What is the force of gravity?
In the 17th century, Isaac Newton discovered that there is a small attractive force between any two objects. It exists even between you and this book, or between you and your classmates. Usually, this force is so small that we do not notice it. However, if one of the objects is the Earth (of which the mass is very large), the force is noticeable. Wherever we are on Earth, there is a force pulling us towards the centre of the Earth. This force is called the force of gravity. It keeps us on the Earth's surface. It causes objects to fall back to the Earth. It also acts on a spacecraft and tends to pull it back to Earth.
The force of gravity is a non-contact force. It can act on an object at a distance. For example, the Earth exerts a force of gravity on a spacecraft even when the spacecraft travels into space. The force of gravity decreases as distance increases. Therefore, the Earth will exert a smaller force on a spacecraft when it is further away from the Earth.
＊ The force of gravity tends to pull objects towards the centre of the Earth.
重力 (gravity) 是一種非接觸力 (non-contact force)，而重力亦可說是地心吸力，而地心吸力則是萬有引力的一種，那麼萬有引力是啥？就是說宇宙中的所有事物，每樣事物皆會有互相牽引的力量，其實我們也有，只是地心吸力太大，我們不太感覺得到而已。
萬有引力 (Gravitation) 又名引力相互作用或重力相互作用。在一般使用上，亦即重力 (Gravity)。在物理學上，萬有引力或重力是指具有質量的物體之間加速靠近的趨勢。