Question 3
In activity 1, what is observed about the reading of Newton balance A, the one connecting the trolley to the wall?
The right answer is the one marked above.
It equals the sum of the readings of B and C. B and C pull in the same direction, so A has to balance their resultant.
The difference applies when forces are set against each other — in activity 1 B and C are not opposed, they pull together. And were A's reading zero, the trolley would be dragged away.