1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

When will the sled attain this speed?

  1. Feb 21, 2013 #1
    a bobsled slides down an icy track making of 29 degrees with the horizontal. Assume that the motion is frictionless.
    a) how far must the bobsled slide in order to attain a speed of 91 km/h if initially at rest?
    I converted km/h to 25.27 m/s and I used d = (v^2)/(2gsin(theta)) where d= 67.1 m, which is correct.

    b)when will the speed be attained?
    I tried d/v -> 67.1/25.27 and had 2.65 s and 25/67.1
    But the answer is wrong. So I am stuck on part b.
     
  2. jcsd
  3. Feb 21, 2013 #2

    PeterO

    User Avatar
    Homework Helper

    d/v presumably means you want to use distance / time to calculate here. The speed you should be using then is the average speed. You have used the final speed.
     
  4. Feb 21, 2013 #3
    how would I find average speed based on the data?
     
  5. Feb 21, 2013 #4

    PeterO

    User Avatar
    Homework Helper

    For motion under constant acceleration, it is fortunate that the average velocity is simply the average of the initial and final velocities.
    Although the path a bobsled takes if far from straight, it is not too wrong to consider that path as straight, and so the direction of velocity is always positive (or negative - depending which way you defined positive) so the average speed is the average of the initial and final speeds.
     
  6. Feb 21, 2013 #5
    so would initial speed equal zero so the average is just 25.27 m/s /2=12.63 m/s ?
     
  7. Feb 21, 2013 #6

    PeterO

    User Avatar
    Homework Helper

    You could try that and see if you get the right answer. Be bold and try it!
     
  8. Feb 21, 2013 #7
    yes! I did 67/12.63 and had around 5 seconds. I didnt try at first because I did not want to run out of chances on the online HW. Thanks a lot!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: When will the sled attain this speed?
  1. Maximum speed attained (Replies: 16)

Loading...