Fig. 1-1 The mass of an MTR train is about 3200 tons. It takes quite a long time to stop it or set it into motion.

In daily life, we notice that it is very difficult to change the motion of a massive object. We can set a bicycle into motion easily, but we may be exhausted after pushing a broken car on a road, and of course it is virtually impossible for us to move an MTR train of 3200 tons without supplying huge electrical power to it. The mass of an MTR train is so large that it is very difficult to set it into motion from rest. On the other hand, if it is in motion, it is also very difficult to stop it (Think about the time and distance for an MTR train to stop in an MTR station).

Objects have a certain kind of "laziness" to stay in their original state of motion either at rest or in constant speed motion along a straight line. The tendency for an object to maintain its motion is called inertia. It is difficult to change the motion of an object with a large inertia. Mass is a measure of inertia, its SI unit is kilogram (kg).