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!

Forces - Solved, but wrong?

  1. Oct 2, 2007 #1
    1. The problem statement, all variables and given/known data

    A 21000 kg helicopter is lifting a 5000 kg truck with an upward acceleration of 2.00 m/s2. Calculate the force the air exerts on the helicopter blades.

    2. Relevant equations

    F=ma

    3. The attempt at a solution

    I did the total force is equal to the sum of the two masses(26000kg) times the acceleration (2) which gave me 52000N. Which is apparently wrong but I have no idea why...

    ----------------------------------------------------------------------------------
    1. The problem statement, all variables and given/known data


    A car that weighs 14200.0 N is initially moving at a speed of 44.0 km/hr ((or 12.222)m/s) when the brakes are applied and the car is brought to a stop in 4.1 s. Find the magnitude of the force that stops the car, assuming it is constant.

    2. Relevant equations

    F=ma, Vf = Vi+ at

    3. The attempt at a solution

    So I solve using kinematics to get the acceleration (-12.22=(a*4.1)) so a = 2.98m/s^2. Which I then multiply by 14200/9.8 (the mass) to get the force equaling 4319.45N. Which is, again, wrong. And as with the above question I don't know why. I've done everything right as far as i can tell...
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Oct 2, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    helicopter

    You found the net force, but the question asks for the force of the air. What other force acts on this system?
     
  4. Oct 2, 2007 #3

    Doc Al

    User Avatar

    Staff: Mentor

    car

    This one looks fine to me.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?