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!

Kinetic Energy

  1. Apr 7, 2007 #1
    1. The problem statement, all variables and given/known data
    A 3 m long canon fires a 14.0 kg projectile with a velocity of 630 m/s [F]. Calculate.
    A) The kinetic energy of the projectile as it leaves the canon barrel.
    B) The average force acting on the projectile in the barrel.

    3. The attempt at a solution
    A) Ek=1/2*14kg*630m/s

    Is that Right?
    Last edited: Apr 7, 2007
  2. jcsd
  3. Apr 7, 2007 #2
    Kinetic energy is not [itex]\frac{mv}{2}[/itex] (though your final number is correct), and its unit is the Joule, not the Newton. Other than that, your approach seems correct.
  4. Apr 7, 2007 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    I can't possibly see the point in exercise B.
    Sure, you can average whatever numbers you want, but that doesn't mean anything if the deviations from that average is humungous, as in this case.

    The rate of momentum transfer in the initial phase (from the explosion of gun powder) is orders of magnitude higher than the forces acting from the barrel upon the ball during its motion through it.
    In addition, those negligible barrel forces are resistive, rather than contributive.

    Bad exercise.
  5. Apr 7, 2007 #4
    your mean B is wrong
  6. Apr 7, 2007 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    No, I mean that the exercise is dumb, not that you have made a mistake.
  7. Apr 7, 2007 #6
    He means that the question is asking you to determine a quantity which has little physical significance. Even if you calculate the average force acting on the bullet as kinetic energy is added to it, you will be stuck with a value that means nothing.

    But this is the fault of the question, and not the fault of your answer.
  8. Apr 7, 2007 #7
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Kinetic Energy
  1. Kinetic energy (Replies: 3)

  2. Kinetic energy (Replies: 2)

  3. Kinetic Energy? (Replies: 5)

  4. Kinetic energy (Replies: 1)

  5. Kinetic energy (Replies: 1)