Final answer:
Using the kinematic equation v² = u² + 2as with the given acceleration of -6 m/s² and stopping distance of 20 m, the initial speed (u) turns out to be approximately 15.49 m/s, which is not listed in the provided options.
Step-by-step explanation:
To determine how fast a car was going before it started slowing down with an acceleration of -6 m/s² and stops in 20 m, we can use the kinematic equation: v² = u² + 2as, where:
- v is the final velocity (0 m/s since the car stops)
- u is the initial velocity (what we are trying to find)
- a is the acceleration (-6 m/s²)
- s is the distance (20 m)
Substituting the given values into the equation: 0 = u² + 2(-6)(20).
Solving for u:
u² = 240
u = √240
u = approximately 15.49 m/s
However, this speed is not one of the provided options (a-d). There's either a mistake in the problem or in the proposed answers.