Everything in the universe moves. To understand physics, we first need a clear way to describe that movement. Let's start with the basics: where something is and how far it has travelled.

Imagine you walk from your home to a shop 500 metres down the street. The total distance you have covered is 500 metres. Simple enough. Now, imagine you walk back home. You've now walked a total distance of 1000 metres (500 metres there, 500 metres back).

But are you actually 1000 metres away from where you started? No, you're right back at your front door. This is where a new concept, displacement, comes in. Displacement is your change in position from your starting point, including direction. After your round trip to the shop, your displacement is zero.

Distance just cares about the total ground covered. Displacement is the straight-line distance and direction from start to finish. In physics, this distinction is crucial.

The difference between distance and displacement introduces us to two important categories of measurement: scalars and vectors.

A scalar is a quantity that has only magnitude (a size or amount). Distance is a scalar. If you say you walked 1000 metres, you've given a complete description of the distance. Other examples include time (30 seconds) and temperature (20°C).

A vector is a quantity that has both magnitude and direction. Displacement is a vector. To fully describe it, you need to say how far and in which direction, like "500 metres east."

Forgetting about direction can lead to very different results, which is why vectors are so fundamental in physics.

Just as we distinguished between distance and displacement, we must do the same for how fast an object is moving.

Speed is a scalar quantity that tells you how fast you're covering distance. It's calculated by dividing the distance travelled by the time it took.

Velocity, on the other hand, is a vector. It's the rate at which an object's position changes. It is calculated by dividing displacement by time. Since displacement has a direction, velocity does too.

Often, an object’s velocity isn't constant. It might speed up, slow down, or change direction. Any change in velocity is called acceleration.

Because velocity is a vector, acceleration is also a vector. You are accelerating if:

1. Your speed increases.
2. Your speed decreases (this is often called deceleration, but in physics it's just negative acceleration).
3. Your direction of motion changes (like the car on the circular track).

Graphs are a powerful tool for understanding motion. The two most common types are position-time graphs and velocity-time graphs.

A position-time graph plots an object’s position (on the y-axis) against time (on the x-axis). The slope, or gradient, of the line on this graph tells you the object's velocity.

• A horizontal line means the object is stationary (velocity is zero).
• A straight, sloped line means the object is moving at a constant velocity.
• A steeper slope means a higher velocity.
• A curved line means the velocity is changing, so the object is accelerating.

A velocity-time graph plots velocity (y-axis) against time (x-axis). The slope of this graph tells you the acceleration, and the area under the line tells you the displacement.

• A horizontal line means constant velocity (zero acceleration).
• A straight, sloped line means constant acceleration.
• The area under the graph gives the total displacement during that time interval.

These basic concepts form the language we use to describe motion. Mastering them is the first step to understanding the more complex reasons why things move the way they do.