Acar travels at a constant speed around a circular track whose radius is 2.9 km. the car goes once around the track in 400 s. what is the magnitude of the centripetal acceleration of the car?

First we need the linear velocity 
 D=2*Pi*r=2*pi* 2.9=18.22 km=18,221.23m 
 V=D/T=18,221.23m/400s= 45.55 m/s 
 A=V^2/r=(45.55 m/s)^2/2900m= 0.72 m/s^2 
 the magnitude of the centripetal acceleration of the car is 0.72 m/s^2