Problem 1:You drive at a constant speed of 20 miles/h around a city block, checking out the neighborhood. Are you accelerating?
Yes, the direction of the velocity vector is changing all the time.
Problem 2:You are traveling east at 30 miles per hour. You see a ball rolling onto the road and you break hard, because you are afraid that a child will come running after the ball. You come to a stop in 0.8 seconds. What is the direction of your average velocity in this short time interval? What is your average acceleration?
The direction of your average velocity is east. You are traveling east with decreasing speed until you come to a stop. The change in your velocity is Dv = v2-v1 = 0 - 30 mph east = -30 mph east = 30 mph west. Let us convert to SI units.
30 mph times 1609 m/mile times 1 h/(3600 s) = 13.4 m/s.
Your average acceleration is Dv/Dt = (13.4 m/s)/(0.8 s) west = 16.8 m/s2 west.