Explain that an object moving in a circular orbit at constant speed has a…

The idea

Velocity is a vector: it has both a size (speed) and a direction. An object in a circular orbit can move at constant speed, but its direction of travel changes continuously as it goes round the circle. Because the direction changes, the velocity changes, even though the speed stays the same. A changing velocity means the object is accelerating, and that acceleration is directed towards the centre of the circle, caused by a centripetal force such as gravity for a satellite. This is qualitative only: you need to explain the reasoning, not calculate values.

What earns the marks

• States that velocity is a vector quantity with both magnitude and direction.
• Explains that the direction of motion of an object in a circular orbit changes continuously.
• Concludes that a changing direction means the velocity changes even when the speed is constant.
• Links the changing velocity to an acceleration directed towards the centre of the orbit.

Common mistakes and how to put them right

• Claims that constant speed means constant velocity.

  Put it right: Recognise that a changing direction changes the velocity.

• Treats velocity as a scalar equal to speed.

  Put it right: Velocity also includes direction.

• Says the object has no acceleration because its speed is constant.

  Put it right: Recognise that a change in direction is an acceleration.

• Places the acceleration along the direction of motion.

  Put it right: Direct it towards the centre of the circle.

Exam tips

• Define velocity as speed in a stated direction before applying it to the orbit.
• Focus on the change of direction as the reason the velocity changes.
• Link the changing velocity to a centripetal acceleration towards the centre.
• Keep the answer qualitative; no calculation is needed.