steevey Posted January 22, 2011 Share Posted January 22, 2011 So if I take a ball and just drop it, when it hits the ground, the ball transfers more kinetic energy to the ground than if I just set the ball on the ground. So, how does gravity cause this increase in kinetic energy being transfered to things and how does it not run out? The energy and momentum has to be coming from somewhere... Link to comment Share on other sites More sharing options...
lemur Posted January 22, 2011 Share Posted January 22, 2011 So if I take a ball and just drop it, when it hits the ground, the ball transfers more kinetic energy to the ground than if I just set the ball on the ground. So, how does gravity cause this increase in kinetic energy being transfered to things and how does it not run out? The energy and momentum has to be coming from somewhere... My impression is that force-fields are stores of potential energy. I.e. they are themselves potential energy waiting to be released as kinetic energy. In a gravity-well, this fact seems somewhat obvious. Where I get stuck is how to convert the PE of, say, a magnetic field into KE. Theoretically, it seems like a magnet is a highly organized system with the potential to decay into a more disorganized system and thereby release energy, but how? Link to comment Share on other sites More sharing options...
steevey Posted January 22, 2011 Author Share Posted January 22, 2011 My impression is that force-fields are stores of potential energy. I.e. they are themselves potential energy waiting to be released as kinetic energy. In a gravity-well, this fact seems somewhat obvious. Where I get stuck is how to convert the PE of, say, a magnetic field into KE. Theoretically, it seems like a magnet is a highly organized system with the potential to decay into a more disorganized system and thereby release energy, but how? What if an object was in perpetual orbit around something? 1 Link to comment Share on other sites More sharing options...
lemur Posted January 22, 2011 Share Posted January 22, 2011 What if an object was in perpetual orbit around something? That is potential energy. Why wouldn't it be? Because it's in motion? If it collides with something, it transfers momentum to that thing and its orbit shifts accordingly. I think you could even say that an orbital position is the same thing as a stabile ground-position at a given altitude, except the orbital position has to maintain a certain speed to retain its altitude. In a vacuum, however, that doesn't cost any propulsion. Link to comment Share on other sites More sharing options...
swansont Posted January 22, 2011 Share Posted January 22, 2011 When you take a ball and drop it, you did work on the ball to get it to where you dropped it. After you dropped it, the potential energy is converted to kinetic energy. Since the force acts equally on the ball and on the earth, the earth also moves toward the ball, conserving momentum. 1 Link to comment Share on other sites More sharing options...
steevey Posted January 22, 2011 Author Share Posted January 22, 2011 When you take a ball and drop it, you did work on the ball to get it to where you dropped it. After you dropped it, the potential energy is converted to kinetic energy. Since the force acts equally on the ball and on the earth, the earth also moves toward the ball, conserving momentum. So my arm is holding up the ball, and its putting a certain amount of force into the bottom of the ball in order to resist Earth's gravity. But, what if I were on a place holding my arm out and I was up 30,000 feet in the air? You could point out the potential energy of the plane, but the plane isn't holding up the ball, my arm still is. Or What if some random piece of ice suddenly started falling towards the Earth and hit the ground? It's been in the vacuum of space for a while and hasn't been gaining much energy. Link to comment Share on other sites More sharing options...
Janus Posted January 22, 2011 Share Posted January 22, 2011 So my arm is holding up the ball, and its putting a certain amount of force into the bottom of the ball in order to resist Earth's gravity. But, what if I were on a place holding my arm out and I was up 30,000 feet in the air? You could point out the potential energy of the plane, but the plane isn't holding up the ball, my arm still is. So what? Force is not energy. The potential energy of the ball was added to it when the plane lifted it to 30,000 ft. The plane had to expend energy to do so. Or What if some random piece of ice suddenly started falling towards the Earth and hit the ground? It's been in the vacuum of space for a while and hasn't been gaining much energy. The ice already had potential energy with respect to the surface of the Earth. For the hunk of ice it would be equal to: [math]E_p = \frac{GMm}{R_e} - \frac{GMm}{R_m}[/math] Where G is the gravitational constant, Re is the radius of the Earth, Rm is the distance of the ice from the center of the Earth, M is the mass of the Earth and m is the mass of the ice Link to comment Share on other sites More sharing options...
steevey Posted January 23, 2011 Author Share Posted January 23, 2011 So what? Force is not energy. Then how do you explain gravitational waves which are carried by what appears to be the fabric of space time itself? Link to comment Share on other sites More sharing options...
insane_alien Posted January 23, 2011 Share Posted January 23, 2011 Then how do you explain gravitational waves which are carried by what appears to be the fabric of space time itself? that has nothing to do with force being energy. and gravitational waves aren't carried by space-time. they move through space-time but then so does everything else. Link to comment Share on other sites More sharing options...
steevey Posted January 23, 2011 Author Share Posted January 23, 2011 (edited) that has nothing to do with force being energy. and gravitational waves aren't carried by space-time. they move through space-time but then so does everything else. Hence the word "appear" But anyway, then what's carrying those gravitational waves unless what gravity is caused by is some energy or particle with energy itself? Edited January 23, 2011 by steevey Link to comment Share on other sites More sharing options...
lemur Posted January 23, 2011 Share Posted January 23, 2011 (edited) So what? Force is not energy. Actually, maybe it is - based on the definition from Wikipedia: In physics, energy (from Greek ἐνέργεια - energeia, "activity, operation", from ἐνεργός - energos, "active, working"[1]) is a quantity that is often understood as the ability a physical system has to produce changes on another physical system.[2][3] F=MA: Isn't acceleration a change in a physical system? The changes are produced when the energy is transferred from a system to another. A system can transfer energy by means of three ways, namely: physical or thermodynamical work, heat transfer, or mass transfer. F=MA: Isn't acceleration a form of (mass) transfer? Edited January 23, 2011 by lemur Link to comment Share on other sites More sharing options...
swansont Posted January 23, 2011 Share Posted January 23, 2011 Actually, maybe it is - based on the definition from Wikipedia: No, it's not. Force and energy are distinct concepts, though they are related to each other. Link to comment Share on other sites More sharing options...
lemur Posted January 23, 2011 Share Posted January 23, 2011 No, it's not. Force and energy are distinct concepts, though they are related to each other. The point of my post was that the common conceptual distinction might be misguided. Why is it logical to differentiate potential energy from field-force? When a force-field causes something to accelerate, how is that different from fueled-propulsion? If a battery is used to store energy, you call it energy, right? But isn't the battery just separating ions in a way that produces electrostatic force between the cells? Link to comment Share on other sites More sharing options...
swansont Posted January 23, 2011 Share Posted January 23, 2011 The point of my post was that the common conceptual distinction might be misguided. Why is it logical to differentiate potential energy from field-force? When a force-field causes something to accelerate, how is that different from fueled-propulsion? If a battery is used to store energy, you call it energy, right? But isn't the battery just separating ions in a way that produces electrostatic force between the cells? Force and energy aren't the same thing. Period. Link to comment Share on other sites More sharing options...
lemur Posted January 23, 2011 Share Posted January 23, 2011 Force and energy aren't the same thing. Period. In principle I agree. Intuitively, force is a static quasi-object to me. Force that is caused by energy-transfer, such as momentum, also seems necessarily distinct from the energy it transfers. Nevertheless, how else would you describe the cause of potential energy except as position within a force-field? And then, what else stores that (potential) energy except the field-force itself? An apple may have potential to fall out of a tree and strike the ground at a certain speed, etc., but the energy to accelerate the apple to t hat speed is not contained within the apple. If the apple was a rock hung from the same branch, it would have the same potential energy. So the energy is stored as position in the gravitational field, no? So what is the source of the energy except the force-field itself? Link to comment Share on other sites More sharing options...
steevey Posted January 23, 2011 Author Share Posted January 23, 2011 (edited) Force and energy aren't the same thing. Period. It doesn't seem like it does it, but I don't think it's a coincidence that gravity and the electro-magnetic force travel at the exact same speed of light which is pure electro-magnetic energy. After all, can't only energy travel at the speed of energy? Otherwise why isn't matter going at the speed of light? Edited January 23, 2011 by steevey Link to comment Share on other sites More sharing options...
Mr Skeptic Posted January 23, 2011 Share Posted January 23, 2011 So if I take a ball and just drop it, when it hits the ground, the ball transfers more kinetic energy to the ground than if I just set the ball on the ground. So, how does gravity cause this increase in kinetic energy being transfered to things and how does it not run out? The energy and momentum has to be coming from somewhere... Gravity, or any force in general, can convert stored potential energy to kinetic energy, or can do the opposite, depending on whether you're moving along or against the force. So you can store energy in a spring by pushing or pulling it, and release the energy again by releasing the spring. Any time you release potential energy you cannot move the system back to the original state without putting in at least as much energy again. Also, any time the net force acting on an object is in the direction the object is moving, the kinetic energy of the object will increase, and anytime the net force acting on an object is opposite the direction of motion, the kinetic energy will decrease. For example, if you throw an object upwards against gravity, its kinetic energy will be decreased by the effects of gravity. Link to comment Share on other sites More sharing options...
lemur Posted January 23, 2011 Share Posted January 23, 2011 Gravity, or any force in general, can convert stored potential energy to kinetic energy, or can do the opposite, depending on whether you're moving along or against the force. Why wouldn't you just say that gravity stores energy in a potential form according to an object's position relative to other objects in the field? Ultimately, isn't matter only convertible to energy because the configuration of the sub-atomic particles changes relative to each other? Isn't such a configuration-change just a change between the two particles' relation to each other's field-force? I.e. the position of each in relation to the other's field force constitutes a certain amount of potential energy that can be converted into some other (kinetic) form, no? Link to comment Share on other sites More sharing options...
Mr Skeptic Posted January 23, 2011 Share Posted January 23, 2011 Why wouldn't you just say that gravity stores energy in a potential form according to an object's position relative to other objects in the field? Ultimately, isn't matter only convertible to energy because the configuration of the sub-atomic particles changes relative to each other? Isn't such a configuration-change just a change between the two particles' relation to each other's field-force? I.e. the position of each in relation to the other's field force constitutes a certain amount of potential energy that can be converted into some other (kinetic) form, no? Because I prefer dealing with small, pointlike particles than with vector fields of infinite extent. 1 Link to comment Share on other sites More sharing options...
swansont Posted January 23, 2011 Share Posted January 23, 2011 In principle I agree. Intuitively, force is a static quasi-object to me. Force that is caused by energy-transfer, such as momentum, also seems necessarily distinct from the energy it transfers. Nevertheless, how else would you describe the cause of potential energy except as position within a force-field? And then, what else stores that (potential) energy except the field-force itself? An apple may have potential to fall out of a tree and strike the ground at a certain speed, etc., but the energy to accelerate the apple to t hat speed is not contained within the apple. If the apple was a rock hung from the same branch, it would have the same potential energy. So the energy is stored as position in the gravitational field, no? So what is the source of the energy except the force-field itself? That's how potential energy is defined, because the work done is not dependent on the path. But that's not true of all all forces and all energy. It doesn't seem like it does it, but I don't think it's a coincidence that gravity and the electro-magnetic force travel at the exact same speed of light which is pure electro-magnetic energy. After all, can't only energy travel at the speed of energy? Otherwise why isn't matter going at the speed of light? Matter not going the speed of light means that energy need not move at the speed of light. Gravity and photons move at c because the exchange boson is massless. Or, you model the exchange boson as massless because the interaction occurs at c. Chicken/egg. Link to comment Share on other sites More sharing options...
lemur Posted January 23, 2011 Share Posted January 23, 2011 Because I prefer dealing with small, pointlike particles than with vector fields of infinite extent. Why? Because of calculation-utility? Because it's more intuitively comforting? Because there's something about that model that has more veracity? Some other reason(s)? That's how potential energy is defined, because the work done is not dependent on the path. But that's not true of all all forces and all energy. Why does that matter? The issue was whether force-fields themselves constitute a form of potential energy. Why or why not? Matter not going the speed of light means that energy need not move at the speed of light. Gravity and photons move at c because the exchange boson is massless. Or, you model the exchange boson as massless because the interaction occurs at c. Chicken/egg. It doesn't seem like it should be a chicken/egg problem of modeling. It seems like there should be something about the relationship between mass and gravity that restricts the acceleration of particles with mass within a gravitational field. Then there should be something else about gravity and/or electromagnetism that causes photons to always travel at C. If all matter is constituted from energy in some way, then gravity would ultimately be a byproduct of energy. In that case, the relationship between EM waves and gravity would be something akin to the relationship between evaporation and precipitation (i.e. co-influential aspects of a single process). Ok, that analogy is maybe far-sought, but what I mean is that the reason gravity limits the motion of EM waves in a consistent way probably has to do with the gravity playing a role in the photons' transport. I.e. gravity and electromagnetism don't seem to be unrelated forces. Link to comment Share on other sites More sharing options...
steevey Posted January 23, 2011 Author Share Posted January 23, 2011 (edited) That's how potential energy is defined, because the work done is not dependent on the path. But that's not true of all all forces and all energy. Matter not going the speed of light means that energy need not move at the speed of light. Gravity and photons move at c because the exchange boson is massless. Or, you model the exchange boson as massless because the interaction occurs at c. Chicken/egg. But, if the Gauge Boson for those forces are traveling at c, then they are some type of pure energy and don't have mass unlike the Gauge Boson for the weak force which has mass. Edited January 23, 2011 by steevey Link to comment Share on other sites More sharing options...
swansont Posted January 23, 2011 Share Posted January 23, 2011 They have energy. They are not energy itself. Link to comment Share on other sites More sharing options...
steevey Posted January 23, 2011 Author Share Posted January 23, 2011 They have energy. They are not energy itself. But if they aren't energy, how do they travel at c? Link to comment Share on other sites More sharing options...
swansont Posted January 23, 2011 Share Posted January 23, 2011 But if they aren't energy, how do they travel at c? They are massless particles. Link to comment Share on other sites More sharing options...
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