Physics › 🚀 Mechanics

What Is Motion?

Position, distance, displacement and velocity — the vocabulary every other physics idea is built on.

High schoolIntro
💡
The big idea: Motion is simply a change in position over time. To describe it precisely you need two things: a reference point to measure from, and a clear difference between how far something travelled (distance) and how far it ended up from the start (displacement).
🎯 By the end, you'll be able to
  • Describe position relative to a chosen reference point
  • Tell the difference between distance and displacement
  • Tell the difference between speed and velocity
  • Read a position–time graph and connect its slope to velocity
📎 You should already know
  • The number line & negative numbers
  • Reading simple graphs

It all starts with a reference point

Ask "where is the car?" and the only honest answer is "compared to what?" Position always needs a reference point (an origin) and a direction. Once you pick one — say, the school gate — every position becomes a signed number: +50 m up the road, −20 m behind it.

Motion is what happens when that position changes as time passes. Nothing more mysterious than that.

🔑 Distance vs displacement
Distance is the total ground covered — always positive. Displacement is the straight-line change in position from start to finish, with direction. Walk 3 m east then 3 m back west: distance = 6 m, displacement = 0. They are not the same thing.

Speed and velocity

Speed tells you how fast — distance per unit time. Velocity adds direction — it's displacement per unit time. A car going round a roundabout at a steady 30 km/h has constant speed but changing velocity, because its direction keeps changing.

\[ v = \frac{\Delta x}{\Delta t} \]
Velocity is the change in position (Δx) divided by the time taken (Δt).
🎮 Interactive: motion & the position–time graph LIVE
Set a speed and press Start. Watch the runner move along the track while the position–time graph draws itself. Notice how a bigger speed makes the graph line steeper — the slope of an x–t graph IS the velocity.
✨ The slope is the velocity
On a position–time graph, a steeper line means faster motion. A flat line means the object is standing still. A line sloping downward means it's moving back toward (and past) the origin. You can read an object's whole story off the shape of its x–t graph.
📝 Worked example: A cyclist rides 120 m east in 15 s, all in a straight line. What is her velocity?
  1. Displacement Δx = 120 m east; time Δt = 15 s.
  2. \(v = \dfrac{\Delta x}{\Delta t} = \dfrac{120}{15}\).
✓ v = 8 m/s east. (Direction matters — that's what makes it a velocity.)

Check your understanding

1. You jog 400 m around a running track and stop exactly where you started. Your distance and displacement are…
Distance is the total ground covered (400 m). Displacement is the change in position start-to-finish — you ended where you began, so 0 m.
2. On a position–time graph, a horizontal (flat) line means the object is…
Flat line = position isn't changing = zero velocity = standing still.
3. A car drives around a circular track at a steady 60 km/h. Which is constant?
Speed (how fast) is constant, but velocity includes direction, which changes continuously around the circle — so velocity is not constant.
✅ Key takeaways
  • Motion is a change in position over time, measured from a reference point.
  • Distance = total path length (no direction); displacement = straight-line change in position (with direction).
  • Speed has no direction; velocity does. v = Δx / Δt.
  • On a position–time graph, the slope of the line is the velocity.
🎓 Go deeper: external courses & trusted references

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