A drag racer crosses the finish line, and the driver deploys a parachute and applies the brakes to slow down, as Figure 2.8 illustrates. The driver begins slowing down when t₀=9.0 s and the car’s velocity is v₀=+28 m/s. When t=12.0 s, the velocity has been reduced to v=+13 m/s. What is the average acceleration of the dragster?

Figure 2.8  (a) To slow down, a drag racer deploys a parachute and applies the brakes. (b) The velocity of the car is decreasing, giving rise to an average acceleration that points opposite to the velocity. (© Geoff Stunkard)

Reasoning  The average acceleration of an object is always specified as its change in velocity, v–v₀, divided by the elapsed time, t–t₀. This is true whether the final velocity is less than the initial velocity or greater than the initial velocity.

Solution The average acceleration is, according to Equation 2.4,