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 |
|
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 |
|
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 |
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
|
or
for motion in a circle radius
in the direction of acceleration