# Minimum stopping distance

1. Sep 14, 2007

### klm

The minimum stopping distance for a car traveling at a speed of 30 m/s is 60 m, including the distance traveled during the drivers reaction time of 0.560 s.
what is the min. stopping distance for the same car traveling at a speed of 38.0 m/s?

tried to use this equation v^2 - u^2 = 2 a s
but then didnt know what to do.

2. Sep 14, 2007

### Staff: Mentor

Use that equation to figure out the car's acceleration. What distance did you use? (How far has the car traveled before the brakes are applied?)

3. Sep 14, 2007

### klm

43.2m is the real distance he first travels with the velocity of 30 m/s. because i subtracted 16.8 m from the 60 m.
so my a= 10.416

4. Sep 14, 2007

### Staff: Mentor

Good! Now apply that--in reverse--to the second case, where the speed is 38 m/s, to find the new total stopping distance. (You'll have to calculate a new "reaction time distance".) The car's acceleration remains the same.

5. Sep 14, 2007

### klm

i dont know how to find a new reaction time distance??
i used this formula :
0^2 - 38^2 = 2(10.416) x

6. Sep 14, 2007

### Staff: Mentor

Do it the same way as before. How far does the car move before the brakes are applied?

7. Sep 14, 2007

### klm

38 x .56?
=21.28 m
but what do i do with that?

8. Sep 14, 2007

### Staff: Mentor

You add it to the other distance. Understand what happens when the person wants to stop: (1) there's a time delay due to reaction time, so the car moves some distance before the brakes are even applied; (2) once the brakes are applied, the car accelerates to a stop.

The total stopping distance is the sum of both of these distances.

9. Sep 14, 2007

### klm

you add 21.28 to 60? to get 81.28m?

10. Sep 14, 2007

### Staff: Mentor

No. 60m was the total stopping distance for the 30m/s case--nothing to do with the 38 m/s case!

Add 21.28 m to the distance you calculated for the de-accelerating phase of the motion.

11. Sep 14, 2007

### klm

to the 69.3?
69.3+21.28 = 90.58?
sorry if i leave really quick i have to run to class in about 2 mins!

12. Sep 14, 2007

### Staff: Mentor

Yes.

13. Sep 14, 2007

### klm

THANK YOU!!! so so much