 Have you ever been on a merry-go-round in a carnival? As you are swinging around, you are actually undergoing circular motion. In this video, we will explain why an object moving in a circle with a constant speed has a changing velocity. First of all, what's the difference between velocity and speed? Speed is a measure of how far something travels a distance in a particular time. For example, a car travelling at 25 metres per second or a person jogging at 4 metres per second are both measures of speed. Velocity is a speed in a particular direction. For example, a van travelling north at 25 metres per second and a raindrop moving down at 1 metre per second. For each of these statements, decide whether they are a measure of speed or velocity. Pause the video while you think. When an object is moving in a circle at a constant speed, it has a certain velocity, v, which is at a tangent to the circle. At the same time, the mass is also accelerating towards the centre of the circle. But you may be wondering, why is the object accelerating when the speed remains constant? To understand this, we must review the factors that cause velocity to change. As you know, a change in velocity results in acceleration from this equation. Acceleration equals the change in velocity over time. The two ways in which velocity can change are a change in speed or a change in direction. So when you're moving in a circle at a constant speed, your velocity is constantly changing because the direction you are moving in is also constantly changing. But you may be wondering, what actually causes the acceleration? As you know from Newton's second law of motion, the net force acting on an object is directly proportional to its acceleration. In the case of circular motion, the net force that causes this acceleration is called the centripetal force and it is always directed towards the centre of the circle. You can easily test this by attaching a ball to a string and swinging it round your head. The speed of the ball is constant, but you need to keep pulling the string to make it go in a circle. What direction do you think the ball will follow if you suddenly let go of the string? Is it in the red direction or the blue direction? The ball will continue to travel in the same direction, so with the same velocity along the red direction. Try it yourself and see. So that's how the velocity can change whilst speed remains the same, by changing direction and not speed.