# Sports car Acceleration problem

1. Oct 6, 2007

### JDR 007

1. The problem statement, all variables and given/known data

A sports car moving at constant speed travels 120 m in 4.7 s. If it then brakes and comes to a stop in 4.0 s, what is its acceleration? Express the answer in terms of g's, where g = 9.80 m/s2. (Take the positive direction to be the direction of travel.)

2. Relevant equations

Change in speed/time of change

3. The attempt at a solution
120/4.7=25.53m/s
end speed 0m/s-start speed 25.53m/s divided by time 4s= -6.38m/s^2 divided by 9.8m/s2= -.651 g

I got this wrong...what did I do wrong?

Last edited: Oct 6, 2007
2. Oct 6, 2007

### Kurdt

Staff Emeritus
I think its probably because you forgot the minus sign. The question specifically states which direction is positive.

3. Oct 6, 2007

### learningphysics

Looks right to me.

4. Oct 6, 2007

### JDR 007

I used a minus sign...still wrong. It is a web based hw system and it is horrible. we have 4 trys to get it right...so i used a - sign then a + sign then scientific notation then changed my sig figs...so it says I am wrong but I don't see how.

5. Oct 6, 2007

### learningphysics

Double check that you posted the question exactly as it appears... you might have mixed up the numbers or something....

6. Oct 6, 2007

### JDR 007

I copy and pasted the question it is exact

7. Oct 6, 2007

### Kurdt

Staff Emeritus
So the question gave g as 9.80m/s2? I'd have expected 9.81m/s2 if it was to two decimal places.

8. Oct 6, 2007

### JDR 007

I emailed my teacher And I will see what she says. I really hate these web assignments because they aren't multiple choice (hence wrong sigs, diff. calculators etc... will mess answers up due to rounding)

9. Oct 6, 2007

### JDR 007

A car slows down from 23 m/s to rest in a distance of 91 m. What was its acceleration, assumed constant?

I only have one more try on this one... is -2.91 right?

10. Oct 6, 2007

### learningphysics

Yes, that's what I get rounded to 2 decimal places.