There are two forces acting on the teacher:
Force due to weight/gravity (Fg)
Force due to drag (Fd), which is a resistance opposite to the direction of motion. Think of an airplane flying through the sky: there will be air that tries to oppose the plane's direction of motion AKA air-resistance.
The force of gravity is always downward (the direction of gravity).
Like we said before, the force of drag is always opposite to the direction of motion. Since the teacher is falling down, the force of drag is exerted upward.
Look at the attached diagram. The teacher is the circle in the middle. The two arrows indicate the two forces and their directions.
Now let's look at numbers:
Fg = mg = 65kg * 9.81 m/s^2 = ??N
Fd = 320N
To find the "Net Force" we must add up all of the forces exerted on the teacher, BUT we have to take into account the direction of forces.
Let's define downward as our "positive" direction. Since downward is positive, that means our force due to gravity is positive = +Fg
But since our force due to drag is UPWARD that means our force is NEGATIVE = -Fd.
So our total net force is
[tex]\Sigma F = F_g - F_d[/tex]
