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: 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.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook