Displacement
Motion & Kinematics - OCR A-Level Physics
Key Definition
Displacement
The straight-line distance from a fixed reference point in a specified direction. It is a vector quantity, measured in metres (m).
The straight-line distance from a fixed reference point in a specified direction. It is a vector quantity, measured in metres (m).
Key Definition
Velocity
The rate of change of displacement. It is a vector quantity, measured in $m s^{-1}$.
The rate of change of displacement. It is a vector quantity, measured in $m s^{-1}$.
$$v = \frac{\Delta s}{\Delta t}$$
Key Definition
Acceleration
The rate of change of velocity. It is a vector quantity, measured in $m s^{-2}$.
The rate of change of velocity. It is a vector quantity, measured in $m s^{-2}$.
$$a = \frac{\Delta v}{\Delta t}$$
- Speed is the magnitude of velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. and is a scalar quantityA quantity that has magnitude (size) only. It has no direction.
- Distance is the total path length travelled and is a scalar quantityA quantity that has magnitude (size) only. It has no direction.
- An object can have constant speed but changing velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. if its direction changes (e.g. circular motion)
- Instantaneous velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. is the velocity at a single moment in time; average velocityTotal displacement divided by total time taken. A vector quantity. = total displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m). / total time
Common Mistake
MEDIUM
Students often: It's tempting to treat displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m). and distance as interchangeable, giving scalar answers when a vector is needed.
Instead: Displacement is a vector (straight-line, with direction); distance is a scalar (total path length). A runner completing a lap has distance > 0 but $displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m). = 0$.
Instead: Displacement is a vector (straight-line, with direction); distance is a scalar (total path length). A runner completing a lap has distance > 0 but $displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m). = 0$.