I would like to ask some questions. I am in a spacecraft (I have grip on the floor) and at one end a spring is attached to the wall. I hold the spring and begin to walk backwards, extending the spring. 1) What will happen to the spacecraft as I walk backwards? 2) What will happen to the spacecraft when I let go of the spring? 3) What will happen to the spacecraft when the spring has returned to its original state?
Quote from: chazemz on 09/16/2017 11:11 amI would like to ask some questions. I am in a spacecraft (I have grip on the floor) and at one end a spring is attached to the wall. I hold the spring and begin to walk backwards, extending the spring. 1) What will happen to the spacecraft as I walk backwards? 2) What will happen to the spacecraft when I let go of the spring? 3) What will happen to the spacecraft when the spring has returned to its original state?Honestly I think it would be more helpful for you to try to work out the answer yourself, keeping in mind that the combined center of mass of the spaceship and all objects inside it will not move. In this case that refers to the hull, you, and the spring. Drawing a diagram may help with this.Note that to simplify things in mass-spring systems, physicists often assume that a spring has no mass, which is fine when masses attached to either end of the spring have significantly more mass than the spring. Since you are letting go of one end of the spring, there is no mass attached to that end, so you can't consider the spring massless if you want to see the dynamics of the spring retracting.
Quote from: chazemz on 09/11/2017 06:56 pmMomentum and energy are not the same thing. Mmmmm, I agree, but is momentum stored energy?No. Momentum and kinetic energy are not the same thing either. They are related, but not at all the same. Kinetic energy is 1/2 the product of the mass times velocity squared. Ek = 1/2 mv^2Momentum is mass times velocity. p = mvTwo objects with the same kinetic energy only have the same momentum if they also have the same mass.
Momentum and energy are not the same thing. Mmmmm, I agree, but is momentum stored energy?
With reference to the EM drive thread there appears to be a lot of “violates this conservation law” and “violates that conservation law” when I am of the opinion that what you are in fact doing is preserving third law. Commenting earlier on conservation of energy/momentum I put forward a simple experiment asking what would happen with a five ball newton's cradle if you raised two balls and let them go so they collided with the remaining three balls. We were in agreement that two balls must come out the other side since if one ball came out at twice the velocity momentum would be conserved but not kinetic energy I then asked if nature treats momentum and energy as the same and when nature conserves “it accounts for”.So on this point I would like to ask a question: if we have a spaceship that has a surrounding skin that you are calling “the boundary”and I fit solar panels to the outside ( we are assuming that we have a device inside the spaceship that transfers energy/ momentum to the spaceship from the solar panels), if the energy/momentum in equals the energy/momentum stored plus losses, since the energy/momentum comes from outside the boundary am I to assume that all is well?For the Newtonians among you we are not looking at third law in this instance, just conservation of energy/momentum.
I am on your wavelength here (no pun intended), although there is a school of thought that says a photon has momentum but no mass?
When I let go of the magnets one magnet will accelerate and the other magnet will decelerate.
If I turn so that I am facing the direction of travel and repeat the experiment, what will now happen when I release the magnets?
What about energy conservation?
Quote from: chazemz on 09/26/2017 06:56 pmWhat about energy conservation?Do you have a specific question about it? The last scenario you described simply involves conversion of potential energy to kinetic energy.
Quote from: meberbs on 09/26/2017 07:35 pmQuote from: chazemz on 09/26/2017 06:56 pmWhat about energy conservation?Do you have a specific question about it? The last scenario you described simply involves conversion of potential energy to kinetic energy.Well there are numerous questions that can be asked but we will start with a specific one. When I let go of the magnets they will, as you have stated, accelerate towards one another. Using inverse cube we know that the rate of acceleration will rise dramatically as the distance between them decreases. So where are the magnets getting their energy/momentum from?
The last scenario you described simply involves conversion of potential energy to kinetic energy.
You are evading the question. The magnets will accelerate at ninety degrees to the direction of travel. As they accelerate, since they have mass, their momentum and kinetic energy will increase. Where is this energy coming from?