An early submersible craft for deep-sea exploration was raised and lowered by a cable from a ship. when the craft was stationary, the tension in the cable was 7500 n. when the craft was lowered or raised at a steady rate, the motion through the water added an 1800 n drag force.

In this example, we would first have to address what the two numbers provided mean. We learn that when the early submersible craft was stationary, the tension in the cable was 7500 n. Moreover, when the craft was lowered or raised, the motion through the water added an 1800 n drag force.

If we want to find out what the tension in the cable was when the craft was being lowered to the seafloor, we would need to get the difference between the two numbers:

7500-1800= 5,700 n

On the other hand, if we want to find out what the tension in the cable was when the craft was being raised from the seafloor, we need to add the two forces:

7500+1800= 9,300 n

a and b sometimes have a positive value