A force is a push or a pull on an object.

力是對象的推動或拉動。

A force changes the motion of a moving object.

力會改變運動物體的運動。

A force can be strong or weak,and it always has a direction.

力可以強或弱，它總是有方向。

The Earth is a planet that goes around the sun in a circular orbit.

地球是一個圍繞太陽旋轉的行星。

The speed of its motion doesn't change,but the direction of its motion does.

它的運動速度不會改變，但它的運動方向確實會改變。

The Earth has a layer of gas above its surface,which helps to support life.

地球表面上方有一層氣體，有助于維持生命。

Atoms are the building blocks of matter.

原子是物質的基石。

At the center of an atom is its nucleus , which has a positive electric charge.

在原子的中心是它的原子核，它具有正電荷。

Outside the nucleus are electrons, which have a negative charge.

核外是電子，具有負電荷。

Asteroids and comets are objects tha orbit our Sun but are smaller than planets.

小行星和彗星是我們太陽運行的物體，但比行星小。

If the collide with Earth they could cause an extinction event.

如果與地球的結合，他們可能會導致滅絕事件。

Asteroids that pass close to the Earth are called near-earth objects.

靠近地球的小行星稱為近地天體。

The layer of gases just above Earth's surface is our atmosphere.

地球表面上方的氣體層是我們的大氣層。

The atmosphere is made up of gases such an oxygen and nitrogen.

大氣由氧氣和氮氣等氣體組成。

Without an atmosphere,there would be no life on Earth.

沒有大氣層，地球上就沒有生命。