Motion
Scalars and Vectors
Physics quantities are either scalars (magnitude only) or vectors (magnitude + direction).
| Scalar | Vector equivalent |
|---|---|
| Distance | Displacement |
| Speed | Velocity |
| Time | Acceleration |
| Mass | Force |
Distance is the total path length travelled; displacement is the straight-line distance from start to finish in a given direction.
Speed is the rate of change of distance; velocity is the rate of change of displacement:
velocity (m/s) = displacement (m) ÷ time (s)
Equations of Motion (SUVAT)
For uniform (constant) acceleration the four kinematic equations apply:
| Equation | Variables |
|---|---|
| v = u + at | no s |
| s = ½(u + v)t | no a |
| s = ut + ½at² | no v |
| v² = u² + 2as | no t |
Where: s = displacement (m), u = initial velocity (m/s), v = final velocity (m/s), a = acceleration (m/s²), t = time (s).
Acceleration is the rate of change of velocity: a = (v − u) / t
A negative acceleration (deceleration) means the object is slowing down in the direction of motion.
Distance–Time Graphs
- Gradient = speed (steep = fast).
- Horizontal line → stationary.
- Straight line → constant speed.
- Curve → changing speed (acceleration or deceleration).
To find speed at an instant on a curve: draw a tangent at that point and calculate its gradient.
Velocity–Time Graphs
- Gradient = acceleration (positive gradient = speeding up; negative = slowing down).
- Horizontal line → constant velocity (zero acceleration).
- Area under the graph = displacement.
For a trapezium-shaped graph: area = ½(a + b) × h where a, b are the parallel sides (times) and h is the height (velocity).
Core Practical 1 — Investigating the motion of objects (trolley on ramp)
Equipment: ramp, dynamics trolley, ticker tape + ticker timer (or light gates + data logger), ruler, power supply.
Method:
- Connect ticker tape to the trolley; thread tape through the ticker timer.
- Release the trolley from rest down the ramp.
- Measure dot spacings on tape (dots every 0.02 s at 50 Hz) to calculate velocity at each interval.
- Plot a velocity–time graph; gradient = acceleration.
Key control variables: ramp angle fixed; same trolley mass; compensate for friction by raising one end slightly so the trolley moves at constant speed on a flat tape test.
Edexcel examiner tip: In a 6-mark QWC question on CP1, state the method, the key measurements, how you process results (gradient of v–t graph), and state any safety precautions (trolley stopping block at end of ramp).
Stopping Distances
Total stopping distance = thinking distance + braking distance.
- Thinking distance depends on reaction time (affected by tiredness, alcohol, drugs, distractions).
- Braking distance depends on vehicle speed (proportional to v²), road condition, tyre condition, brake condition.
At 30 mph: thinking ≈ 9 m, braking ≈ 14 m → total ≈ 23 m. At 70 mph: thinking ≈ 21 m, braking ≈ 75 m → total ≈ 96 m.
AI-generated · claude-opus-4-7 · v3-edexcel-physics