Final answer:
To find the initial velocity of the car, we use the acceleration formula with the given values of -5 m/s² for acceleration and 5 s for time. The calculation reveals the initial velocity to be 25 m/s.
Step-by-step explanation:
The student asked what the initial velocity of a car was if it started to brake with an acceleration of -5 m/s² and took 5 seconds to stop. To solve this problem, we can use the formula for acceleration: a = (vf - vi) / t, where a is acceleration, vf is the final velocity, vi is the initial velocity, and t is time. Since the car comes to a stop, vf = 0 m/s, and acceleration is given as a = -5 m/s². The time taken to stop is t = 5 s.
Plugging these values into the formula, we get: -5 m/s² = (0 m/s - vi) / 5 s. Solving for vi, we find that the initial velocity vi = 5 s * -5 m/s² = -25 m/s. The negative sign indicates that the car is decelerating, so the initial velocity magnitude is 25 m/s. Hence, the correct answer is (a) 25 m/s.