1. Standing passengers are pushed backwards when a bus suddenly starts due to the inertia of rest of the upper part of the body which tends to remain at rest.
2. Fast moving bullet cuts a hole in a window pane, because that portion of glass which is hit by the bullet comes in motion while the neighbouring region of glass remains at rest due to inertia.
3. In card and coin experiment, when the card placed over a glass tumbler is flipped quickly, then the coin falls into the tumbler due to the inertia of rest of the coin.
4. Due to the large inertia of the heavy ball, sudden jerk breaks the lower string fastened below the ball.
5. Standing passengers fall towards the front when a bus stops suddenly, because the upper portion of the body tends to remain in motion even though the lower portion in contact with the floor of the bus has come to rest as the bus comes to a halt. For a similar reason, the sitting passengers fall backwards when a bus starts suddenly.
6. The force (F) which causes an acceleration (a) in a moving object of mass (m) is given by F=ma
7. The SI unit of force is Newton.
8. To every action there is always an equal and opposite reaction and action-reaction forces act on different bodies.