英语流利说 Level 4 Unit1 Part2 Vocabu

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

A force changes the motion of a moving object.

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

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

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

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

Atoms are the building blocks of matter.

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

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

Asteroids and comets, are objects that orbit our sun, but are smaller than planets.

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

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

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

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

(The electric force between the nucleus and the electrons holds the atom together.)

(If they collide with Earth they could cause an extinction event).