Speed and velocity

1. Speed is a scalar quantity, so it tells you only how fast an object is moving.
2. Velocity is a vector quantity, so it tells you both how fast an object is moving and in which direction.
3. Two objects can have the same speed but different velocities if they are moving in different directions.
4. The velocity of an object therefore changes if its speed changes, if its direction changes, or if both change.
5. An object moving at a constant speed can still have a changing velocity, as long as its direction of motion is changing.

Moving in a circle

1. A planet in a circular orbit stays the same distance from its star and moves at a constant speed.
2. At every instant, the planet's velocity points along a tangent to the circle, at right angles to the line joining the planet to the centre.
3. As the planet moves round, the direction of this tangent keeps turning, so the direction of the velocity keeps changing.
4. The size of the velocity, which is the speed, stays the same, but its direction is never the same from one moment to the next.
5. Because the velocity is changing, the planet is accelerating, even though it is not speeding up or slowing down.

Gravity provides the inward force

1. By Newton's first law, an object's velocity changes only if a resultant force acts on it.
2. Without any force, a planet would carry on in a straight line along the tangent and drift off into space.
3. The force that stops this is the gravitational force of attraction between the star and the planet.
4. This force acts along the line joining the planet to the star, so it always points towards the centre of the orbit.
5. The force continuously pulls the planet away from its straight-line path and bends its motion into a circle.
6. The planet's acceleration is in the same direction as the force, so the planet is accelerating towards the centre of the circle.
7. Whirling a ball on a string shows the same idea, because the tension in the string pulls the ball towards your hand and keeps it moving in a circle.
8. If the string snaps, the inward force disappears and the ball flies off in a straight line along the tangent.
9. The same reasoning applies to the Moon orbiting the Earth and to artificial satellites, with the Earth's gravity providing the inward force.

Why the speed does not change

1. In a circular orbit, the gravitational force is always at right angles to the direction in which the planet is moving.
2. A force at right angles to the motion has no part pulling forwards or backwards along the path, so it cannot speed the planet up or slow it down.
3. The force does no work along the direction of motion, so the planet's kinetic energy stays the same and its speed stays constant.
4. The only effect of the force is to change the planet's direction, which is why the velocity changes while the speed does not.