What is the difference between speed and velocity at N(A)-Level?

Speed is how fast an object moves and is a scalar, so it has size only. Velocity is speed in a stated direction and is a vector, so it has both size and direction. A car going at 20 metres per second has a speed of that value; saying 20 metres per second due north gives its velocity. The N(A)-Level marker expects you to add the direction when a question asks for velocity.

How do you find acceleration from a speed-time graph?

Acceleration is the gradient (slope) of a speed-time graph. You read off the change in speed on the vertical axis and divide it by the time taken on the horizontal axis, using $a = (v - u)/t$. A straight sloping line means constant acceleration, a flat horizontal line means constant speed (zero acceleration), and a line sloping downward means deceleration.

What does the area under a speed-time graph represent?

The area between the line and the time axis on a speed-time graph gives the distance travelled. For a flat line this is just speed times time (a rectangle); for a sloping line you find the area of the triangle or trapezium underneath. This is a favourite N(A)-Level question because it tests graph reading rather than a memorised formula.

What is free fall, and why do objects fall at the same rate in a vacuum?

Free fall is motion under gravity alone, with no air resistance. In a vacuum all objects, whether heavy or light, accelerate downward at the same rate, about $10\ \text{m s}^{-2}$ near Earth, because gravity gives every mass the same acceleration. In air, a feather falls more slowly than a coin only because air resistance affects the feather more, not because gravity is weaker on it.

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Measurement and Kinematics for Singapore N(A)-Level Science (Physics) 5105/5106: SI base quantities and units, prefixes, measuring length and time, speed, velocity and acceleration, and reading distance-time and speed-time graphs including free fall

A Singapore N(A)-Level Science (Physics) overview of Measurement and Kinematics (SEAB 5105/5106). It covers the SI base quantities and units, common prefixes and measuring instruments, the definitions of speed, velocity and acceleration with one-step calculations, and how to read distance-time and speed-time graphs, including free fall and air resistance.

Generated by Claude Opus 4.86 min readSEAB-5105Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this module covers
Physical quantities and SI units
Speed, velocity and acceleration
Motion graphs and free fall
How this module is examined
Check your knowledge

What this module covers

Measurement and Kinematics is the starting point of N(A)-Level Science (Physics) 5105/5106. It gives you the language of measurement, the SI base quantities and units, and the prefixes and instruments you use in the laboratory. It then describes motion using speed, velocity and acceleration, and shows how distance-time and speed-time graphs turn a story of movement into a picture you can read.

Everything later in the course is measured, so getting units and instruments right here pays off across the whole syllabus. The dot points below each carry full worked answers and practice questions.

Physical quantities and SI units

Physical quantities and SI units sets out the base quantities you need: length in metres, mass in kilograms and time in seconds. It also covers prefixes such as milli, centi and kilo, and how to choose the right instrument, a ruler or measuring tape for length and a stopwatch for time, with sensible precision.

Speed, velocity and acceleration

Speed, velocity and acceleration defines the three core quantities of motion. Speed is distance divided by time, velocity is speed in a stated direction, and acceleration is the rate of change of velocity. The one-step formulas are

\text{speed} = \frac{\text{distance}}{\text{time}}, \qquad a = \frac{v - u}{t},

where

u

is the starting speed and

v

is the final speed.

Finding acceleration

A car speeds up from $5.0\ \text{m s}^{-1}$ to $20\ \text{m s}^{-1}$ in $3.0\ \text{s}$ . Find its acceleration.

step Identify the quantities

The starting speed is $u = 5.0\ \text{m s}^{-1}$ , the final speed is $v = 20\ \text{m s}^{-1}$ , and the time is $t = 3.0\ \text{s}$ .

step Apply the acceleration formula

a = \frac{v - u}{t} = \frac{20 - 5.0}{3.0} = \frac{15}{3.0} = 5.0\ \text{m s}^{-2}.

step State the result clearly

The car accelerates at $5.0\ \text{m s}^{-2}$ , meaning its speed rises by $5.0\ \text{m s}^{-1}$ every second.

Motion graphs and free fall

Motion graphs and free fall shows how graphs describe motion. On a distance-time graph the gradient is the speed: a steeper line means faster motion and a flat line means the object is at rest. On a speed-time graph the gradient is the acceleration and the area underneath is the distance travelled.

The module also covers free fall, the motion of an object under gravity alone. Near Earth this acceleration is about $10\ \text{m s}^{-2}$ . With air resistance, a falling object speeds up until the air resistance grows to equal its weight, after which it falls at a steady terminal velocity.

How this module is examined

Units and prefixes. Quote the correct SI unit and convert prefixes such as $1\ \text{km} = 1000\ \text{m}$ before calculating.
Read the gradient. Speed is the gradient of a distance-time graph; acceleration is the gradient of a speed-time graph.
Use the area. The distance travelled is the area under a speed-time graph, found as a rectangle, triangle or trapezium.

Check your knowledge

Recall and calculation questions across the module. Attempt them, then check the worked solutions.

State the SI unit of length, mass and time. (3 marks)
A runner covers $100\ \text{m}$ in $20\ \text{s}$ . Calculate the average speed. (2 marks)
A cyclist slows from $12\ \text{m s}^{-1}$ to $0\ \text{m s}^{-1}$ in $4.0\ \text{s}$ . Calculate the acceleration. (2 marks)
State what the gradient of a distance-time graph represents. (1 mark)
State what is meant by free fall. (1 mark)

Sources & how we know this

Singapore-Cambridge GCE N(A)-Level Science (Physics, Chemistry) Syllabus 5105/5106 — Singapore Examinations and Assessment Board (2026)