Compared to displacement and velocity, acceleration is like the angry, fire-breathing dragon of motion variables. It can be violent; some people are scared of it, and if it’s big, it forces you to take notice. That feeling you get when you’re sitting in a plane during take-off, or slamming on the brakes in a car, or turning a corner at high speed in a go-kart are all situations where you are accelerating.

Acceleration is the name we give to any process where the velocity changes. Since velocity is a speed and a direction, there are only two ways for you to accelerate: change your speed or change your direction or change both.

If you’re not changing your speed and you’re not changing your direction, then you simply cannot be accelerating no matter how fast you’re going. So, a jet moving with constant velocity at 800 miles per hour along a straight line has zero acceleration, even though the jet is moving fast since the velocity isn’t changing. When the jet lands and quickly comes to a stop, it will have acceleration since it’s slowing down.

Or, you can think about it this way. In a car, you could accelerate by hitting the gas or the brakes, either of which would cause a change in speed. But you could also use the steering wheel to turn, which would change your direction of motion. Any of these acceleration, since they change velocity.