Angular momentum is about how things spin. It's a sort of “spinning power” of something that’s turning or rotating. Big heavy things that spin quickly have a lot of spin power.

Imagine you’re on a playground merry-go-round. If you’re sitting near the edge, and the merry-go-round is spinning, you feel it pulling you outwards. If you move closer to the centre, the merry-go-round starts to spin faster without anyone pushing it more. That’s because you’re helping it spin faster by moving closer, and that is related to angular momentum.

The size or strength of the angular momentum depends on three things:

• How massive the spinning object is. The more massive the object, the greater its angular momentum. A heavier object spinning at the same speed as a lighter one will have more angular momentum.

• How fast the object is spinning (its speed): If the object spins faster, its angular momentum increases, meaning it has more "spin power."

• How far the mass is from the centre of rotation (its distance from the centre): This is the distance from the centre of the circle, like when a ball spins around on a string. If the mass is farther from the centre, or if the spinning object is wider, the angular momentum also increases. This is because having more mass farther out means more "spin power."

In the figure below the objects on the right will have a smaller angular momentum than the items on the left, if they all rotate at the same speed.

The angular momentum is a vector quantity, because the spinning direction is important. We represent a vector quantity with a little arrow. Do you remember what is a vector? We introduced them in the article Measurement scales and vectors. For angular momentum, the vector will show the “strength” of the spinning and the “direction” of the spin.

What is the spinning direction?

Imagine you’re spinning a ball around on a string. The direction of the angular momentum is not along the string or the circle: it’s along the rotation axis, which is an imaginary line around which something rotates. When an object spins, all the parts around it move in circles, but that center line, the rotation axis, just stays put and doesn’t go anywhere.

We find the direction of the angular momentum using the “right-hand rule”:

• Hold your right hand out and curl your fingers in the direction of the spinning.
• Your thumb, sticking out, shows the direction of the angular momentum vector!

So, if a top is spinning clockwise on a table, the vector of angular momentum points straight up.

Here are some examples:

Angular momentum follows a special rule called the law of conservation of angular momentum. This means that angular momentum doesn’t just disappear: it stays constant unless an outside force changes it. So, if something starts spinning, it will keep spinning with the same angular momentum unless something stops it or slows it down.

A great way to understand this is by looking at ice skaters or dancers spinning with their arms stretched out.

When they pull their arms in close, they start to spin faster. When they stretch their arms out again, they slow down. Even though their speed changes, their angular momentum stays the same. They control their speed by changing how close their arms are to their body: pulling their arms in makes them spin faster (because they get closer to the centre), while stretching their arms out slows them down (because their body becomes wider). Try it yourself!