Answer: 180 Newtons.
Step-by-step explanation:
The alternate form of Newton's second law states that force (F) is equal to mass (m) multiplied by acceleration (a):
F = m x a
In this problem, the mass of the runner (m) is 90 kg, and the acceleration (a) can be found by dividing the change in velocity (8 m/s) by the time interval (4 s):
a = (8 m/s) / (4 s) = 2 m/s^2
Substituting the values into the equation for force:
F = 90 kg x 2 m/s^2
F = 180 N
Therefore, the net force on the runner is 180 Newtons.