# Homework Help: Car stops

1. Aug 31, 2008

### Phoenixtears

1. The problem statement, all variables and given/known data
The minimum stopping distance for a car traveling at a speed of 30 m/s is 60 m, including the distance traveled during the driver's reaction time of 0.50 s.

(a) What is the minimum stopping distance for the same car traveling at a speed of 60 m/s?
m

2. Relevant equations
Vf^2= V0^2 + 2ax

3. The attempt at a solution
Now I have my equation set up, but I can't figure out the change in x. Instead I have (60m-x) in my equation, but I'm looking for acceleration so that I can use that in my next equation when it comes to going 60 m/s. How is one to find the other x if it is not given?

~Phoenix

2. Aug 31, 2008

### Dschumanji

The relevant equation should be written as:

Vf2 = Vi2 +2a$$\Delta$$x

$$\Delta$$x is the distance the car goes to stop.

3. Aug 31, 2008

### Phoenixtears

Gotcha... wow, now I feel rather dense. That makes perfect sense- I've no idea why I skipped over that. All right, I worked the equations and got 7.5 m/s/s as my acceleration. I used that in my equation involving 60 m/s (0^2= 60^2 + 2(7.5)x) and got 240 as my answer. Yet this is wrong, is there something in my calculations that is incorrect?

4. Aug 31, 2008

### Dschumanji

Did you include the distance you travel before you hit the brakes?

5. Aug 31, 2008

### Phoenixtears

Yes, using the equation: deltax= V0*t + (.5)(a)(t^2). My numbers being: V0=60; t=.5; a=7.5. totalling to 270.94, and yet that did not work either.

6. Aug 31, 2008

### Dschumanji

The car is not accelerating before the brakes are applied.

The equation you should use is x = vt.

7. Aug 31, 2008

### Phoenixtears

Yikes. That's just basic... I think I'm a bit tired. Right, well then, that would mean it travel 60m*.5s leaving 120m total during that time. Adding that to my original 240 brings me to 270m total.... well. That was just depressing, as that didn't work. :/ Now something must be wrong. I've done everything wrong along the way, then correcting it, and yet 270 seems as if that is correct...

8. Aug 31, 2008