# Average acceleration.

1. Aug 24, 2011

### cneedshelp

An Olympic sprinter can accelerate from rest out of the blocks to a top speed of about 11.5 m/s. This is accomplished in the first 15 m of a race. What is the average acceleration of the first sprinter?
Given- vi- 11.5 m/s
Distance-15m
Vf-?
t-?
Which formula do I use to find these missing terms?

2. Aug 24, 2011

### tiny-tim

Last edited by a moderator: Apr 26, 2017
3. Aug 24, 2011

### cneedshelp

Last edited by a moderator: Apr 26, 2017
4. Aug 24, 2011

### tiny-tim

s is always distance

5. Aug 24, 2011

### cneedshelp

Oh haha thank you. :)
Now how do you find the different times?

6. Aug 24, 2011

### tiny-tim

what times??

(i don't see any times in the question )

7. Aug 24, 2011

### cneedshelp

You have to find acceleration, the formula is vf-vi over tf-ti right?

8. Aug 24, 2011

### tiny-tim

that formula certainly works if you know t

but you only know vi vf and s, and there is a formula using that information

9. Aug 24, 2011

### cneedshelp

d = vt + (1/2)at^2
is that the formula?

10. Aug 24, 2011

### tiny-tim

no of course not

11. Aug 24, 2011

### cneedshelp

Im not sure which formula to use...

12. Aug 24, 2011

### lightgrav

what's their _average_ speed during that 15m ?
so how long should it have taken them to go 15m, at that average speed?

13. Aug 25, 2011

### tiny-tim

Last edited by a moderator: Apr 26, 2017