Forces.
部队。
Force is a push or a pull on an object.
力是对物体的推或拉。
If you push or pull something, you can change its position.
如果你推或拉某物，你可以改变它的位置。
It takes more force to move a heavy object than a light object.
移动重物比移动轻物需要更多的力。
If you kick a ball, the ball will move away from you.
如果你踢球，球就会离开你。
The harder you kick the ball, the farther it will move.
你踢得越用力，球就会移动得越远。
If you kick a ball up into the air, it will follow a curved line to the ground.
如果你把一个球踢向空中，它会沿着一条曲线落地。
Gravity is pulling the ball down.
重力把球拉下来。
The amount of force that pulls something down toward Earth is called its weight.
将某物拉向地球的力称为它的重量。
Gravity always pulls objects towards the ground.
重力总是把物体拉向地面。
If you bounce a ball, it will go up and then down.
如果你弹球，它会先升后降。
Friction is a force that makes an object slow down when it moves over another object.
摩擦力是一种使物体在移动时减慢速度的力。
Smooth surfaces cause less friction than rough ones.
光滑的表面比粗糙的表面产生的摩擦力小。