so just some elementary physics here

这里仅涉及到一些基础物理学知识
which most of you probably remember from high school physics,

估计你们在高中物理课上都学过
is that if an object,

就是对于一个物体
the object here is this green oval.

这个物体以绿色椭圆表示
If it's going to be moving with a constant speed what does that mean?

如果它以匀速运动这意味着什么
Moving with a constant speed means not accelerating.

匀速运动意味着物体没有加速
There has to be some force applied in order to create the motion

必须有力作用于物体它才能运动
but the forces must balance,

但受力必须平衡
so the overall forces acting are zero.

因此物体所受合力为零
For an object that's not accelerating the sum of the forces is zero

一个没有加速的物体所受合力为零
because the acceleration is zero.

因为加速度是零
If we think about this green object moving to the right,

如果我们考虑这个绿色物体向右边运动