(HT only) Students should be able to explain qualitatively, with examples,…

The idea

Speed is a scalar quantity indicating how fast an object moves, whereas velocity is a vector quantity representing speed in a stated direction. For an object moving in a circle, such as a car on a roundabout or a satellite in orbit, its direction of travel changes continuously. Because velocity depends on direction, the velocity is constantly changing even if the object's speed remains perfectly constant. The instantaneous velocity is always directed along the tangent to the circular path. Understanding that a change in direction constitutes a change in velocity is crucial for explaining circular motion.

What earns the marks

• States that speed is a scalar quantity (magnitude only) while velocity is a vector quantity (magnitude and direction).
• Explains that an object moving in a circular path continuously changes its direction of motion.
• Concludes that because the direction changes, the velocity must be changing even if the speed remains constant.
• Provides a valid example of circular motion, such as a satellite in orbit, a car on a circular track, or a whirling bung.

Common mistakes and how to put them right

• Stating that velocity is constant because the speed is constant.

  Put it right by emphasising that velocity is a vector and changes if the direction changes.

• Confusing the terms speed and velocity in explanations.

  Put it right: Strictly defining speed as a scalar and velocity as a vector in a given direction.

• Failing to provide a concrete example when asked to explain the concept.

  Put it right by memorising and citing a specific example like a fairground carousel or an orbiting moon.

Exam tips

• As this is a Higher Tier concept, always explicitly state that velocity includes direction, which continuously changes during circular motion.
• Use a specific example, such as a car on a roundabout, and consider drawing a quick sketch with tangent arrows to illustrate the changing direction.