Fig. 1-2 A person tends to fall backward in an accelerating MTR train.

In an MTR train you may find it difficult to balance your body when the train starts to move or about to stop (accelerate or decelerate). When the train departs, your body tends to stay at rest while the train is accelerating forward, hence you tend to fall back (Fig. 1-2). Similar situation arises when the train is about to arrive at a station. The train begins to slow down, but you feel as if your body is still moving forwards at a high speed. In fact you are! If you were not attached to the train you would continue to move forwards until you collided with the front end of the train. In reality, of course, the floor and perhaps the handrail exert forces on your body and slow you down together with the train.

Fig. 1-3 Holding the stray-ring (left) or leaning on the glass besides the seat (right) may help passengers to balance their body in an accelerating/decelerating train.

The above experience in an MTR train is an example of inertia, that any body has the tendency to remain at rest or in uniform motion, unless it is disturbed by an external force. These results can be summarized by Newton's First Law:

The video clips and activity below may help you to understand the meaning of Newton's First Law.

	1.	Inertia of a person in an MTR train   Play [Low | High]    Download [Low(345kb) | High(610kb)]
		The posture of a person in an MTR train travelling between two stations. Note that the person tends to fall backward/forward when the train accelerates/decelerates.
	2.	Throwing a ball on a horizontal escalator   Play [Low | High]    Download [Low(69kb) | High(121kb)]
		A ball is thrown vertically upward on a moving horizontal escalator. The ball maintains its forward motion in air due to its inertia.
	3.	Hanging an object on a mini-motorbike   Play [Low | High]    Download [Low(38kb) | High(67kb)]
		An object is hanged by a string on a mini-motorbike. When the motorbike begins to accelerate, the ball swings backward due to its inertia.
		Activity: Feeling the inertia of our bodies inside a bus