## College Physics (4th Edition)

The average frictional force is $8400~N$ and this force is exerted in the opposite direction of the car's motion.
We can find the acceleration: $v_f = v_0+at$ $a = \frac{v_f-v_0}{t}$ $a = \frac{0-28~m/s}{4.0~s}$ $a = -7.0~m/s^2$ The magnitude of acceleration is $7.0~m/s^2$ We can find the average frictional force: $F = ma$ $F = (1200~kg)(7.0~m/s^2)$ $F = 8400~N$ The average frictional force is $8400~N$ and this force is exerted in the opposite direction of the car's motion.