Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Nuts about Newton

  1. Mar 7, 2007 #1
    This is something I read recently

    e / Example 5.
    Momentum and kinetic energy in firing a rifle example 6
    The rifle and bullet have zero momentum and zero kinetic energy to
    start with. When the trigger is pulled, the bullet gains some momentum
    in the forward direction, but this is canceled by the rifle’s backward
    momentum, so the total momentum is still zero. The kinetic energies
    of the gun and bullet are both positive numbers, however, and do not
    cancel. The total kinetic energy is allowed to increase, because both objects’
    kinetic energies are destined to be dissipated as heat—the gun’s
    “backward” kinetic energy does not refrigerate the shooter’s shoulder!

    Wrong ( in my opinion)>>>>>The single explosion in the bullet creates a symetrical sphere of force which then expands outward (the opposite of gravity) like a shock wave. The bullet travels faster due to it's smaller mass, it's easier to push, or put another way it offers less resistance to the force. The mass of the Gun + person > bullet. This force I described, what is it? The force that is created from the explosion that is, not the explosion itself. I think a shock wave travels faster than the expansion of gases. I thought it was kinetic energy but I don't see it in this example from "conceptual physics" page 45 by Benjamin Crowell

    Newtons third law doesn't explain this. It looks to me like the action is the explosion then there are two reactions. We can create this spherical force when we strike a nail with a hammer or light a firecracker or 2 billiard balls. This example 5 just isn't logical... It looks like they overthink it.....This is why I'm nuts... Help me Stimpy!
     
  2. jcsd
  3. Mar 7, 2007 #2
    .........wow this is horrible physics.

    What....

    What.........


    We can help you if you dont understand the physics, but that does not mean you can make up your own physics every time something does not make sense to you.
     
    Last edited: Mar 8, 2007
  4. Mar 8, 2007 #3
    I didn't write it. Benjamin Crowell did.

    after :Wrong ( in my opinion)>>>>> I did write. I honestly see something missing. What happens before the bullet and the gun begin to exhibit momentum? To me the gas expansion is a reaction. The momentum created both the gun and the bullet are reactions too. What is the original action called?
     
  5. Mar 8, 2007 #4

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I've read your post about a half dozen times, and I still don't understand what you're not understanding, or what you're trying to figure out.

    When a gun fires, the following things happen:

    1) The firing pin impacts the primer. The primer deforms, and its explosive contents detonate, igniting the rest of the bullet's charge.

    2) The charge burns, rapidly changing phase from a dense solid to a much larger volume of gas.

    3) The gas exerts an equal pressure in all directions on the walls of the casing, and the rear end of the bullet.

    4) The large, strong mass of the gun's firing chamber prevents the casing from exploding sideways or backwards, but the bullet has no structure in front of it to resist the force, and the bullet is propelled down the barrel.

    5) When the gun and bullet are separated, momentum can be observed to have been conserved. The backwards momentum of the gun is equal and opposite the momentum of the bullet. The difference in size is, as you've said, what causes the difference in velocities. The gun and bullet have very different final velocities, because of their very different masses.

    6) The kinetic energy imparted to the gun and bullet by the explosion was originally contained in the chemical bonds in the propellant inside the bullet.

    - Warren
     
  6. Mar 8, 2007 #5

    rbj

    User Avatar

    hey Warren (completely OT),

    where can i get a nice .jpg of the pic that is your avatar?

    i have one called "helix-nebula.jpg" that looks similar but not the same. i imagine both are from the HST.

    thanks for any info.
     
  7. Mar 8, 2007 #6
    i cant understand anything that post said. Is this the book your school gives you? it seems like this book was written by someone who could barely speak the language it was written in and then had it translated it to english by someone who could barely speak english. there is no such thing as "backward" kinetic energy and i dont see how anyone would expect it to refrigerate anything to begin with.
     
    Last edited: Mar 8, 2007
  8. Mar 8, 2007 #7
    Forget it. Thanks anyway.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Nuts about Newton
Loading...