A red car and a green car, identical except for the color, move toward each other in adjacent lanes and parallel to an x axis. At time t = 0, the red car is at xr = 0 and the green car is at xg = 220 m. If the red car has a constant velocity of +20 km/h, the cars pass each other at x = 44.6 m, and if it has a constant velocity of +40 km/h, they pass each other at x = 76.9 m.
(a) What is the initial velocity of the green car? (Indicate direction with the sign of your answer.) in m/s
(b) What is the acceleration of the green car? (Indicate direction with the sign of your answer.) in m/s2
I think I'm supposed to use:
Xf = Xi + Vxi*t + 0.5at2
The Attempt at a Solution
I'm having a hard time with this one - even knowing where to start.
For part (a), it suggested setting the equations equal to each other for each set of data. I converted the constant velocity to m/s (20 km/hr = 5.56 m/s; 40 km/hr = 11.11 m/s) to avoid conversion errors later.
So I figured out how much time it would take the car at 5.56 m/s to reach the 44 m mark:
∆x/V = t --> 44.6m / 5.56m/s = 8.02 sec. So I thought I could take the green car from the opposite end and see how fast it would have to go to clear the distance and meet the red car at 8.02 seconds. So ∆x=-175.4m (accounting for direction), t=8.02 and V is unknown.
When I plug that into the equation I get ∆x/t = V (-175.4/8.02), so V=-21.87m/s... but when I submitted the answer online it was wrong and it said the right answer was -13.3m/s.
And then for part (b), I thought if you have a constant velocity, you have an acceleration of 0. As far as the problem seemed to me, both cars are moving at a constant velocity when the problem "starts". But my homework program says the answer is -2.12 m/s^2. so... huh?
Any help would be awesome, guys!