where vy is the velocity of the student just before hitting the ground. Solving this expression for the height yields
(1)
At this point we do not know the impact velocity vy. However, it can be determined by examining the collision of the student with the ground. Just before the collision, the velocity of the student is vy. Since the student comes to rest, the final velocity is vf = 0 m/s. The average force exerted on the student by the ground is = +18 000 N, and the time of collision is = 0.010 s. The impulse-momentum theorem, Equation 7.4, relates these variables:
(2)
Now v0 is the impact velocity of the student, which was labeled as vy in Equation (1); therefore, v0 = vy. Thus, Equation (2) becomes . Solving this equation for vy and substituting the result into Equation (1) yields
= +18 000 N, and the time of collision is 
= 0.010 s. 
. 