When a body is going around a circle,

如果一个物体做圆周运动
they say that's some centrifugal force acting.

他们就说有离心力作用于这个物体
There is no such thing.

根本就没有这种力
Be careful.

一定要注意
Whenever there is a force,

如果有作用力
it can be traced back to a tangible material cause,

那它一定能找到一个明确的实体施力物体
which is all the time a force of contact,

也就是无论何时物体受的都是接触力
with the exception of gravity.

引力除外
Okay, so with that, if you write the right forces,

本着这个原则如果你受力分析画对了
you will be just fine.

那就功德圆满了
You will be able to solve all the problems we have in mechanics.

你就可以解决我们即将碰到的所有力学问题
I'm going to now start doing simple problems in mechanics.

现在我来讲解几个简单的力学题目