A-Level Physics — 3) Motion & Forces (IP-Friendly Guide)
Download printable cheat-sheet (CC-BY 4.0)14 Jul 2025, 00:00 Z
TL;DR
Graphs, Kinematics and Dynamics sit at the heart of every Motion & Forces question.
Nail the language (position vs displacement), pick the right graph tool (area or gradient) and keep a one-page suvat cheat-sheet handy. This post turns the SEAB bullet-points into WA-ready check-lists and timing hacks.
1 Kinematics warm-up — language matters
| Term | Scalar / Vector | One-liner you must be able to recite |
| Position \((x)\) | — | Where the object is relative to a chosen origin. |
| Distance \((d)\) | Scalar | Path length travelled, ignores direction. |
| Displacement \((s)\) | Vector | Change in position with direction. |
| Speed \((v)\) | Scalar | Distance per unit time. |
| Velocity \((\vec v)\) | Vector | Displacement per unit time. |
| Acceleration \((\vec a)\) | Vector | Rate of change of velocity. |
1.1 Quick mnemonic
Dis-place-ment starts with D and P — Direction and Position. Burn it in; exam setters love swapping “distance” and “displacement”.
2 Graphical super-powers
2.1 Position-time (x-t)
- Gradient \(\frac{\Delta x}{\Delta t}\) gives instantaneous velocity.
2.2 Velocity-time (v-t)
- Gradient → acceleration.
- Area under curve → displacement.
IP hack: Photocopy a single v-t diagram with five different slopes and areas; annotate s and a for each. Spend 3 min every Sunday redrawing it from memory until Prelims.
3 SUVAT — Five Equations
Uniform acceleration in a straight line means the following quartet always applies:
\[ \begin{array}{l c} \text{No displacement }&s& \hspace{5em} & v & = & u + at \hspace{10em} \\[6pt] \text{No initial velocity }&u& \hspace{5em} & s & = & vt - \tfrac12 at^2 \hspace{10em} \\[6pt] \text{No final velocity }&v& \hspace{5em} & s & = & ut + \tfrac12 at^2 \hspace{10em} \\[6pt] \text{No acceleration }&a& \hspace{5em} & s & = & \tfrac12(u + v)t \hspace{10em} \\[6pt] \text{No time }&t& \hspace{5em} & v^2 & = & u^2 + 2as \hspace{10em} \end{array} \]
Derivation rides purely on the definitions of velocity and acceleration plus triangular areas under the v-t graph.
3.1 suvat flow-chart
- List the five symbols \((s,u,v,a,t)\).
- Strike out what the question does not give.
- Pick the single equation that skips that symbol — no simultaneous equations, no tears.
3.2 Classic free-fall
Take \(g = 9.81 \space \pu{m.s-2}\) downward; set \(u = 0\) when an object is released from rest.
4 Mass, inertia and why bowling balls don 't dodge
Mass is the built-in stubbornness against acceleration — the bigger the mass, the bigger the reluctance.
Class demo: roll a tennis ball and a bowling ball with the same push; ask students to time how fast each stops. The disparity screams “inertia”.
5 Linear momentum — the motion coin
Define
\[
\vec p = m\vec v
\]
Vector, conserved in closed systems, core to collision problems.
5.1 Elastic vs inelastic crash mini-drill
Give two carts totalling \(2 \space \pu{kg}\) at \(3 \space \pu{m.s-1}\) and \(1 \space \pu{kg}\) at \(-2 \space \pu{m.s-1}\). Compute total \(\vec p\) before and after; verify they match when KE is lost.
6 Newton 's trilogy
| Law | Exam-level phrasing | Teaching tip |
| 1 | Object stays at rest or constant velocity unless acted on by a resultant external force. | Use an air-track puck to visualise. |
| 2 | Resultant force proportional to rate of change of momentum, directionally same. | Connect to \(\vec F = m \vec a\) when \(m\) constant. |
| 3 | Forces between two bodies are equal in magnitude, opposite in direction, same line of action. | Balloon-rocket demo works. |
6.1 Common Misconceptions Around Newton's Third Law
There can be tricky questions to testing your understanding for Newton's Third Law 3.
Many students misidentify the correct force-reaction pairs in free-body diagrams (FBDs).
A classic example is the reaction force of the weight of a box resting on a table as seen in the FBD below.
The opposite reaction force to the weight of a box \(\text{W}_\text{b}\) resting on a table is the gravitational force of the ball pulling on the Earth \(\text{F}_\text{gravity, box on Earth}\).
It is NOT the normal reaction force of the table on the ball. The normal reaction force is the opposite reaction to the ball pushing against the table.
7 Vector diagrams — tip-to-tail never fails
Add forces graphically: draw the first vector, start the second at its tip, resultant runs from the original tail to the new tip.
Student challenge: sketch an object on an incline, resolve weight into \(mg \sin \theta\) down-slope and \(mg \cos \theta\) perpendicular. Plug into \(F = ma\) to find acceleration, then suvat for time to slide 2 m.
8 Three WA timing rules (Motion edition)
- Spend 30 s drawing the right sketch before touching equations.
- Highlight given variables in yellow — prevents mixing up u and v.
- For “show that” proofs, copy the target result first; reverse-engineer which suvat gets you there.
9 Further reading
10 Call-to-action
Parents: schedule a 1-h Motion & Forces clinic two weeks before WA 1; we cover graph reading, suvat drills and Newton's Laws MCQs.
Students: print this post, fold to A5, stick inside your formula booklet — revisit every bus ride.
Last updated 14 Jul 2025. Next review after SEAB releases the 2027 draft syllabus.
