# Find the least acceleration and least retardation?

1. Jun 22, 2013

### Douna2nd

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?

2. Jun 22, 2013

### tiny-tim

Hi Douna2nd!

Show us what you've tried, and where you're stuck, and then we'll know how to help!

(use the standard constant acceleration equations)

3. Jun 22, 2013

### Douna2nd

It was in my final exam today actually and I did in the first case S=Vot + 1/2 a t^2 and I have the distance, the intial velocity and time and I got the a then in the second case I didn't actually know what to do but I assumed that S =0 and used the same rule used above and got the acceleration but I wanted to know what is the correct answer for this problem

4. Jun 22, 2013

### tiny-tim

nooo, s is still 40, v is 0

you knew u v and s, and you wanted a, so the correct formula would be … ?

5. Jun 22, 2013

### Douna2nd

V^2 =Vo^2 +2 a S
I solved the problem in a wrong way in the exaaaaam!! :O

6. Jun 22, 2013

### tiny-tim

'fraid so!

but hopefully you've done well in the rest of the exam

7. Jun 22, 2013

### Douna2nd

Why is the velocity zero in the second case? The car will be moving but moving slowly

8. Jun 22, 2013

### tiny-tim

no …
… if she stops at (or before) the light, her speed is 0 at (or before) the light

the least deceleration will be the one that makes her speed 0 at the light

9. Jun 22, 2013

### Douna2nd

Aahh! Thank you !! :D