Speed and velocity

Speed and velocity answer the same question, how much ground something covered and over what time, but speed gives you a number and velocity gives you that same number with a direction attached. The HESI A2 physics subtest builds its entire motion unit on this distinction, so reading a value correctly as a velocity, rather than mistaking it for a plain speed, decides whether later questions on acceleration and projectile motion make sense or feel like guesswork.

Distance and displacement

Distance is the total length of the path something actually covers. It only ever adds up, metre by metre, no matter how the path curves or doubles back on itself. Displacement measures something else: the straight-line change from where an object started to where it ended up, reported together with a direction. The two agree only when a trip runs in one straight line without doubling back.

Step counter versus displacement. A nurse's fitness tracker can log several kilometres of walking across one busy shift, all of it real distance. If that nurse starts and ends the shift at the same nurses' station, the shift's net displacement is still zero, however high the step count reads.

Why velocity needs a direction

A quantity that stands complete as a number and a unit is a scalar. Distance is a scalar, and so is speed. A quantity that only becomes complete once a direction is attached is a vector. Displacement is a vector, and so is velocity, which is speed reported together with a direction of travel.

This is not a technicality the exam drops for convenience. Two ambulances can both be travelling at exactly 60 km/h and still have completely different velocities if one heads north out of the depot while the other heads south. Their speedometers would read identically. Their velocities would not.

Calculating average speed

Average speed only ever asks for two numbers, the total distance covered and the total time it took, one divided by the other.

Quantity	Formula	Vector or scalar
Distance	total path length covered	Scalar
Displacement	straight-line change in position	Vector
Average speed	v = d / t	Scalar
Average velocity	v = displacement / t	Vector

Any consistent pair of units works, metres and seconds or kilometres and hours. The one habit worth automating before exam day is checking that the distance and the time you are dividing belong to the same stretch of the trip, since pairing distance from one leg of a journey with time from a different leg is the fastest way to turn an easy mark into a wrong one.

Speed tells you how fast something is moving; velocity tells you how fast and in which direction.

Once distance versus displacement and speed versus velocity both feel automatic, the next lesson in this unit builds straight on top of them: acceleration, the rate at which velocity itself changes over time. The 9.8 m/s² you just met does not disappear. It becomes the constant behind every falling or thrown object you work through next.