1. Not finding help here? Sign up for a free 30min 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!

Physics 12 Momentum and work, energy question

  1. Jun 28, 2013 #1
    Q:A boy of mass 45 kg is travelling without a seat belt, in a car with a velocity of 14 m/s.
    When the car stops suddenly, he is stopped by the windshield in a distance of 0.024m.
    What force does he exert on the windshield?



    i set the impulse of the vehicle (mv)= to FD (because (delta)E=W=Fd) and got a force of 2.6*10^4N. That seems to high of a force to me.

    Calculations: p=mv ∇E=p=45(14)=630N*s ∇E=Fd 630=F*.024 solving for F i got 26250
     
    Last edited by a moderator: Jun 28, 2013
  2. jcsd
  3. Jun 28, 2013 #2

    gneill

    User Avatar

    Staff: Mentor

    Momentum is not the same as energy. The units are not the same. So ∇E=p is false.

    However, you could use kinetic energy. Set the change in kinetic energy to the work done making the change (F x d).
     
  4. Jun 28, 2013 #3

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    mass * velocity cannot be equated with force * distance; the first gives momentum, the second energy.
    Since the question provides a distance and asks for the force acting over that distance, energy is the right quantity to consider. The answer should be much higher.
    (However, it is not true that the force will be constant over the impact distance. The question ought to ask for a lower bound on the peak force.)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Physics 12 Momentum and work, energy question
Loading...