This is a fun question - your science teacher is creative!
First is easy, 25000 feet converts to 7620 meters
Potential energy is a product of mass, gravity and height. In this case, mass is 90Kg and height is 7620m. gravity on earth is 9.8m/s2.
Potential energy = 90(9.8)(7620) = 6720840J
By Newton 2nd law, force is equal to mass times acceleration.
Gravity force = mass*gravity
= 90(9.8) = 882N
Acceleration when he leaves the plane is gravity, 9.8m/s2
With no wind resistance, the acceleration is constant at 9.8.
Using kinematic equation, v2 = u2 + 2as where u is initial velocity, v is final velocity, a is acceleration and s is distance traveled.
In this case, u=0, a=9.8, s=7620
v^2 = 2(9.8)(762) = 149352
final velocity = sqrt (149352) = 386.5m/s
150MPH converts to 67m/s.
If he is falling at a constant speed of 150MPH, then he is not accelerating.
As he is not accelerating, the net force = gravity force - wind resistance = 0
wind resistance = gravity force
= 90(9.8) = 882N
If there is no wind resistance, by conservation of energy,
final Kinetic Energy = initial Potential energy
= mass * gravity * height
= 90*9.81*7620
= 6720840J
He is unhurt because the safety net provides a soft landing. It decelerates his momentum gradually over time to avoid injury.
