A-Level Physics Mechanics Revision: SUVAT, Momentum, Work and Energy

Overview

If you are revising mechanics for A-Level Physics, the most useful improvement is usually not memorising more facts. It is learning how to recognise the model behind the question.

Many mechanics problems can be sorted into one of these patterns:

• Constant acceleration motion where SUVAT applies
• Interaction or collision questions where momentum is the main idea
• Force acting through a distance where work done and energy changes are more efficient than force equations alone
• Mixed questions where one stage uses SUVAT and another uses momentum or energy

A strong revision method is to ask the same short sequence of questions every time:

1. What is the system?
2. What happens before, during, and after the event?
3. Is acceleration constant?
4. Are there external forces that matter over the time involved?
5. Is momentum conserved, or only approximately conserved?
6. Would an energy approach be shorter than a force approach?
7. What assumptions is the question expecting me to state or use?

This matters because examiners often reward method before final accuracy. If your setup is clear, your units are consistent, and your model is sensible, you can collect marks even if arithmetic goes wrong later.

For specification differences across boards, it can help to compare wording and assessment style in AQA vs Edexcel vs OCR A-Level Physics: Specification and Assessment Comparison. For broader planning, Best Order to Revise A-Level Physics Topics for Year 12 and Year 13 is useful alongside this mechanics guide.

Template structure

The easiest way to make mechanics revision reusable is to use the same answer template each time. You do not need to write every heading in the exam, but you should think in this order.

1. Define the known quantities

Write down the given values with symbols and units. This sounds basic, but it stops one of the most common mistakes: mixing information from different stages of motion.

For SUVAT, label:

• u = initial velocity
• v = final velocity
• a = acceleration
• s = displacement
• t = time

Be careful with signs. If upward is positive, downward acceleration near Earth is negative. If motion reverses, velocities may switch sign even when speed is positive.

2. State the model and assumptions

This is especially important in written and multi-step questions. Typical assumptions include:

• particle model applies
• air resistance is negligible
• acceleration is constant
• the collision time is short, so external forces have negligible impulse
• the surface is smooth, so friction is negligible
• the string is light and inextensible

You do not need to force assumptions into every answer, but when a question asks about validity or modelling, these statements often separate average responses from strong ones.

3. Choose the principle before the equation

This is the heart of good mechanics revision.

• Use SUVAT only when acceleration is constant.
• Use momentum when objects interact internally, especially in collisions and explosions.
• Use work done and energy when a force acts through a distance, or when comparing start and end states is simpler than analysing every force over time.

A useful decision rule is this:

If the question is mainly about motion over time, think SUVAT. If it is mainly about an interaction, think momentum. If it is mainly about start and finish states, think energy.

4. Show substitutions clearly

Write the equation symbolically first, then substitute. For example:

v = u + at
18 = 6 + a(4.0)
a = 3.0 m s^-2

This earns method marks and makes checking easier.

5. Check units and physical sense

Before moving on, ask:

• Does the sign make sense?
• Is the value too large or too small?
• Have I written velocity or speed appropriately?
• Are the units in standard SI form?

If unit conversion is slowing you down, revisit Physics SI Units and Prefixes Revision Guide: kilo, mega, milli, micro and nano.

Core mechanics relationships to keep active

These are not the whole topic, but they are the formulas many questions circle back to:

• v = u + at
• s = ut + 1/2at²
• v² = u² + 2as
• s = 1/2(u + v)t
• p = mv
• F = dp/dt and in simple constant-mass cases F = ma
• W = Fs for force parallel to displacement
• E_k = 1/2mv²
• ΔE_p = mgΔh close to Earth
• P = W/t

The revision goal is not just recall. It is knowing when each one is the shortest route.

How to customize

The same template becomes much more useful when you adapt it to the exact style of mechanics question in front of you.

Customizing for SUVAT questions

Start by checking whether acceleration is constant throughout the motion. If it is not, SUVAT cannot be used across the whole journey, though it may still work for one stage.

For a standard SUVAT question:

1. Choose a positive direction.
2. List the five variables and identify which three or four are known.
3. Select the equation that avoids unnecessary unknowns.
4. Keep signs consistent from start to finish.

Common mistake: using distance instead of displacement. If an object changes direction, displacement is not total distance travelled.

Another common mistake: applying one SUVAT equation over a journey with different accelerations, such as powered motion followed by braking.

Customizing for momentum questions

Momentum is usually the cleanest method during a short interaction. Define the system carefully. If the system includes both colliding objects, the internal forces between them cancel in the momentum balance.

Use this structure:

1. Write total momentum before interaction.
2. Write total momentum after interaction.
3. Set them equal if external impulse is negligible.
4. Only then solve for speeds or directions.

Common mistake: assuming kinetic energy is conserved in every collision. Momentum is conserved in isolated systems; kinetic energy is only conserved in elastic collisions.

Useful exam phrase: “Total momentum is conserved because the external resultant force is negligible during the short collision time.”

Customizing for work and energy questions

Energy methods are often faster than force methods when the question only cares about initial and final states. They are especially helpful on slopes, falling objects, springs, and situations involving braking or resistive work.

Use this structure:

1. Identify the starting energy store and ending energy store.
2. Decide whether energy is conserved, transferred, or dissipated.
3. Write the balance equation.
4. Substitute values and solve.

Common mistake: saying energy is “lost”. In physics exam language, energy is transferred or dissipated, not destroyed.

Another common mistake: forgetting that work done against friction or drag reduces useful mechanical energy.

Customizing for mixed mechanics questions

The strongest students do not force one method onto the whole problem. They split the motion into stages.

A mixed question may look like this:

• Stage 1: object accelerates uniformly, so use SUVAT
• Stage 2: object collides with another object, so use momentum
• Stage 3: object rises to a maximum height, so use energy

When revising, practise drawing a line between stages and writing one sentence about the main principle for each stage. That habit makes longer exam questions far more manageable.

For more targeted practice after revising a topic, use A-Level Physics Topic Questions by Topic: The Best Practice for Each Paper Area. If your difficulty is understanding command words rather than content, Physics Command Words Explained: Calculate, Describe, Explain, Evaluate and More can help you turn working into the kind of answer examiners expect.

Examples

The aim of these examples is not only to get an answer, but to model the decision process.

Example 1: SUVAT selection

A car increases speed uniformly from 8.0 m s^-1 to 20 m s^-1 in 6.0 s. Find the acceleration and displacement.

Step 1: Identify known values
u = 8.0 m s^-1
v = 20 m s^-1
t = 6.0 s

Step 2: Find acceleration
Use v = u + at
20 = 8.0 + 6.0a
a = 2.0 m s^-2

Step 3: Find displacement
Use s = 1/2(u + v)t
s = 1/2(8.0 + 20)(6.0) = 84 m

Why this route is good: the second equation avoids carrying acceleration forward unnecessarily. In the exam, shorter valid routes reduce error.

Example 2: Momentum in a collision

A 0.50 kg trolley moving at 4.0 m s^-1 collides head-on with a 0.30 kg trolley moving at -1.0 m s^-1. They stick together. Find their final velocity.

Step 1: Define direction
Take the first trolley’s direction as positive.

Step 2: Momentum before
p = (0.50 x 4.0) + (0.30 x -1.0) = 2.0 - 0.30 = 1.7 kg m s^-1

Step 3: Momentum after
Combined mass = 0.80 kg
0.80v = 1.7
v = 2.125 m s^-1

Answer: about 2.1 m s^-1 in the positive direction.

Exam insight: because they stick together, this is not an elastic collision. Momentum is conserved, but kinetic energy is not fully conserved.

Example 3: Work and energy on a vertical rise

A 2.0 kg object is projected vertically upward at 10 m s^-1. Neglect air resistance. Find the maximum height reached.

Why choose energy? The question compares launch and highest point. Time is not required, so an energy method is efficient.

Initial kinetic energy
E_k = 1/2mv² = 1/2(2.0)(10²) = 100 J

At maximum height
Final kinetic energy = 0, so gain in gravitational potential energy = 100 J

Use mgh = 100
2.0 x 9.81 x h = 100
h ≈ 5.1 m

Alternative route: you could use v² = u² + 2as with final velocity zero and acceleration negative, but energy is often cleaner.

Example 4: Mixed method question

A ball falls from rest through 1.8 m, rebounds, and leaves the ground at 4.0 m s^-1. Find the speed just before impact and comment on the collision.

Stage 1: Falling
Use SUVAT or energy.

Using v² = u² + 2as
v² = 0 + 2(9.81)(1.8)
v ≈ 5.9 m s^-1

Stage 2: Collision with the ground
The rebound speed is 4.0 m s^-1, which is smaller than the impact speed.

Comment: the collision is not perfectly elastic because kinetic energy decreases. Some energy is transferred to heating, sound, and deformation.

This is exactly the kind of question where one formula is not enough. The marks come from choosing one method for each stage.

When to update

This page should be something you revisit, not read once and forget. Mechanics improves through repeated use of the same framework on new questions.

Come back to this guide when:

• you keep choosing the wrong formula even when you know the content
• you lose marks on sign conventions or unit conversion
• you are moving from topic learning into past-paper practice
• you notice that longer mechanics questions are really several shorter stages joined together
• your exam board wording or formula sheet expectations need checking

A practical update routine looks like this:

1. After each revision session, add one mechanics question you found difficult and sort it into SUVAT, momentum, energy, or mixed.
2. After each marked paper, note whether the error was conceptual, mathematical, or exam-technique based.
3. Every few weeks, rewrite your personal “formula selection checklist” in one page.
4. Before mocks and final exams, practise under timed conditions using topic questions and full papers.

Your checklist might be as short as:

• Constant acceleration? Use SUVAT.
• Short interaction? Check momentum.
• Start-to-finish state change? Try energy.
• Several stages? Split the problem.
• Need explanation marks? State assumptions.

That one page is often more useful than dozens of disconnected notes.

To keep your revision practical, combine this article with topic practice and specification-aware planning. Helpful next steps include A-Level Physics Topic Questions by Topic: The Best Practice for Each Paper Area and Best Order to Revise A-Level Physics Topics for Year 12 and Year 13.

Action step: build a mechanics revision sheet with four boxes labelled SUVAT, momentum, energy, and mixed. Each time you complete a question, file it into one box and write one line explaining why that method was appropriate. Over time, that turns mechanics from a memory test into a recognition skill, which is exactly what improves exam performance.