Velocity, v, in metres per second, m/s

The idea

Velocity, v, is the speed of an object in a stated direction, measured in metres per second (m/s). It is a vector quantity, meaning a change of direction alters the velocity even if the speed remains constant. In the context of momentum (which is assessed at Higher Tier only), v represents the velocity of the moving object, and the equation p = m × v relies on this value. For example, a cyclist travelling at 6 m/s due north has a velocity of 6 m/s north. If she turns and rides at 6 m/s due south, her speed is unchanged but her velocity has reversed. When substituting into p = m × v, use the velocity value with its direction. A negative sign indicates motion in the opposite direction.

What earns the marks

• States that velocity is speed in a stated direction, making it a vector quantity.
• Identifies the standard unit of velocity as metres per second (m/s).
• Applies velocity, v, correctly in the Higher Tier momentum equation p = m × v.
• Recognises that changing direction changes velocity, even at a constant speed.
• Interprets a negative velocity value as motion in the opposite direction to the chosen positive axis.

Common mistakes and how to put them right

• Treating velocity and speed as identical.

  Put it right by stating that velocity includes a specific direction, whereas speed is a scalar.

• Writing the unit as m/s².

  Put it right by using m/s for velocity, as m/s² is the unit for acceleration.

• Ignoring direction when comparing velocities.

  Put it right by checking whether the direction of travel has changed before concluding the velocity is identical.

Exam tips

• Always check if a direction is specified in the question before using a velocity value in a momentum calculation.
• If a calculation results in a negative velocity, explain that the negative sign indicates motion in the opposite direction.