Scalars vs Vectors, SI Units & Measurement: MYP Physics Explained
Physics starts with measurement: if you cannot put a number and a unit on something, you cannot test a prediction about it. Once you can measure, the next question is whether direction matters — and that single question separates scalars from vectors, one of the most useful ideas in the whole subject.
Try it yourself: four short missions on what can be measured, SI units, scalar vs vector quantities, and a vector navigation lab where you plot a route. Your progress saves on this device.
What makes something measurable?
A physical quantity is something you can measure with a number and a unit — length, mass, time, temperature. Feelings such as "how scary a film is" are not physical quantities, because there is no agreed unit or instrument. A useful test: could two different people measure it and get the same result?
SI units: one shared language
Scientists use the International System of Units (SI) so results can be compared anywhere. The base units you meet most often are the metre (length), kilogram (mass), second (time), kelvin (temperature) and ampere (electric current). Other units are combinations — speed is metres per second (m/s), force is the newton. Always write the unit; a number without one loses most of its marks.
Scalars and vectors
Some quantities need only a size, and others need a direction too:
- •Scalar — magnitude only: distance, speed, mass, time, energy, temperature.
- •Vector — magnitude and direction: displacement, velocity, force, acceleration.
Distance vs displacement — a worked example
Walk 3 km east, then 4 km north. The distance you travelled is 3 + 4 = 7 km, a scalar. Your displacement is the straight line from start to finish: because the two legs are at right angles, Pythagoras gives √(3² + 4²) = 5 km, at an angle of about 53° north of east. Same journey, two different answers — and both are correct, because they answer different questions.
Adding vectors
Vectors are added head to tail: draw the first, start the second where the first ends, and the resultant runs from the very start to the very end. If vectors point in opposite directions they partly cancel, which is why two equal forces pulling in opposite directions give a resultant of zero. This is the idea behind navigation, forces and, later, motion in two dimensions.
How this is assessed
Expect Criterion A questions that ask you to classify quantities, convert or state units, and apply vector addition to a simple scenario. Show the direction as well as the size whenever the question involves a vector.
Frequently asked questions
A scalar has magnitude only (e.g. speed, mass). A vector has magnitude and direction (e.g. velocity, force, displacement).
Distance is the total path length travelled (a scalar). Displacement is the straight-line change in position from start to finish, with a direction (a vector).
A single international system means measurements made anywhere can be compared and combined without conversion errors.