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: Horizontal Force

  1. Oct 12, 2005 #1
    A 50kg sprinter, starting from rest, runs 50m in 7.0s at constant acceleration.
    a) What is the magnitude of the horizontal force acting on the sprinter?
    b) What is the sprinter's power output at 2.0s, 4.0s, and 6.0s?
    I think maybe the 'magnitude' part is confusing me.
    F = ma
    V(avg) = (50-0)/(7-0) = 50/7
    A = (50/7)/7 = 50/49
    F= 50kg * 50/49
    F is exactly half of what the answer in my book says, what do I need to do and why?
    Thank You,
    in advance =o
     
  2. jcsd
  3. Oct 12, 2005 #2

    mezarashi

    User Avatar
    Homework Helper

    Because the question explicitly states that there is a constant acceleration you cannot use the average velocity principle. You should instead refer to your kinematics equations:

    [tex] d = v_it + \frac{1}{2}at^2[/tex]

    and solve for the acceleration. Once you get acceleration, as you have shown you understand, you may apply it to Newton's second law.
     
  4. Oct 12, 2005 #3
    works so much better that way, ty ;p
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook