## Definitions

Terms commonly used everyday by ordinary people may have a slightly different meaning in physics. Many people for example confuse velocity and speed. These are not the same, the difference being that velocity is the speed of a body in a certain direction. The difference is important therefore if the direction is changing, for instance if the motion is in a circle. A body moving at constant speed in a line experiences no acceleration and is subject to no net force, but a body moving is a circle has a velocity that is constantly changing. This body does have an acceleration AND experiences a net force. It is necessary therefore to define acceleration in terms of the velocity, not the speed.

Some common definitions used in kinematics are given in the table below, with some of their properties.

 Quantity Symbol Definition Example SI Unit Vector or Scalar Displacement Distance moved in a particular direction The displacement from London to Birmingham is 120 miles North West Metre (m) Vector Velocity or Rate of change of displacement The average velocity of a 2 hour car journey from London to Birmingham is 60 mph North West Metres per second (m/s) Vector Speed V or u Rate of change of distance The average rate speed of a two hour journey from London to Birmingham is 65 mph. Notice this is higher than the velocity. The distance travelled is at least equal to the displacement. Metres per second (m/s) Scalar Acceleration Rate of change of velocity for motion in a circle radius m/s/s Vector Momentum Product of mass and velocity A particle mass 1 kg moving in the x direction with speed 3 m/s has momentum 3 kg m/s Kg m/s Vector force Rate of change of momentum, also expressed as If the particle above accelerates from 0 to 3 m/s in 2 s then the momentum has changed from 0 to 3 kg m/s in 2 s so the force is in the direction of acceleration newton (N) vector Impulse Change in momentum, often during collisions or when a body experiences a force The body above receives an impulse of 3 kg m/s kg m/s Vector