# Acceleration problems

1. Sep 7, 2009

### magdalenagirl

1. The problem statement, all variables and given/known data
I) A car slows down from 23 m/s to rest in a distance of 80 m. What was its acceleration, assumed constant?

II) A car slows down uniformly from a speed of 26.0 m/s to rest in 5.50 s. How far did it travel in that time?

2. Relevant equations

what am i doing wrooong? am i using the right formula?

3. The attempt at a solution
I) I used this formula: a=Vf-Vi/tf-ti but the problem doesn't give me time so it didn't work

II) I used the same formula ^^ 0-26.0/0-5.50=4.72 is not the answer

2. Sep 8, 2009

### Jebus_Chris

Well, you can find the time it took to travel 80m using Vavg and then find the acceleration. Or v^2 = vo^2 + 2ax
For the second part, you need to find distance not acceleration. Which is equal to Vavg*t

3. Sep 8, 2009

what's Vavg?

4. Sep 8, 2009

### rl.bhat

(v)avg = (v + vo)/2

5. Sep 8, 2009

### magdalenagirl

Ok, I got problem # II.

I still don't understand # I...
So I find the Vavg = (0+23)/2 = 11.5
am I right here?
Then I guess I plug that into the formula:
11.5^2=0^2+2 a (80?) >> I don't know what's my X

Help!!

6. Sep 8, 2009

### kuruman

Since the time is not given, use the kinematic equation that relates acceleration, displacement and the squares of the final and initial velocity.

7. Sep 8, 2009

### magdalenagirl

that's what I'm doing and I'm not getting the right answer
v^2 = vo^2 + 2ax

0=23^2+2a(80)
0=689a

8. Sep 8, 2009

### Integral

Staff Emeritus
How did you combine those factors? The first does not have an a in it so cannot simply be added.

9. Sep 8, 2009

### kuruman

0 = 232+2*a*80 is OK. It's the next step that is incorrect. Doing the number multiplications gives

0 = 529 + 160*a

You can't add 529 and 160*a to get 689*a !! To get 689*a you need to add 529*a + 160*a, which is not what you have here.

160a = -529

a = -529/160 m/s2

10. Sep 8, 2009

### magdalenagirl

ok,
0(final)=23^2(initial) + 2 a (80)
I got the 23 ^2 which is 529
then I multiplied 80 * 2 =160
then I added 529 +160 =689

11. Sep 8, 2009

### magdalenagirl

oooooooh ok,
thanks a lot kuruman!! I really appreciate it.