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 questions

  1. Feb 9, 2008 #1
    1. The problem statement, all variables and given/known data
    1. A pendulum is set swining on the moon from a frictionless anchor point. Will it stop moving? Why or why not?
    2. A grandfather clock has a swinging pendulum. Why does it need to be wound up every few days?
    3. How can a rocket change the direction when it is far out in space in a vacuum? Include momentum, impulse, and force in your discussion.

    2. The attempt at a solution
    Well, I'm really lost but here goes--
    1. According to Newton's First Law once a force is applied to an object it accelerates in the same direction and there's no gravity or friction to stop it?
    2. No idea...
    3. For every force, there is an equal and opposite force, so when a rocket takes off, it ejects a large amount of gas and creates a momentum upwards?
  2. jcsd
  3. Feb 9, 2008 #2
    1. The only force acting on it would be the force applied by the gravitational field of the moon, so i think it would stop but it would take a long time.
    2. don't know anything about pendulum clocks, but you can google on how it works
    3. if the rocket doesnt have propellers to accelerate it backwards, it can fire missiles - "For every force, there is an equal and opposite force"
  4. Feb 9, 2008 #3
    1. I don't think it will stop moving. Once it has been set into motion, and someone has given it some potential energy, the pendulum will continue to change from potential energy to kinetic energy and back to potential energy. The gravitational field is what creates the potential energy. So given no sources of friction, it should continue to swing back and forth.
    2. I'm guessing this has to do with the friction that exists. Air resistance and friction in the anchor point will slow it down, and the initial energy will be lost slowly to thermal energy. So you need to wind it up and give it back it's potential energy.
    3. The rocket is far out in space, so they are not talking about when it launches. I think it has to do with releasing gas through thrusters, and since momentum has to be conserved, if you send gas particles in one direction, you will have a momentum in the other. My teacher told my class that if you were on a frozen lake, and ignoring all friction, if you spit, you will go in the other direction (very slowly, of course, but you will be moving in the opposite direction).
  5. Feb 10, 2008 #4
    thanks guys! this has been so helpful :)
  6. Feb 10, 2008 #5
    Well, one thing that normally that kind of clocks have is a weight that is attached to it with a rod. As long as the clock is moving, the weight is falling. So, you only need to put the weight back up. (once more, is giving him potential energy. But this way you only have to give him like 5-5 days...^_^)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook