"Black line" to connect the battery negative (Ground);

“黑线”接电瓶的负极（搭铁）；

youdao

Remember, you've got to keep track of the sign of acceleration, so if you're picking up speed towards the ground, a is negative, so it will be g plus a, but a is a negative number.

记住,一定要注意加速度的符号,所以当你加速下降的时候,a 是负的,还是 g 加上 a,但是 a 是负数

耶鲁公开课 - 基础物理课程节选

In the upswing, consumers took on mucho debt, the savings rate at one point dwelled in negative ground, normally 6 percent of personal income.

FORBES: How To Play Housing's Recovery

As it stands, it's unclear whether anyone, including Brennan, knows what negative consequences are emerging on the ground because of remote drones.

CNN: Bring drones out of the shadows