L4-U1-P2 Science 🔬科学

A force is a push or a pull on an object 力是作用在物体上的推货拉

L4-U1-P2 Science 🔬科学

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 力可以变强也可以变弱，它总是有方向的

The earth is a planet that goes around the sun in a circular orbit 地球是以环形轨道绕着太阳运转的行星

The speed of its motion doesn't change，but the direction of its motion does 它运动的速度没有改变，但是它运动的方向会改变

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帮助维持生命

L4-U1-P2 Science 🔬科学

Atoms are the building blocks of matter 原子是物质的组成模块

L4-U1-P2 Science 🔬科学

At the center of an atom is its nucleus，which has a positive electric charge 原子中心是带有正电荷的原子核

Outside the nucleus are electrons which have a negative charge 原子核外面是带有负电荷的电子

Asteroids and comets are objects that orbit our sun ，but are smaller than planets 小行星和彗星是绕着太阳转但比行星小的物体

L4-U1-P2 Science 🔬科学

Asteroids that pass closed to the Earth are called near-earth objects 靠近地球的小行星被称为近地物体

The layer of gases just above Earth's surface is our atmosphere 地球表面上方的一层气体是我们的大气层

The atmosphere is made up of gases，such as  oxygen and nitrogen大气层是由气体组成的，比如氧气和氮气

Without an atmosphere  there would be no life on Earth没有大气层，将没有生命在地球上

A force can increase or decrease the speed of an object，and it can change the direction of motion力能使物体的速度变大或变小，且能改变运动的方向

If they collide with Earth they could cause  an extinction event 如果它们与地球碰撞，会导致毁灭事件

The electric force between the nucleus and the electrons holds the atom together 原子核与电子之间的电力（电场）使原子结合在一起