Velocity Question - Odd answers

[hiddenlife5009]

Homework Statement

Part A

A cheetah, the fastest of all land animals over a short distance, can accelerate from zero to 18.0 m/s in three strides and to a full speed of 31.3 m/s in seconds. Assuming the first three strides are each 4.2 m long and that acceleration is constant until the cheetah reaches full speed, what is the cheetah's acceleration (in m/s2)?

Part B

If in part (a) of this question the cheetah's first three strides were each 4.6 m long, how many seconds would it take to reach its full speed?

Homework Equations

The Attempt at a Solution

Ok so for Part A I use the above equation:

31.3^2 - 18^2 = 2*a*12.6 (12.6 = 3 strides)

979.69 - 324 = 25.2a

655.69 = 25.2a

a = 26.02

Ok this is where it gets weird. That is not the answer, but when I divide it by 2, I get 13.01 which is within 0.1 of the answer showing on the online test. Every time I try this question with different velocities, I am within 0.1 of the online test's correct answer. The test gives me the mark, but I can not help but wonder if it's me doing something wrong or is the programming on the online test a bit off.

I am not really concerned about Part A, but Part B I just can't understand. I have tried using the rearranged velocity equations and the simple speed equation, but I still can't get the answer shown. The answer for Part B is 2.67 seconds.

EDIT: Sorry about the attachment, not sure how to make it appear within the post.

 

[hage567]

For (a):

If the cheetah is accelerating from rest, your initial velocity is zero, not 18 m/s. The question says in the first three strides the cheetah goes from 0 to 18 m/s. You don't know how many strides it took the cheetah to get from 18 m/s to 31.3 m/s, and I don't think you should assume it's three.

 

[nrqed]

Homework Statement

Part A

A cheetah, the fastest of all land animals over a short distance, can accelerate from zero to 18.0 m/s in three strides and to a full speed of 31.3 m/s in seconds...

There seems to be a missing word before "seconds"... without that I can't help with the question.

 

[hiddenlife5009]

There seems to be a missing word before "seconds"... without that I can't help with the question.

I'm quite sure its not missing, I think its trying to say "to a full speed of 31.3 m/s within seconds", meaning it can get to the max speed within a general time frame of a few seconds (insignificant). I guess its just trying to tell you its max speed.

EDIT: Hage567 - Thanks for that, by doing 18^2 / 25.2, it give me the exact answer shown. Still funny that the online test gives me the mark for a completely wrong method that is always within the 0.1 boundary.