Big Idea
Motion describes how far something travels, how fast it's moving, the direction it's going, and how its speed is changing. You can understand any object's movement using formulas, and visualise it instantly on graphs.
Key Concepts at a Glance
Speed & Velocity
Speed is how fast (no direction). Velocity includes direction.
Acceleration
How quickly velocity changes. Positive = speeding up. Negative = slowing down.
Distance-Time Graphs
Show position over time. Gradient (slope) = speed.
Speed-Time Graphs
Show speed over time. Gradient = acceleration. Area under = distance.
Freefall
Objects fall with constant acceleration (g = 9.8 m/s²) unless air resistance acts.
Terminal Velocity
Maximum speed when air resistance equals weight. Object stops accelerating.
Speed & Velocity
What's the difference?
This is the biggest idea to nail down first:
- Speed tells you how fast something is moving. It only has a number: "I'm going 10 m/s." It doesn't care about direction.
- Velocity tells you how fast something is moving and in which direction. "I'm going 10 m/s north." It's the same speed value, but with a direction attached.
Analogy: Imagine driving 60 km/h on a straight road. Your speed is always 60 km/h. But your velocity changes every time you change direction—if you turn a corner, your velocity is now "60 km/h southwest" instead of "60 km/h north."
In physics language: Speed is a scalar quantity (magnitude only). Velocity is a vector quantity (magnitude + direction).
Speed Formula
What this means: If you travel 100 metres in 10 seconds, your speed is 100 ÷ 10 = 10 m/s. The further you travel in less time, the faster you're going.
Average Speed
In real life, objects don't always move at a constant speed. You speed up, slow down, stop at traffic lights. So we use average speed:
Example: You walk to a shop 1 km away, which takes 15 minutes (900 seconds). Your average speed is 1000 m ÷ 900 s ≈ 1.1 m/s. You might have walked faster sometimes and slower other times, but on average, that's the number.
Real-World Context: When you see "Average running speed = 3 m/s" or "Average cycling speed = 6 m/s," these are average speeds for a full journey, not instantaneous speeds.
Velocity Formula
The formula looks identical to speed, but it uses displacement instead of distance:
Key difference: If you walk 100 metres east, then walk 100 metres back west, your distance is 200 m, but your displacement is 0 m (you're back where you started). So:
- Average speed = 200 m ÷ time
- Average velocity = 0 m ÷ time = 0 m/s
Common Mistake:
Representing Velocity Direction
Acceleration
Speed–Time Graphs