Kinematics: Displacement, Velocity & Acceleration

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Discover the differences between displacement, velocity and acceleration: three of the most important terms in kinematics, which are easy to mix up. Answer questions to test your knowledge.

What Is Kinematics?

Everything on Earth moves. Life is movement, and the Earth is brimming with it. So an important step in understanding the world is understanding movement, and that's what kinematics is all about.

Kinematics is the study of motion, without addressing any of the forces that cause the motion. In kinematics, we simply talk about how things move: how their position changes, how fast they go, and how that speed itself changes. We map out motion using graphs, numbers and calculations.

Key Kinematics Terms

To study kinematics, there are certain key terms we need to understand. Those terms are ones people often get mixed up, so let's go through them carefully.

Displacement is the change in position of an object or person, measured in meters. Displacement is a vector, which means that it has magnitude (size) and direction. For example, 50 miles is considered to be a distance, but 50 miles north of the origin is a displacement. Displacement ignores the path you took, and only considers the start and end points. It is said to be path independent. You can travel a huge, windy path and go a great distance, but if you end up where you started, your displacement is zero.

One thing that makes displacement confusing is that it can be used in two subtly different ways. You might say that over a period of six seconds, your displacement was 4 meters in the negative x-direction. When you say that, you mean that you ended up 4 meters away from where you started after the six seconds.

But you might also say that at the end of the 6 seconds your displacement is 7 meters in the x-direction. Those two numbers might seem to contradict each other, but they don't necessarily disagree at all. That's because displacement can be measured relative to a fixed origin, or alternatively it can be measured relative to where you started. Maybe you started at the coordinate (11,0) and ended at the coordinate (7,0). Your displacement at the end of the 6 seconds was 7 meters in the x-direction, but you displaced 4 meters during that time. Both of those facts are true. It's all about the way that you say it. So it's super important to be clear about what you mean when you talk about displacement.

Velocity is the rate at which your displacement is changing; it's how fast you're moving, and in what direction. Velocity is measured in meters per second. Strictly speaking, 25 meters per second is a speed, but 25 meters per second north is a velocity. However, sometimes in everyday life people miss mentioning the direction and still call it a velocity. If your velocity is 45 meters per second, it means your displacement (position) is changing by 45 meters every second.

Last of all, acceleration is the rate at which your velocity is changing, measured in meters per second per second (or meters per second squared). Your acceleration tells you how much your velocity is changing by each second, or the number of meters per second it changes by per second. That's why it's measured in meters per second, per second. For example if you're currently moving at 6 meters per second, and have an acceleration of 4 meters per second per second, then one second later you'll have a velocity of 10 meters per second. A second after that, you'll have a velocity of 14 meters per second. Then 18, and so on.

Race cars can accelerate rapidly.
Race cars can accelerate rapidly.

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