A force is a push or a pull on an object.
力是对物体的推或拉。
A force changes the motion of a moving object.
力改变运动物体的运动。
A force can be strong or weak and it always has a direction.
力可以是强的，也可以是弱的，它总是有方向的。
The earth is a planet that goes around the sun in a circular orbit.
地球是一颗以圆形轨道绕太阳运行的行星。
The speed of its motion doesn't change, but the direction of its motion does.
它运动的速度不变，但运动的方向不变。
The earth has a layer of gas above its surface, which helps to support life.
地球表面有一层气体，有助于维持生命。
Atoms are the building blocks of matter.
原子是物质的组成部分。
At the center of an atom is its nucleus, which has a positive electric charge.
原子的中心是原子核，它带一个正电荷。
Outside the nucleus are electrons, which have a negative charge.
原子核外是带负电荷的电子。
Asteroids and comets, are objects that obey our sun, but are smaller than planets.
小行星和彗星是服从太阳的天体，但比行星小。
Asteroids that pass close to the Earth are called near-earth objects.
靠近地球的小行星被称为近地天体。
The layer of gases just above Earth's surface is our atmosphere.
地球表面上的气体层就是我们的大气层。
The atmosphere is made up of gases, such as oxygen and nitrogen.
大气是由气体组成的，如氧和氮。
Without an atmosphere, there would be no life on Earth.
没有大气层，地球上就没有生命。
A force can increase or decrease the speed of an object, and it can change the direction of motion.
力可以增加或减少物体的速度，它可以改变运动的方向。
If they collide with Earth they could cause an extinction event.
如果它们与地球相撞，就可能导致灭绝事件。
The electric force between the nucleus and the electrons holds the atom together.
原子核和电子之间的电力能使原子结合在一起。
