At what instant(s) do the two cars have the same velocity?

1. When your car first passes the Porsche.
2. When the Porsche passes you.
3. At both of the above instants.
4. At some time between 1. and 2.

Answer 4 is correct. You pass the Porsche because at that time you're going faster than it, and the Porsche later passes you because it's going faster than you. At some point between these instants the cars have the same speed.

Looking at the graphs for both cars at once can actually give us the answers to the sample problem we did earlier. This is another good way to check the answers.

The position graph shows that there are two locations where the cars pass each other. The red car passes the black one around 1.8 s, at x = 6 m, and the black one passes the red one around 5.7 s, at x = 50 m. This confirms our hard work earlier, solving the quadratic.

Finding the time when the black car passes the red one, you can then figure out how fast the black car is going at that time by reading it directly off the velocity graph. At t = 5.7 s, the black car's speed is about 17 m/s, about 50% more than the red car's speed at that time.