Once you have started the boulder rolling, it develops momentum, which is defined by its mass and velocity.
一旦你开始让巨石滚动，巨石就会产生动量，这是由其质量和速度决定的。
2
And in terms of equations that we use, it's sometimes easier to plug in the fact, since momentum is equal to mass times velocity.
在我们使用方程这方面，事实上有时是很容易代入的，因为动量等于质量乘以速度。
3
They're different because they don't exist behind a screen; they're in our world, built from real physics, not simulations, they have mass and velocity and smell.