# Refrigerators are like rockets, right?

1. Jul 28, 2010

### Dreslan

A refrigerator has coils in back that get hot, and heat is molecular motion. These moving molecules have both energy and momentum. Why doesn't the refrigerator need to be tied to the wall to keep it from recoiling from the momentum it loses out the back?

2. Jul 28, 2010

### Dreslan

One more, related question that I have been grappling with:

A ballistic pendulum is a device which can be used to calculate the velocity of a bullet to a reasonable degree of accuracy. The end of the pendulum is large, massive, and designed with a material that renders the collision the bullet makes with it completely inelastic. The bullet becomes embedded in the pendulum, losing the largest amount of kinetic energy it could possibly lose, in other words. A figure I have heard regarding the loss of original kinetic energy to heat and sound is 99.7 percent.

To find the velocity of the bullet, one can apply conservation of momentum on the bullet, pendulum system, calculate the the horizontal distance the bullet travels, and apply a small angle approximation to come up with a logical figure.

My question regards the validity of applying this conservation law to the bullet and pendulum system. When the bullet coheres to the pendulum, it produces a considerable amount of heat, and as stated above, heat is molecular motion. The system is therefore giving up some of its momentum to the outside environment, because it has lost no mass, would this not reduce the final value of the velocity that would be gotten using the above method, in order to preserve total momentum? Or is the amount of heat dissipated, and therefore, the amount of momentum transferred to the air, very negligible in the time period during which the measurement is made?

Last edited: Jul 28, 2010
3. Jul 28, 2010

### RonL

I would think, the same reason you wouldn't worry about a half dozen ants carrying it out the back door.

4. Jul 28, 2010

### Dreslan

Thank you for your quick response. I think I understand:

Any momentum that is lost is too negligible to overcome the force of friction holding it to the ground, then?

If the ground were very smooth, would it indeed begin to scoot forward?

Edit:

Any momentum that is lost is too negligible for the refrigerator as a whole to feel a recoil force large enough to overcome the force of friction acting on it, or that, the amount that it does lose is too negligible to ever matter?

Last edited: Jul 28, 2010
5. Jul 28, 2010

### talk2glenn

There is much more holding the refrigerator in place than simply friction, and the momentum of its waste heat gives off is very small. Simply greasing the floor wouldn't do much about the fact that the atmosphere itself is pushing on the fridge, and gravity is pulling it down. Your fridge could in principle move (it is converting electrical energy into heat energy, and ejecting that heat into space and away from your perishables, so yes the heat must be pushing on the fridge), but in practice it is not going anywhere.

6. Jul 28, 2010

### Dreslan

Well said, thank you.

7. Jul 28, 2010

### Staff: Mentor

The kinetic energy of hot objects is random (it moves in all directions), so there is no recoil at all - all that stuff about friction and greasing the floor is irrelevant.

8. Jul 28, 2010

### Dreslan

I have considered that, yet perhaps inadequately or with error:

If a small portion of coil is considered, then momentum and kinetic energy will be lost in all directions (though the momentum as a whole will always be zero for the air/refrigerator system). The momentum lost toward the back of the coils will head in the direction of the refrigerator and be transferred back to it. So there, no momentum will be lost. However, in every other direction, momentum and energy will be lost to the surrounding environment. If it is lost in equal amounts, the losses will balance in all directions. However, the momentum lost along the line perpendicular to the refrigerator will be split; some momenta will leave the refrigerator completely out one side of the coil and some will be transferred back into it from the opposite side of the coil (still amounting to zero total momentum), resulting in a net momentum on the fridge along that line which would create a forward force, despite the random dissipation of heat and random transfers of momentum.

Last edited: Jul 28, 2010
9. Jul 28, 2010

### DaveC426913

Have you attempted to figure out just how much momentum a few cubic feet of slightly warm air might have?

10. Jul 28, 2010

### talk2glenn

This is quite wrong; it does not follow that because heat is lost in all directions, hot objects can not move. After all, hot gas tends to expand in all directions, but rockets clearly take flight.

In this case, the refrigerator is not hot, its radiator is. The placement of the radiator on the fridge, and the dimensions of the unit itself, will determine the vector of the heat loss (it is hotter behind your fridge than in front of it, obviously), and inertia in turn determines the vector the fridge WANTS to go in.

It is the other forces which hold it in place.

11. Jul 28, 2010

### DaveC426913

The rockets are able to take flight because the gases are prevented from expanding in all directions except rearward.

12. Jul 28, 2010

### Staff: Mentor

No. Heat is transferred off the coils via convection. The only associated force is an upward force on the coils due to aerodynamic drag, which is rediculously small.

This is not the same as the idea that the heat energy of molecules gives a group of molecules linear motion, which is completely wrong. Momentum is not a 'thing' that gets thrown from the coil to the refrigerator to bounce back - it is a property of the coil itself and it is zero.

Last edited: Jul 28, 2010
13. Jul 28, 2010

### Staff: Mentor

The heat of the gases causes them to expand in all directions unless constraied (as Dave said), but the rocket example is even worse than that: rocket propulsion requires a mass flow rate out of the back of the rocket. The hot refrigerant gas in a refrigerator coil isn't being ejected out the back of the refrigerator. It's just circulating inside the coil.

This bears no resemblance whatsoever to a rocket.

14. Jul 29, 2010

### Dreslan

I agree, and I did not think I made a claim to the contrary. Though, I suppose it's possible.

Momentum, as I understand it, is a way of quantifying relative motion. That it can be transferred does not make it material in the sense that there is a particle of momentum, it means that momentum is the amount of motion an object possesses, and this amount is subject to change. When two objects of equal mass collide, one with relative movement and one with no relative movement, the moving object loses all its momentum, or motion, and the stationary object receives an equal amount and takes off in the direction of the momentum vector. This is the result which explains Newton's cradle. This is a transfer of momentum, as I purported above, but does not require momentum to be anything but a property of the object, in agreement with your statement.

First, take a pipe by itself running from ceiling to floor, radiating heat. It radiates heat equally in all directions and so too does it do this with molecular momentum. The pipe feels no net force.

Now place this pipe, vertically, on a flat cart with wheels. The situation has not changed. Now place a brick along the pipe. Situation has not changed. Now place a fridge along the pipe instead of the brick. Still, no net movement.

Is this a more correct way of thinking about the situation, or is something still convoluted about my approach?

Thank you, by the way, for taking the time to answer my above replies.

15. Jul 29, 2010

### Staff: Mentor

Note that in Newton's cradle, the ball is moving and therefore it has non-zero momentum in a specific direction. Again, the total momentum of the coil is zero. There is no momentum to bounce off the refrigerator as you implied here, if such a thing is even possible: "The momentum lost toward the back of the coils will head in the direction of the refrigerator and be transferred back to it."
Well, radiating heat is a little different because radiated heat is actually emitted photons (much different from a discussion of the molecular kinetic energy), so if you block some of the photons, you can produce a thrust. But most of the heat loss here is convection: the radiated component of the heat loss is very small and the radiation pressure associated with it is too small to measure with any scale. The vertical force due to convection is at least large enough to measure (but still a fration of a Newton).

For a comparison of the magnitude of radiation pressure, the radiation pressure from the sun is about 0.0000046 N/sq m.

16. Jul 29, 2010

### Andrew Mason

Momentum and energy are always conserved in any interaction. Kinetic energy is not always conserved, however. This is because kinetic energy can be converted to heat energy, so the total kinetic energy of the bodies after collision is not the same as before the collision. But momentum is ALWAYS conserved - even if much of the energy is turned into heat.

AM

17. Jul 29, 2010

### Andrew Mason

Momentum and energy is conserved in any interaction. Momentum has only one form -> mass x velocity. So the sum of all the mass x velocity terms before the collision is always equal to the sum of the mass and velocity terms after the collision. But energy can be converted from kinetic energy into thermal energy so kinetic energy is not always conserved in a collision.

AM

18. Jul 29, 2010

### RonL

I hope my answer to the OP didn't sound too dismissive, the second question came while I was responding to the first.
I can't list all the ways I have observed heat raidiating from different things but blacktop roads, wood burning stoves and cutting and welding metal, are the most common for me. In a few cases with hot metal I have seen smoke from a welding rod being affected by the heat radiating from iron that has been heated in order to form it in some shape.

My point is, there is a small energy force and it can be seen under the right conditions, but as you say, it is so small the six ants I mentioned would be more likely to move the fridge.

Last edited: Jul 29, 2010
19. Jul 29, 2010

### Dreslan

My second question was not intended to call into question the validity of the law of conservation of momentum, but rather the validity of the system it was being applied on. I thought that, since some of the particles were interacting with the air, the surrounding environment would have to be considered part of the system as well as the pendulum and bullet to account for the momentum lost to the air (I understand now that the system is just fine defined the way it is).

I have read your replies and thought about this some more, and I think that, when the bullet and pendulum collide, this "lost" momentum I was referring to is the result of some of the kinetic energy turning into increased molecular motion. But, as Russ said much earlier, the motion of the molecules would be totally random, and the momentum from the vibrations would be zero as a whole. So even though the object would indeed be interacting with the air molecules, causing them to vibrate more vigorously and thus increasing their average kinetic energy, it would be doing so symmetrically. There would be no momentum lost to the environment; all those little mv's the molecules had would add up to zero, but kinetic energy lost would not be zero, as many of you have said, since it is a scalar.

When I said radiating heat, I should not have. The sense I meant it in is not in line with the definition used in physics. In all these discussions, when I mentioned 'radiated', I meant the vibration of the object's molecules would be interacting with the surrounding air molecules, increasing the air molecules movement, while decreasing the movement of the object's molecules. So, in a sense, they are "giving off" motion, but because this motion is not in any one direction, but spreads out evenly in all directions, increasing the average kinetic energy of the surrounding air and thus its temperature, the air's momentum has not actually increased. It has not increased because all the random movements have to balance out to zero. So all the momentum is still within the bullet pendulum system. This same idea would apply to the refrigerator.

Though this lost kinetic energy could not by itself cause motion in any one direction, it could affect other objects. For instance, if the experiment were done under water, the heat generated might cause water to undergo a phase change, if the kinetic energy produced were large enough, but it could not cause water to move in any specific direction, it would not increase the momentum the water had before the collision.

Everyone else who replied to this topic, thank you for your time and consideration, all of the replies here helped me to think about the situation through other eyes. Kinetic energy and momentum are completely different things. I had learned that but not really understood it until now, with the help of many of you whom have taken the time to point out the errors in my argument and to offer your own arguments, in some cases. I feel much more confident in my understanding of momentum and kinetic energy, as they apply to these two situations and as a whole.

To answer my own questions with a better understanding:

The refrigerator would not move, except maybe imperceptibly due to photon radiation, if some of those photons were somehow blocked so that there were a net direction in which this radiation pressure occurred. The momentum of the coil would remain constant, even while it gave off kinetic energy to the air.

The bullet-pendulum system's velocity will not be affected due to loss of what is really just kinetic energy, but what I continued, in error, to call momentum.

-Tony

Last edited: Jul 29, 2010
20. Jul 29, 2010

### brainstorm

you think you're worried now? Wait until you realize the possibility that your stove could be driving your house into the ground and the ventilation fan for the fume hood is helping it along.