1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Initial Velocity Question

  1. Nov 3, 2011 #1
    So the question I'm asking about is actually from my analytical mechanics class, but I think my question is really more of a gen. physics question.

    Basically, in this question we have a cannon ball, diameter(EDIT:but not mass!), fired with a given muzzle velocity, and we are given the cannon mass which is free to recoil. Does the fact that the cannon is free to recoil make it so that I can not use the given muzzle velocity for my initial velocity? EDIT:I would say I should use cons. of momentum (which i did at first)
    , but the mass of the cannonball is not given.
    Thanks guys,
    Last edited: Nov 3, 2011
  2. jcsd
  3. Nov 3, 2011 #2


    User Avatar
    Homework Helper

    Interesting point. Usually the mass of the gun/cannon is so much bigger than the bullet/ball that any difference is too small to worry about.

    We often have a cannon of mass XX tonnes, while the ball only 1 or 2 kg, if that?

    I would tend to use conservation of momentum just to be on the safe side: they did stress the ability to re-coil.
  4. Nov 3, 2011 #3
    Thank you for the response, sorry but I completely messed up when stating my givens. All I know about the cannonball is that it is a "10cm diameter steel cannonball," I do not know its mass.

    So unless my teacher expects us to look up the density of steel and calculate the mass (which I highly doubt), do you now think it would be safe to assume that the cannons weight and ability to recoil is irrelevant?
  5. Nov 3, 2011 #4


    User Avatar
    Homework Helper

    What mass did they give for the cannon. Some people give very unrealistic masses in questions. I have seen people claim cars of mass 600kg and trucks of mass 1200 kg before - along with train carriages of mass 2 tonnes.
  6. Nov 3, 2011 #5

    the mass of the cannon is 40kg.

    muzzle velocity of the cannonball is 100m/s
  7. Nov 3, 2011 #6


    User Avatar
    Homework Helper

    Not much of a cannon!!!!

    density of steel is up to 8000kg per cubic metre, so a cannon ball that size has a mass around 30 kg.

    You would definitely have to allow for the recoil.
  8. Nov 3, 2011 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Actually, that is a pretty reasonable thing for a teacher to expect. See if your physics textbook has a table or list of densities for various materials (check in the index under "density").
  9. Nov 3, 2011 #8


    User Avatar
    Homework Helper

    By not much of a cannon, I mean notice that the entire cannon is moulded from 1.3 cannon balls worth of steel!!!!
    As I said, people often use very unrealistic masses for objects in Physics problems.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook