freefall

If we expressed a system's momentum as a scalar (e.g. added up all of the |mg|'s), would that quantity be conserved? What if we turned energy into a vector that points in the direction of a system's momentum [e.g. (1/2)mv^2 * (mv/|mv|) ] ? Would that vector be conserved, as momentum is? Just curious, because it seems that we could easily have defined an "energy vector" or a "momentum scalar."

I do not completely understand it. I see how, if fluid is running though a pipe, the pressure decreases as the radius decreases because the acceleration of the fluid requires the unbalanced force of the pressure differential. What I don't see is why the pressure must decrease absolutely (instead of relatively) as the radius decreases. For example, if you have a pipe with water flowing though it, and you add a narrower pipe to the end, it seems that the pressure in the larger pipe would increase. I'm pretty sure it would, because you can do the experiment with your mouth. Hold your mouth wide open and blow air out, then close your mouth almost entirely, and your cheeks will bulge outwards. The pressure in the larger-radiused area increased because it had to accelerate the air though the small gap. So I guess what I'm confused about is: to me, the bernoulli equation proves that there must be a relative pressure decrease as radius of a pipe decreases, but that does not justify the fact that faster moving fluids have absolutely lower pressures, which everybody always says.

yes, but why is it that, say, gasoline can store a certain amount of energy and not a certain amount of ability to to impulse?

Instead of impulse, for example? They are both conserved. When putting energy into an object, its \delta velocity is not directly proportional to the KE added (but is directly prop. to impulse). For this reason we say that a car needs more gas to accelerate by a constant amount the faster its moving (see other thread). But it doesn't take more impulse. If we based energy on impulse, we wouldn't say it takes more gas. Right? Can someone explain? thanks!

okay guys you gotta help me out here. It seems to me that in order for the car to increase its speed by, say, 1 m/s, it takes more work the greater the velocity of the car. KE=1/2mv^2. If v is small, KE doesn't have to increase much to get v to increase by 1 m/s. But as v gets large, KE has to increase by a larger margin to get v to increase by 1 m/s. How am wrong? Because MattC says it doesn't take more work. Thanks very much.

assume no friction. When you accelerate your car, the road forces it forward, right? The faster you're moving, the longer the distance the road has to force it to increase your speed by some constant amount. f x d = work. So, how does that not correlate to more work done on you car by the road (which means more work done by the engine)?

right, I mean you accelerate the object.

Say you're driving in a car pretty fast. You hold out an object in your hand and push it out in front of you, the same direction that the car is moving. You would feel no more resistance than if you were travelling at 0 m/s right? But aren't you doing more work on the object? You're forcing it over a greater distance because the system is moving faster? But it wouldn't feel any different... Consider you and the car to be the external system. Could someone explain? This is related to what someone told me: that in a car, it takes more gas to increase your speed by a constant increment when you're moving faster because the engine needs to do more work on the car. But it doesn't seem like it would be an "harder" for the engine to speed it up. Thanks very much!

interesting. thanks for the response, athiest. I see what you're talking about on the surface (I'm missing the fundamentals). I'm just going to go read about relativity; I think that's the best thing to do.

I do not know the math. I am currently studying newtonian physics but read an article in scientific american which breifly mentioned the phenomenon. I looked around a bit online but could not find a suitable answer without delving into the subject. I posted here hoping for an answer that could temporarily satisfy me until I begin studying this subject.

How is it possible for an object to appear to slow down as it approaches the event horizon of a black hole? Gravity redshifts the light to lower energy so the object appears more red to the observer, but the light is always travelling at the same velocity right? Each photon emitted by the object would still reach the observer in the same amount of time as usual (no redshifting), so how could it look like it's slowing?

i suppose the problem is akin to asking about a collision between an object of infinite mass and an object of finite mass.

I've been arguing with some people about a situation in which two completely rigid bodies surrounded in a vacuum collide... they keep saying they will bounce off each other but that doesn't seem right. It's written up here http://midnightwiki.com/index.php?title=Inelastic_Collision. What do you guys think? Also, would there have to be some sort of new energy released if they were to collide?

Hey everyone, MidnightWiki is a website based on wiki same technology and allows any sort of contribution. You can post scientific ideas that are your own that you would like to share with the world. It's basically wikipedia without restrictions. All contributions are licensed under a creative commons license. The site just started, so there is hardly any content on it right now. Please help make it better! Thanks, freefall P.S. sorry for spamming Feel free to send hatemail to krygenzero@yahoo.com edit_again: no! I did NOT notice that just below my post, another wiki is being spoken about. Well... contribute to both!