Q: Kinematics: Understand and use the language of kinematics: position; displacement; distance travelled; velocity; speed; acceleration.

The idea

Kinematics describes motion without considering its causes. Position is where an object is relative to a chosen origin, often given as a coordinate or vector. Displacement is the change in position: a vector with magnitude and direction. Distance travelled is the total length of path covered, a scalar, so it is never negative. Velocity is the rate of change of displacement, a vector; speed is the magnitude of velocity, a scalar. Acceleration is the rate of change of velocity, a vector, so an object can accelerate by changing direction even at constant speed. In problems, choose a positive direction, then use signs consistently: negative displacement means motion in the opposite direction.

For example: A ball thrown up and caught has zero displacement but non-zero distance travelled.

What earns the marks

• Defines position as location relative to an origin and displacement as change in position, a vector.
• Distinguishes distance travelled (scalar path length) from displacement (vector change in position).
• Defines velocity as rate of change of displacement and speed as the magnitude of velocity.
• Defines acceleration as rate of change of velocity, including change of direction.
• Uses sign conventions correctly to represent direction in one-dimensional motion.
• Applies these definitions to interpret graphs or simple motion scenarios.

Common mistakes and how to put them right

• Confusing distance and displacement.

  Put it right by sketching the path and marking start and end positions.

• Treating speed and velocity as interchangeable.

  Put it right by stating direction when giving velocity.

• Assuming acceleration only means speeding up.

  Put it right by defining it as any change in velocity.

Exam tips

• State your positive direction at the start of a solution and keep to it throughout.
• When asked to compare speed and velocity, mention both magnitude and direction explicitly.
• Use the wording of the question: 'distance travelled' requires total path length, while 'displacement' requires a straight-line vector from start to finish.