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!

Homework Help: Find the power of a boat that accelerated

  1. Oct 13, 2011 #1
    A 3.5x107 kg ship is travelling at 7 m/s. The ship is accelerated forward with a F of 4.5 x 105 N over a distance of 2 km.

    What is the final speed? Vf = 10 m/s

    How long does it take the ship to traverse the 2 km? T = 233.33 seconds

    What is the average power which the engines had to produce to accelerate the ship in part 1? I need help on this please.

    You can do P = W/t = (4.5x105 x 2000) / 233.333

    P = 3.9 x 106 watts which is the right answer.

    However, how come you can't do P = (F)(avg velocity)

    P = 4.5x105 * (3/2) = 675000 watts??
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 13, 2011 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member


    Where'd you get your avg velocity from?

    You can use this formula, and in fact the two methods are equivalent, since:

    P = W/t = Fd/t = F(d/t) = Fvavg

    vavg = d/t = 2000 m / 233.33 s = 8.57 m/s

    4.5e5 N * 8.57 m/s = 3.9e6 Nm/s (= J/s = W)
  4. Oct 13, 2011 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Ah I see. It looks like you were confused in your formula for avg. velocity. It's (vf + vi)/2 NOT (vf - vi)/2.

    (10 m/s + 7 m/s)/2 = 17/2 m/s = 8.5 m/s
  5. Oct 14, 2011 #4

    Oh my goodness!!! Your eyes are so good!!!! I have been using energy formulas all day that do Xf-Xi all day!! Such a silly mistake thank you so much!!! it is so late and my brain if frazzled :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook