A car is coasting backwards downhill at a speed of 3.2 m/s when the driver gets the engine started. After 2.5 s, the car is moving uphill at 5.0 m/s. Assuming that the uphill is the positive direction, what is the car's average acceleration?

Answer 1

Divide the velocity change, (5.0 -(-3.2)) = 8.2 m/s, by the time interval (2.5 s).

The answer will have dimensions of m/s^2

Note the sign change in the velocity.