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: What net external force is requiered?

  1. Jan 28, 2007 #1
    My wife is taking a Physics class and is having some trouble. I've never taken physics but I've tried to help, with no luck.

    1. The problem statement, all variables and given/known data
    What net external force is required to accelerate a 1500kg car from rest to 100km/h in 10 seconds?

    2. Relevant equations
    She believes that the right equation is:

    F=ma

    3. The attempt at a solution

    100km/1hour x 1hour/60sec = 100/60 = 1.7km/s

    a=(1.7 km/s)/10s
    a=0.17s/km

    F=1500kg x 0.17s
    F=255
    -------------------------------
    So we got '255N'


    The book says the answer is '4167N'
    But it does not say how to get this.

    She is studying for a test tomorrow.
    Any help would be great.

    Thanks
     
    Last edited: Jan 28, 2007
  2. jcsd
  3. Jan 28, 2007 #2

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You just changed the clock. There are 3600 seconds in an hour! Also, watch your units...your acceleration should be in meters/sec^2, not km/sec^2, to get the proper unit of the external force in newtons.
     
  4. Jan 28, 2007 #3
    Thank you

    after posting we did convert it to metters but did not catch the min and seconds mixup.

    Thanks again
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook