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: Power needed to keep car moving

  1. Nov 22, 2007 #1
    1. The problem statement, all variables and given/known data
    A 1200 kg car slows down from 90km/h to 70 km/h in about 5.0s on the level when it is in neutral. Approximately what power ( in watt's and hp ) is needed to keep the car travelling at a constant 80km/h?

    2. Relevant equations
    P = W / t

    3. The attempt at a solution
    I really have no idea ............ and
    how much does 1hp = in watts and in joules ?
    550 ft lbs/s = in joules and watts?
  2. jcsd
  3. Nov 22, 2007 #2


    User Avatar
    Homework Helper


    if you have the change in velocity and the time it acts for....you can find acceleration and F=ma...so you can find the power
  4. Nov 22, 2007 #3
    what does each of ur variables mean?? cause i think i use a different variables than u do and i find it confusing to understand
  5. Nov 22, 2007 #4


    User Avatar
    Homework Helper

    Oh well from the definition of power, the rate at which work,W, is done =[itex]\frac{dW}{dt}[/itex] since work done= Force(F)*distance(s) i.e.[itex]\frac{dW}{dt}=\frac{d}{dt}(Fs)[/itex] since force is constant you can then re-write it like [itex]\frac{d}{dt}(Fs)=F\frac{ds}{dt}[/itex] and well [itex]v=\frac{ds}{dt}[/itex]

    In short: F=Force of car
    v=velocity of car
  6. Nov 22, 2007 #5
    wha ??? so the answer is????
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook