A car is travelling at 10 m/sec. When it is 40 meters from a crosswalk, there is still 3 seconds before the light changes to red. The driver has two choices: the first is to accelerate to reach the crossing before the light changes to red and the second is to try to stop at the light. What is the least acceleration which would be necessary in the first case? And what is the least retardation which would be necessary in the second case?