Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Velocity Question - Odd answers

  1. Jun 12, 2007 #1
    1. The problem statement, all variables and given/known data

    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?

    2. Relevant equations

    attachment.php?attachmentid=10258&stc=1&d=1181702481.gif

    3. 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.
     

    Attached Files:

  2. jcsd
  3. Jun 12, 2007 #2

    hage567

    User Avatar
    Homework Helper

    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.
     
  4. Jun 12, 2007 #3

    nrqed

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    There seems to be a missing word before "seconds"... without that I can't help with the question.
     
  5. Jun 12, 2007 #4
    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.
     
    Last edited: Jun 12, 2007
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook