# Chapter 7 - Newton's Third Law - Exercises and Problems: 11

(a) The friction between the foot and the ground is static friction. (b) The magnitude of the friction force is 440 N

#### Work Step by Step

(a) When the foot is in contact with the ground, the foot does not slide on the ground. There is no relative movement between the foot and the ground, so the friction between the foot and the ground is static friction. (b) We can find the sprinter's acceleration. $a = \frac{v-v_0}{t}$ $a = \frac{2.0~m/s-0}{0.25~s}$ $a = 8.0~m/s^2$ The friction force provides the force to accelerate the sprinter forward. We can find the magnitude of the friction force. $F_f = ma$ $F_f = (55~kg)(8.0~m/s^2)$ $F_f = 440~N$ The magnitude of the friction force is 440 N

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