consistent units such as metres, seconds and metres per second.
Forces and momentum
Use the resultant force in F = ma and show the forces on a free-body diagram. Momentum is mass x velocity, and in an isolated system total momentum is conserved before and after an interaction.
Impulse equals change in momentum. Include direction when comparing velocities before and after a collision.
Explain the model
If a question says to ignore air resistance, treat it as a modelling assumption rather than a fact about the real world. Explain how friction, string mass, pulley size or a non-uniform field would change the model when evaluation is requested.
Quick check
What is the gradient of a velocity-time graph?
Answer: Acceleration.
When is momentum conserved?
Answer: When the system is isolated so there is no resultant external force or external impulse affecting the total momentum.
Common questions
Can SUVAT equations be used when acceleration changes?
The standard SUVAT equations assume constant acceleration. Use a graph or another method when acceleration varies.
Why should I draw a free-body diagram?
It makes the forces, directions and resultant force explicit before you choose an equation.