r/AskPhysics • u/AlwaysWearingACap • 11h ago
Why wouldn't this machine work?
Hi all. I was thinking about perpetual motion machines. Why wouldn't this one work? Imagine a ball encased into a tube. Ball falls down due to gravitational force, and a magnet "strikes" the ball up with enough strength to get to the original point + enough to overcome the friction and other forms of energy loss in the process. Then the ball sets in the original position, and cycle repeats.
I'm trying to figure out why it wouldn't work perpetually, can you help me? Thanks!
5
u/Crudelius 10h ago
The magnet is the problem. Its a common misconception that a magnet has some Kind of infinite energy that can be transfered onto other objects.
If you have a permanent magnet the Ball will get into an equilibrium and therefore no energy can be gained.
If you have an electromagnet, you need to put more energy into that than you will gain from the falling ball
3
u/mooremo 10h ago edited 10h ago
Imagine a ball encased into a tube. Ball falls down due to gravitational force
Ok
and a magnet "strikes" the ball up
A little unclear exactly what you mean, but ok.
with enough strength to get to the original point + enough to overcome the friction and other forms of energy loss in the process.
You answered your own question right at the end. Energy is lost in this process. For it to keep going you need to put more energy into the system to replace what was lost. No matter how big of an energy source you have, it will run out eventually and then this will stop.
Even if you were able to eliminate all the sources of energy loss (you can't actually do this in real life, there will always be tiny losses) you would only have created perpetual motion, which physics is fine with. But it's not a machine. You can't extract energy from the system to do something useful (power a toaster) and still have the system maintain its original motion.
1
u/AlwaysWearingACap 10h ago
Okay so magnets don't add energy into a system, they just move the energy around?
2
u/mooremo 10h ago
Correct, the magnet isn't bringing any new energy into the system.
0
u/AlwaysWearingACap 10h ago
Why not? Isn't the magnet like some sort of batter (baseball) that gives their stored energy to the ball? Ensentially introducing energy into the system?
2
u/mooremo 8h ago
Nope, all of the energy is in the system as a whole, not in the magnet or in the ball. The form that energy takes changes over the course of the balls motion.
- At the top (maximum height):
The ball has maximum gravitational potential energy (because it’s high up).
It also has minimum magnetic potential energy (because repelling forces store the least energy when the objects are farthest apart).
The ball falls towards the magnet because the attractive force of gravity is overpowering the repulsive force of the magnet at this distance.
- As it falls downward toward the magnet:
It loses gravitational potential energy and gains kinetic energy (speed).
At the same time, it is moving into a region of stronger repulsion, so its magnetic potential energy increases.
Some energy leaks away as heat and sound through friction.
- At the bottom (closest to the repelling magnet):
Gravitational potential is at a minimum, it's fallen as far as it can.
Magnetic potential is at a maximum (compressed, like a spring being squished).
All of the kinetic energy from the fall, that wasn't lost as heat and sound through friction, has been converted in to magnetic potential energy.
- As it rises again:
The repelling magnet pushes it upward, so magnetic potential energy is converted into kinetic energy and then into gravitational potential energy as it climbs.
But again, some energy is lost along the way.
- Back near the top:
In theory, it could reach the same height (same gravitational potential, same minimum magnetic potential).
But because of the unavoidable energy losses, it won’t quite make it. With each bounce will be a little lower until it stops at a point where the repulsive force from the magnet balances the attractive force from gravity.
To simplify:
Gravity pulls it down (gravitational potential → kinetic → magnetic potential).
Magnet pushes it back up (magnetic potential → kinetic → gravitational potential).
But friction, heat, and sound always bleed energy out of the system. There’s no new energy being added, just trading between forms.
1
u/AlwaysWearingACap 7h ago
Thanks for the detailed explanation. I believe that the part I got wrong is that I thought of the magnet as a batter hitting a ball, not like a perfectly elastic body, like you guys are saying it behaves like. Again, just to clarify my doubt: If I throw a ball (like on a table) at 1m/s onto a very potent magnet, will the ball always get repulsed from the magnet at <1m/s (accounting for friction), no matter the magnet or the conditions?
1
u/mooremo 7h ago
Correct.
A more repeatable experiment, should you actually want to try this, would be to drop it from a certain height rather than throwing it at a certain speed. It removes the inconsistency of your throwing force. Gravity will always apply the same force to the ball, so as long as you're careful to drop it from the same height you'll always get the same result. And that result would be that the highest the ball ever gets is where you release it from and the fastest it ever travels is as it's falling on the first drop. Every oscillation afterwards the ball will not go as high or be traveling as fast because of energy loss to friction.
2
u/KaptenNicco123 Physics enthusiast 10h ago
How does the magnet "strike" the ball up? Does the magnet launch the ball up? If so, you need to power the magnet. Does the magnet repel the ball? Then it will repel it to a lower height than it started at.
-2
u/AlwaysWearingACap 10h ago
It repels the ball. Why would it repel the ball to a lower height? Can't magnets add energy into a system? (From their stored energy that is)
3
u/D-Alembert 10h ago
You should play with magnets some more, your intuition about them doesn't match what they do (and playing with magnets is fun!)
1
u/AlwaysWearingACap 10h ago
Yeah, idk, I'm bored at work thinking about magnets. I just watched at this video https://youtu.be/bA1l3l1PYDM?si=imZX8-zL_NAqPvZs (1:05 timestamp for repulsion scenario), and it seems like the magnet on the right side gained more speed compared to the one on the left on the approach. Doesn't that mean that the system gained energy? (Energy coming from the magnets, that is)
Edit: link
2
u/D-Alembert 10h ago edited 9h ago
The system did not gain energy, magnets are like springs, the one on the right was stationary as the other was pushed into it so the distance closed before it could get up to speed (compressing the "spring"), so when it did get moving it sprang away, faster, but not with more energy than the person's hand pushed into it.
Or from a different perspective, the system gained energy from the person's hand pushing it
This should be intuitive, but I guess you haven't played with them enough
-1
u/AlwaysWearingACap 9h ago
Well, with springs we need to compress them so they have energy that they can give to the ball.
With magnets, we don't have to do that, they have their energy stored, no?
2
u/D-Alembert 9h ago edited 9h ago
No, magnets don't have energy stored, you have to compress them like an invisible spring
Once you have pushed them together despite the repulsion (compressed the invisible spring) then the repulsion is storing the energy you imparted when you pushed them together, like compressing a spring
You should play with some magnets, this should be intuitive not mystifying
Do you know the difference between kinetic energy and potential energy? That's another way to think about it that might help.
2
u/gmalivuk 9h ago
You add energy by pushing the object deeper into the magnetic field, and then that same energy is what bounces it back up.
3
u/KaptenNicco123 Physics enthusiast 9h ago
Magnets don't store energy. If you somehow could store energy in a magnet, and use it to repel the ball higher than it started, well now the magnet doesn't have energy, and we're back to where we started.
2
u/LordCanoJones Quantum field theory 10h ago edited 10h ago
Ok, first of, what you are describing is perpetual motion, not a machine. Machines exert work upon an external system, and thus looses energy over time. Perpetual motion is not a problem, it happens all the time! Just think about planets orbiting around a star, they do not stop.
Now, the main "problem" of what you describe is that you cannot build something that "overcomes friction", since it will always be there. The ball and the casing will heat up from the collision, losing energy over time. Both the ball and the casing will deform from the collision (totally rigid bodies do not exist) which takes energy from the falling ball into elastic vibration of the bodies.
Magnets are not magical artifacts that will add energy to a system.
3
u/Lumpy-Notice8945 10h ago
Perpetual motion is not a problem, it happens all the time! Just think about planets orbiting around a star, they do not stop.
I thought even planets orbits decay over long enought timeframes like sure it takes millions or billions of years but eventualy the moon would either crash i to earth or be thrown out of earths orbit. Nothing is realy forever.
1
u/LordCanoJones Quantum field theory 10h ago
That is because of external or more complex interactions. Like interplanetary dust, tidal forces... But gravitational force conserves energy (we aren talking about Newtonian mechanics here, in GR you get gravitational waves that do emit energy away from the system), two perfect pointlike bodies orbiting eachother, do orbit eternally.
2
u/thefooleryoftom 10h ago
So even that system requires “assume a spherical cow” like circumstances?
1
u/LordCanoJones Quantum field theory 9h ago
If you want to completely describe any system, you need to make approximations. Otherwise you're considering two plastic pswudospheres in a fluid, you gotta consider thermal radiation, relativity, post quantum corrections...
But I mean, if two planets orbit eachother in reality, yes, in theory they will fall into eachother or part ways at some point I guess, but it's more likely that their star will die long before that if the system is stable enough.
1
u/thefooleryoftom 9h ago
Cool, thank you. I was thinking of the earth/moon system where we can extract energy from the tides, but because of the drag it’s slowing earths rotation and the moon is moving away making an unstable system.
I wasn’t sure if there was any actual planetary system (even an imaginary one) that could theoretically stay that stable.
1
u/LordCanoJones Quantum field theory 9h ago
That would depend on what you call "stable" and what is the system. Just consider earth, we build dams that slow earth rotation; that doesn't mean that earth as a system isn't stable, it just means that it's a dynamic system.
The moon might be distancing earth, but it's not like it's going its own separate way... When we talk about astronomical scales, solar systems are incredibly stable considering how many-body systems work (like... With all possible states, most 3 body systems do crash or throw one body away).
1
u/thefooleryoftom 8h ago
I see, I’m probably being to absolutist about it and thinking since the moon is drifting away it isn’t stable as in billions of years Earth will have slowed enough to no longer have tides. That’s still quite a long time…
1
u/Lumpy-Notice8945 4h ago
But thats kinda the point of perpetual motion, that they slowly lose energy to minor effects that you normaly ignore in an idealized model calculation. A pendulum would be a perpetual motion machine if you ignore friction, basicaly anything would move forever if you ignore that.
1
u/AdLonely5056 10h ago
If your magnet "overcomes energy loss" that means that you are essentially inputting energy into the system, so it’s not isolated and not a perpetual motion machine because whatever source of energy you use to power the magnet will eventually run out.
1
u/clintontg 10h ago
I think eddy currents caused by the change in the magnetic field as the ball falls towards the magnet will reduce the kinetic energy of the ball and eventually make it stop.
1
u/GXWT 10h ago
The answer is airways: either you are not actually overcoming friction and there is energy loss, or you are powering the system in some manner such that the input balances out the energy losses. In this case one loss is your (metal) ball being pushed up by the magnet will induce heating within the ball and magnet.
In any case, you can have perpetual motion per-say. If a kick a football way out in space, in a simple empty universe, it will just travel in the same direction forever. The issue is extracting energy for ‘free’, this can’t be done.
1
u/beingsubmitted 10h ago edited 10h ago
It seems like you understand in general why perpetual motion doesn't work. If I bounce a ball, it's never perfectly bouncy and the ball is always losing energy from friction so each time it bounces it reaches a point a little below the last bounce.
I think what's tripping you up is the magnet. If I drop a metal ball over a large magnet, what's actually happening? The magnet isn't letting the ball get closer and then pushing it away. Really, this system has an equilibrium, where the ball hovers perfectly still above the magnet. This is where the magnetic force of the magnet is equal to the force of gravity pushing the ball down, and so it hovers there. But what if I drop the ball from a bit higher? Well, since we're above the equilibrium point, the force of gravity on the ball is greater than the force of the magnet, so the ball can accelerate downward. Eventually, the ball reaches that equilibrium point, except now instead of just have the force of gravity pushing down and the force of the magnet pushing up, we also have the momentum of the falling ball pushing down. Since the downward forces are now greater, the ball will go past the equilibrium point, but as it does, the magnetic force pushing up increases, which decelerates the ball, and the ball decelerating means it loses momentum. When it finally runs out of momentum, it's now just the force of gravity pushing down an the magnetic force pushing up, but that momentum made it so this is closer to the magnet than it's equilibrium, so there's more magnetic force pushing up than there is gravitational force, so the magnetic force accelerates the ball back upward. Again, because it's in motion on it's way back up, when it gets back to equilibrium, it'll have magnetic force and momentum pushing up and gravitational force pushing down, so it overshoots until it runs out of momentum, then it's the same two forces, but we're above equilibrium, so we go back down, etc, etc, etc.
Only, this whole time we've also been losing a little momentum from, for example, air resistance. So each time it passes that equilibrium with the two forces and a little bit of momentum, that little bit of momentum is smaller, so it overshoots equilibrium a little less until it's eventually just sitting there at equilibrium with no momentum left.
2
u/Great_Dependent7736 10h ago
And since magnets have two poles and the ball is magnetic, the ball will turn around and be stuck to the other magnet, north to south. Or?
1
u/TheLapisBee 9h ago
A lot of people have already explained why this machine wouldnt help you. But if you'd like to explore it deeper, perpetual motion machines are technically possible, because energy isnt always conserved (look up noether's theorm, or visit the top of all time question on this sub)
1
u/anisotropicmind 8h ago
The question is always, where does the energy come from to “strike” the ball? Is this a permanent magnet? Then the energy is stored in the already-set-up magnetic field, which will retard the fall of the ball. You are making the ball climb up a potential energy curve higher than the one you want it to roll down. If it’s an electromagnet you can turn on and off, then you’re applying external energy to the system to set up and take down the magnetic field.
1
u/ScienceGuy1006 5h ago
If it's a permanent magnet, then a magnet strong enough to pull the ball up will also be strong enough to stop it from falling all the way back down.
If it's an electromagnet, then it may work - but then you have reinvented the electric motor, and not created a perpetual motion machine!
7
u/Skusci 10h ago
It takes just as much energy to leave the magnet as you gain from approaching it. Or vice versa if it is repelling.
Maybe think of magnets more like wireless springs in terms of where the energy comes from. Like when a ball hits a spring energy gets stored and the ball slows down. Then the spring extends released that energy and the ball speeds back up. The end speed is the same as the start speed.