Question

Consider the four free body diagrams. In which case does the net force equal 15N up?

Answer 1

D is the correct answer.  Since the forces are in opposite directions, we must subtract them.  So, 40 up - 25 down = 15 up.

Hope this helps!

Answer 2

Answer: D

Explanation:

The net force acting on an object is equal to the resultant of the forces acting on the object itself.

In object A), we see that the vertical forces are balanced (+3 N upward and -3 N downward), as well as the horizontal forces (+5 N right and -5 N left), so the net force is zero.

In object B), the vertical forces are balanced (+3 N upward and -3 N downward). The horizontal force (5 N left) is not balanced, so it produces a net force, however this net force is towards left.

In object C), there are only two vertical forces which are balanced (+20 N upward and -20 N downward), so the net force is zero.

In object D), there are only two vertical forces which are not balanced, and the net force is +40 N (upward) - 25 N (downward) = +15 N upward.