it's not going into the block,

不会到斜面里面去
nor is it flying out of the block; it's moving along the block.

也不会飞离斜面它会沿着斜面运动
That's the reason we chose a coordinate to be perpendicular to the block,

所以我们才选择了一个垂直于斜面的坐标系
because that coordinate,

因为在这个坐标系中
the y coordinate, it's not changing.

y 坐标是不变的
Even though the block is sliding down the hill,

即使物体从上面滑下来
when it's sliding down the hill

当它从上面滑下来
the usual x and y coordinates,

如果用通常的 x 和 y 坐标系的话
if you define them this way,

如果这样定义坐标系的话
they're all changing.

它们都会变化
But by this clever choice,

但因为这个聪明的选择
it's always at y = 0.

物体总是保持 y = 0