A force is a push or a pull on an object.
力是对象的推动或拉动。
A force changes the motion of a moving object.
力会改变运动物体的运动。
A force can be strong or weak,and it always has a direction.
力可以强或弱，它总是有方向。
The Earth is a planet that goes around the sun in a circular orbit.
地球是一个围绕太阳旋转的行星。
The speed of its motion doesn't change,but the direction of its motion does.
它的运动速度不会改变，但它的运动方向确实会改变。
The Earth has a layer of gas above its surface,which helps to support life.
地球表面上方有一层气体，有助于维持生命。
Atoms are the building blocks of matter.
原子是物质的基石。
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，它具有正电荷。
Outside the nucleus are electrons, which have a negative charge.
核外是电子，具有负电荷。
Asteroids and comets are objects tha orbit our Sun but are smaller than planets.
小行星和彗星是我们太阳运行的物体，但比行星小。
If the collide with Earth they could cause an extinction event.
如果与地球的结合，他们可能会导致灭绝事件。
Asteroids that pass close to the Earth are called near-earth objects.
靠近地球的小行星称为近地天体。
The layer of gases just above Earth's surface is our atmosphere.
地球表面上方的气体层是我们的大气层。
The atmosphere is made up of gases such an oxygen and nitrogen.
大气由氧气和氮气等气体组成。
Without an atmosphere,there would be no life on Earth.
没有大气层，地球上就没有生命。
