A 1351.9 kg car is traveling at 22 m/s when the driver takes his foot off the gas pedal.......

Question

A 1351.9 kg car is traveling at 22 m/s when the driver takes his foot off the gas pedal. It takes 4.2 s for the car to slow down to 20 m/s. How large is the net force slowing the car? Answer in units of N.

Answer 1

The velocity changes from 22 m/s to 20 m/s, a change of -2 m/s, over 4.2 seconds.  That's an acceleration of -2/4.2 = -0.48 m/s².

 

According to Newton's law, F=ma, so the force is 1351.9 * (-0.48) = -643.76 N.  The negative sign is because the force is in the direction opposite to the motion.  But since they ask "how large," they want the magnitude, so you can just say 643.76 N.