Recall that velocity is speed in a stated direction

The idea

Velocity is a vector quantity: it tells you how fast something moves and also which way it moves. Speed is a scalar, so it gives only the magnitude, for example 8 m/s. Velocity adds a stated direction, for example 8 m/s due north, or 8 m/s along the positive x-axis. Two cars can travel at the same speed but have different velocities if they head in different directions, and a car going round a bend at constant speed still changes velocity because its direction changes. In calculations, treat a reversal of direction as a change in velocity even when the speed is unchanged. When you answer a question, check whether a direction is stated; if it is not, the quantity is speed, not velocity.

What earns the marks

• Identifies velocity as speed together with a stated direction.
• Distinguishes speed as a scalar quantity from velocity as a vector quantity.
• States that a change of direction changes velocity even when speed stays constant.
• Recognises that equal speeds in different directions are different velocities.

Common mistakes and how to put them right

• Treats velocity and speed as interchangeable.

  Put it right: Add the stated direction to describe velocity.

• Assumes a car moving round a bend at steady speed has constant velocity.

  Put it right: Note that its changing direction changes its velocity.

• Gives a direction without a magnitude when asked for velocity.

  Put it right: Supply both the speed value and the stated direction.

Exam tips

• Read the question for a direction word such as north, upwards or along a bearing before deciding whether the quantity is speed or velocity.
• When a multiple-choice option gives only a number, check whether a direction is also needed to make it velocity.
• Use the phrase 'speed in a stated direction' when explaining the difference in a written justification.