Question

Which of these shows acceleration? How much is the net force? Question 6 options:

C is accelerating, the net force is 40 N up.
  D is accelerating, the net force is 15 N up.
  D is accelerating, the net force is 15 N down.
  C is accelerating, the net force is 0 N.

Answer 1

C is not accelerating. The forces shown are balanced.

Looking at Block-D, the 25N downward force cancels 25 of the 40N upward force. But that still leaves another 15N of upward force. So there's a net force of 15N up, and D is accelerating up. (2nd choice)