#### Answer

The peak force on the foot of the runner is a vertical force of 2500 N

#### Work Step by Step

We can find the runner's weight:
$weight = mg = (85~kg)(9.80~m/s^2) = 833~N$
We can find the peak force, which is three times the runner's weight:
$F_{peak} = 3\times 833~N = 2500~N$
The peak force on the foot of the runner is a vertical force of 2500 N.