Does matter and/or energy

[jjjjj]

Always leave a system with moving parts. If the objects try to go in a straight line then there would have to be a force making them stay in the system, but forces require more motion, which would then require more force to not go in a straight line. (note: I'm only 14, so I don't know that much, but I'm pretty sure electromagnetism, gravity, etc. haven't been studied enough to assume them to be axioms).

[swansont]

Motion in a straight line requires no added force once the item is moving. This is one of the main messages of Newton's laws of motion. You need a force to cause an acceleration, which causes a change in velocity (slow down, speed up or change direction, since velocity is a vector)

[jjjjj]

Yes, I know, also if a piece of matter moves in a straight line, it will leave the system. Is it correct to say that a system with moving parts will always lose matter/energy in that way (or another)?

[DJBruce]

No, if the system in which the object is moving is a isolated one then there will be no loss of energy or matter. Of course there is really only one true isolated system, and that is the universe as a whole. Why would motion in a straight line necessarily mean that the object must leave the system? You could define the system in such away that the object doesn't leave it.

[jjjjj]

Wouldn't there need to be a force to make the object not keep going out of the system? The force would then cause more movement.

[...]

I think I understand what you're asking (but maybe not).. You're saying if you have a system defined in such a way that it contains moving pieces at some time t_o and then observe it..the moving pieces will separate from each other as time advances unless there is some force to alter their motion in such a way to keep them together..such as electrostatic attraction or gravity (which isn't a force but may be treated as one), or even nuclear forces if your system was say an atomic nucleus.

 

I think if your system is defined as a spatial region and not based on the components of the system themselves, say the volume encompassing our solar system right now and affixed to the sun's reference frame..and then gravity ceased to be..everything but the sun would immediately continue traveling in straight lines tangent to their current elliptical paths..thereby eventually exiting the predefined spatial system and taking energy with them.

 

So I think the answer to your question as you intended it is yes. However you could define your system to be the components in which case the system would simply expand as the pieces moved away from each other, but would retain the same matter and energy.

[DJBruce]

Wouldn't there need to be a force to make the object not keep going out of the system? The force would then cause more movement.

 

 

So I define my system as the entire universe, no force is need for the object to leave the system because by definition it is impossible for something to leave the universe.

 

Also it is important to note that a force does not necessarily cause movement. A force causes an acceleration. If the acceleration is in opposition to the objects velocity the object will slow down, and possibly stop, slow down, or change directions.

[swansont]

A force perpendicular to the motion does not change the energy of the system

[jjjjj]

... is right, I was referring to a spatial region, and by leaving I meant losing contact/interaction with other parts.

[swansont]

Do you have a particular example in mind? It might make it easier to visualize what you mean that way.

 

A wheel spinning is a moving part, but ideally it loses no energy because of the motion. In reality, there will be friction and something will heat up, and it will radiate or conduct energy away.

[jjjjj]

If the spinning wheel is alone, then of course it won't lose matter/energy. If it spins on an axle, it will due to friction. I mean any system with more than one moving part.

[swansont]

Friction isn't limited to spinning things, though. Nothing special about rotation in this matter. All systems will lose energy; this is one of the most important concepts of thermodynamics.

[jjjjj]

What if a system was surrounded by an indivisible sphere (if that's eve possible), would matter/energy NOT reflect off it since it isn't made of parts and has no properties? If it was completely surrounded by matter that "stuck" to it, would that matter reflect other matter/energy?

[jjjjj]

OK, seems you can't answer that one. 1 more question: doesn't that prove the universe is infinite (unless it all had a beginning)

[ninus maximus]

jjjjj

 

consider a radio controlled truck , a toy like the ones you can buy at radio shack.

 

you place the truck inside a sphere , the sphere can be seperated like one of the small

plastic easter eggs that people put candy in.

 

you turn the truck on so the the wheels of the truck are turning.

you place the truck inside the sphere and completely seal it up.

 

then place the sphere on the ground.

 

matter does not cross the boundary of the closed system.

 

and in a closed system only matter is denied crossing a boundary in physics.

 

so the truck wheels turn pressing against the inside of the sphere , the sphere then turns pressing against the ground.

 

the sphere moves , this is called reactionless propulsion because there is no force presented

to the ground due to a force that is pressed against the outside of the sphere.

 

A reactionless drive or inertial propulsion engine (also reactionless thruster, reactionless engine, bootstrap drive, and inertia drive) is

any form of propulsion not based around expulsion of fuel or reaction mass.

 

there is no mass or fuel being expelled from the sphere.

 

The name comes from Newton's Third Law of Motion, usually expressed as: "For every action, there is an equal and opposite reaction." Such a drive would use a hypothetical form of thrust that does not require any outside force or net momentum exchange to produce linear motion, and therefore necessarily violates the conservation of momentum, a fundamental principle of all current understandings of physics. In addition it can be shown that conservation of energy is violated.

 

In spite of their physical impossibility, such devices have been often proposed in recent history.

 

my my sorry jjjjj

I guess it wouldnt work afterall.

 

better luck next time.

I guess we understand now

LOL

Edited June 26, 2010 by ninus maximus

[swansont]

The sphere is in contact with the ground. It translates because there is friction at the point(s) of contact. No friction, no translational motion.

[AlphaSheeppig]

The sphere is in contact with the ground. It translates because there is friction at the point(s) of contact. No friction, no translational motion.

That is only assuming a rigid body, but in this case it is true. Without friction, the sphere would merely rotate on the spot and the truck would go nowhere.

 

But there are cases where friction is not necessary for motion. Didn't you ever sit in a cardboard box and move around by throwing your weight forward when you were a kid?

[swansont]

That is only assuming a rigid body, but in this case it is true. Without friction, the sphere would merely rotate on the spot and the truck would go nowhere.

 

But there are cases where friction is not necessary for motion. Didn't you ever sit in a cardboard box and move around by throwing your weight forward when you were a kid?

 

Friction is required for that, too.

[AlphaSheeppig]

Yeah, now that I think about it, you're right. Friction does play a role because without the friction, the box moves back at the same time... I can't think of any examples that give linear motion right now, but I can think of several that give rotational motion - helicopters for one.

[swansont]

No, helicopters aren't an example. Angular momentum is conserved there — the tail rotor is needed precisely because of that.

[AlphaSheeppig]

Haha... OK... I think I've got some serious thinking to do...

---

Merged post follows:

Consecutive posts merged

To answer the original question, this is the initial misunderstanding:

 

Wouldn't there need to be a force to make the object not keep going out of the system? The force would then cause more movement.

 

The force does not necessarily cause more movement. It cancels or alters the straight line motion of that component that would happen if the force wasn't there. Don't forget Newton III - something else in the system needs to exert the force, and the original component will exert a force on that. From a point of view outside the system, these two forces cancel each other out, so there is no energy leaving the system.

 

Think of a simple solar system with one planet and a star. The planet wants to drift off into space in a straight line, but the gravity makes it move toward the star. This means that the planet's path curves toward the star, but the star also moves toward the planet (much less though, since it's so much heavier). If the planet is too slow, it will crash into the star, and if it is too fast, it will drift off into space, and matter will indeed leave the system like you say. But if the speed is right, the planet will do neither and just drift around the star in a stable orbit. No matter leaves the system, and no energy needs to be added or escapes because the force the pull the star exerts on the planet to keep it in orbit is cancelled exactly by the pull the planet exerts on the star. Does that make sense?

 

In a mechanical system of cranks and levers, it is exactly the same. The force that linkage A exerts to keep linkage B in the system is cancelled exactly by the force linkage B exerts to keep linkage A in the system, so there is no energy loss.

 

Friction and losses like that are irrelevant for this argument, since energy escapes in the form of heat, and this energy can be forced to remain in the system by defining the entire universe as the system.

Edited June 29, 2010 by AlphaSheeppig
Consecutive posts merged.

[swansont]

Main rotor goes one way, so the body has to go the other way to conserve angular momentum. That's why you need the tail rotor. Another way to cancel this is counter-propagating blades.

[AlphaSheeppig]

I know that. It's a rotational example of what I was trying to say, and I can think of millions of rotational examples. I can also think of dozens of translational examples, but they all oscillate... I can't think of any that don't oscillate. I guess if there really was an easy one, it would be used all over the place.

[jjjjj]

Question: wouldn't forces have to come from other forces (maybe in the structure) so there would always be something trying to travel in a straight line whenever there's a force canceling out the previous one?

[AlphaSheeppig]

Think of it this way: both object A and B want to travel in straight lines, but both are prevented from doing so by a force from the other object. The force stopping object A from leaving the system comes from object B, and the force stopping object B from leaving the system comes from A, so it goes in a circle, and they balance out.

 

The forces don't cancel from object A or B's perspective, only from some point outside the system. All object A feels is the force B exerts on it, and not the force it exerts on B (because technically speaking, they're the same thing, but don't worry about that)... If that makes any sense?