Kinematics · 20 min
Speed & Velocity
More than speed — direction matters.
Why should I care?
Two cars pass a checkpoint at the same speed, heading in opposite directions. Are they moving the same way?
Speed alone can't tell these cars apart. Velocity can — because velocity is a vector, and direction is part of the answer, not an afterthought.
Explore
Read the slope
On a position-time graph, velocity is the slope. A steeper line means a faster velocity; a negative slope means the object is heading back the way it came.
The idea
Rate of change, with a direction
Speed is how fast something moves — a scalar, always positive. Velocity is the rate at which displacement changes with time — a vector, which can be positive or negative depending on direction. Average velocity over an interval is total displacement divided by total time.
True or false?
An object can have zero velocity while still moving.
Worked example
Worked example
A car travels 150 m east in 10 s, then reverses and travels 50 m west in 5 s. Find its average velocity for the whole trip.
- 1
Find total displacement: 150 − 50 = 100 m east.
Your turn
Which quantity is a vector?