The three questions kinematics answers
Kinematics is the branch of physics that describes motion — without worrying about what causes it (that’s a job for forces, covered elsewhere). It answers three basic questions about a moving object: Where is it? How fast is it going? Is that speed changing?
Position, velocity, and acceleration
- Position is where an object is, relative to some starting point.
- Velocity is how fast position is changing, and in what direction. A car going 60 mph north has a different velocity than one going 60 mph south, even though both have the same speed.
- Acceleration is how fast velocity is changing. Stepping on the gas, hitting the brakes, and turning a steering wheel are all forms of acceleration — even braking, since acceleration just means velocity is changing, not necessarily speeding up.
The kinematic equations
For motion with constant acceleration (a very common starting case), a handful of equations connect position, velocity, acceleration, and time. The most useful one to start with:
v = v₀ + at
In words: your final velocity (v) equals your starting velocity (v₀), plus however much your acceleration (a) has changed it over time (t).
A common mix-up
Speeding up and “positive acceleration” aren’t always the same thing. If a car is moving backward and slowing down, its acceleration actually points forward — it’s still accelerating, just in the direction that reduces its (negative) velocity. Paying attention to direction, not just size, is most of what makes kinematics tricky at first.
Why this matters
Kinematics is the language the rest of mechanics is written in. Before you can figure out why a ball follows the path it does (forces, covered in other Learn topics), you first need a precise way to describe the path itself — that’s exactly what kinematics gives you.