Rotation motion

What do a spinning top and a ceiling fan share?

Some things spin around a point. A fan, a wheel and a top all turn without moving away.

Rotation means an object turns around a fixed point or line, and that point stays still. On a flat page, the fixed point is called the center of rotation. The turn can go clockwise or counterclockwise.

• Rotation = turning around a fixed point
• Fixed point = center of rotation

In three dimensions, an object turns around an imaginary line. We call it the axis of rotation. Think of a door. The hinges are the axis, and the door swings around them.

When the axis passes through the object's own middle, we call that a spin.

Earth does this. It spins around its own axis, and that gives us day and night. If the axis is outside the object, the motion is called a revolution, or an orbit.

• Earth spins on its axis
• Earth spin = one day

So Earth does both at once. It spins each day and revolves around the Sun each year.

Rotating things also have angular velocity, which simply means how fast they turn.

Here is one example. A fan blade near the middle moves slowly, but the tip travels a longer circle. Let us see what you remember. Rotation is turning around a fixed point or axis, called a spin if the axis is inside and a revolution if outside.