The average speed of a moving object is defined as the distance travelled divided by the time taken:

We take the KCR trip from University Station to Tai Po Market Station as an example (Fig. 2-1). It takes about 6 minutes to travel a distance of 6.7 km. Thus,

Fig. 3-1 Speedometer in a car.
	Speedometer
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The SI unit for speed is metre per second ( ). To convert our result to SI unit, we multiply 1.12 by 1000 and divide by 60. Hence . The speed is an "averaged" value because the train may have travelled faster or slower than during the trip. In particular, when the train has just departed from the University Station, or is about to arrive at Tai Po Market Station, its speed is lower than the average value.

On the other hand, the instantaneous speed tells us how fast the train is travelling at a given instant. When travelling in a car, you might have seen a speedometer which displays the car's instantaneous speed (Fig. 3-1).

Some typical speeds are shown below:

Speed is a scalar quantity which has a magnitude only. Velocity tells how fast an object moves and its direction of motion. Velocity is a vector quantity. The average velocity of an object is defined as

Take the KCR train as an example. The displacement of Tai Po Market Station from University Station is 5.4 km, so

,

in the North-West direction. This is smaller than the average speed of the train.

Similar to speed, the velocity of an object may also change during a trip. Therefore we use instantaneous velocity to describe the velocity of the object at a given instant. The instantaneous speed and the direction of motion specify the instantaneous velocity of an object

If the instantaneous velocity of a moving object is constant throughout the trip, then the object is said to be in uniform motion. The instantaneous velocity at any instant will be equal to the average velocity. For uniform motion, average velocity and instantaneous velocity are the same, and the displacement is related to the velocity and the travelling time by