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: Momentum formula help

  1. Jul 24, 2007 #1
    1. At the circus, a 100 kilogram clown is fired at 15 meters per second from a 500 kilogram cannon. What is the recoil speed of the cannon? My question is what does 'recoil speed' mean? that's it.
    2. There are two identical carts, attached by a cord moving to the right at speed V. If the cord is cut, what would be the speed of cart A? Just imagine that there are two carts, and between them there is a cord attached to each other, and i think this cord moves to the right at the speed of V. How can i find cart A, what formula can i use? momentum?
  2. jcsd
  3. Jul 24, 2007 #2


    User Avatar
    Homework Helper
    Gold Member

    1. The recoil speed is how fast the cannon will move in the opposite direction after the blast.

    2.Yes, momentum will work for this problem.
  4. Jul 24, 2007 #3
    but which formula of momentum does it relate to the question? change in momentum, as in impulse? or conservation of momentum? Can you sort of give me some ideas, i really don't understand it.
  5. Jul 24, 2007 #4
    Heres my 2 cents about the 2nd question.

    If the two cars are connected with a cord (such that the first car is pulled by the second), then consider their mass to be say "m", and since the two cards are identical, they have the same mass.

    Find the momentum of the whole system, i.e. MV. Where M is the sum of the masses of individual cars i.e. 2m. And the total velocity of the system is given (V). If the cord is cut, the cars are no longer attached. But by conservation of momentum, the sum of their individual momentums should be equal to the total momentum of the system. I hope this helps.

  6. Jul 24, 2007 #5
    so i use this formula: m1v1i+m2v2i=m1v1f+m2v2f? what's the initial velocity?
  7. Jul 24, 2007 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    The clown and cannon start at rest. It is an impulse and the time is the same for the clown and cannon.

    Equal and opposite force. Equal and opposite momentum.
  8. Jul 24, 2007 #7
    I meant the second question about the cord.
  9. Jul 24, 2007 #8


    User Avatar
    Homework Helper
    Gold Member

    You start with both objects as part of a bigger object at some speed,V. The total mass will be the mass of the two carts, or 2m, since the carts are identical. So, you start with on object, whose momentum is , p=2mV, and end with 2, whose momenta are: p_A=mv_A and p_B=mv_B.

    The big hint here is that the carts are identical and moving at the same speed. If you think about this, it should make the problem much easier.
    Last edited: Jul 24, 2007
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook