# Can you move in space?

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A friend and I were having a debate as to wether this theory holds true:

Please try to imagine that you were motionless in free space (relative to whatever....) and you were standing on one end of a plank with your feet firmly secured to the plank, and at the other end of the plank there was a board much like a basketball back board firmly secured to the plank facing you. Please Imagine you had pockets full of good sized rocks, if you were to throw a rock at the back board, you and the plank along with the back board would move in the opposite direction to the rock until the rock hit the back board, then at that point the rock hitting the backboard would cancel out the motion bringing the plank to a stand still again? For the purpose of this theory, please exclude any energy loss from flexibility of the man throwing the rock or any energy loss due to the absorption of energy by the back board or any energy loss due to rotation, just try to imagine that 100% of the rock’s energy is transferred to the backboard on impact. If you had an infinite amount of rocks in your pocket, could you continue to move in a start stop fashion by continually throwing the rocks at the back board?

Additional notes: My friend was caught up on the blowing your own sail paradox where the forces cancel each other out, but in my theory, the forces only cancel out when the rock hits the back board, while the rock is travelling toward the backboard, the plank and everything fixed to it will move in the opposite direction of the rock until the rock hits, if this process can be repeated many times with infinite rocks then the plank should continue to move in the same direction as long as the rock keep getting thrown at the board?

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Posted (edited)

Nope, you wouldn't go anywhere.  If you didn't hit the backboard you could go in the opposite did though.

Edited by Bufofrog

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If the rocks stuck to the board, there would be no overall motion. If the rocks bounced off, there would be.

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35 minutes ago, swansont said:

If the rocks stuck to the board, there would be no overall motion. If the rocks bounced off, there would be.

Good point, I was wrong on my initial post.

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I understand that the rock hitting the backboard cancels out the force but can someone please explain why the platform wouldn’t move while the rock was in flight on its way to the backboard?

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Posted (edited)
29 minutes ago, MPMin said:

I understand that the rock hitting the backboard cancels out the force but can someone please explain why the platform wouldn’t move while the rock was in flight on its way to the backboard?

You have to look at the two extreme cases:

1. The rock sticks to the board (a perfect inelastic collision): then the whole would move backwards. But only as long as the rock is in flight. So you make a translation, but you do not get continuing velocity. The total momentum would still be zero. So, yes, you move while the rock is on its way to the board.
2. The rock bounces without losing any energy in the collision (a perfect elastic collision). The end result would be that the rock flies away in the opposite direction in which you throw it, and so, because of conservation of momentum, you would move forwards, and keep your velocity.

The third case, with infinite rocks, would be that you do not move at all, because the mass would also be infinite. Unless you throw an infinite number of rocks...

Edited by Eise

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33 minutes ago, MPMin said:

I understand that the rock hitting the backboard cancels out the force but can someone please explain why the platform wouldn’t move while the rock was in flight on its way to the backboard?

It would. But all you are doing is rearranging the mass distribution, but the center of mass will not have changed. By motion I (and I assume others) meant that you will not have a sustained velocity of the center of mass.

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1 hour ago, MPMin said:

I understand that the rock hitting the backboard cancels out the force but can someone please explain why the platform wouldn’t move while the rock was in flight on its way to the backboard?

We will assume that the backboard "catches" the rocks ( they don't rebound).  You start with X number of rocks, each of mass M.  Thus you have X times M total mass in rocks.    Now the total mass of the system is XM. ( for this particular example we will assume that your mass and the mass of the platform is zero)

You throw a rock at speed v.  Total momentum of the system is conserved, so the platform moves in response at -V.  The the platform, plus the rocks you still hold now mass (X-1)M and -V will be of a magnitude such that vM -V(X-1)M = 0. After a time T, the rock hits the backboard, and all velocities stop.  The backboard and you will have moved a slight bit from where you started.  You throw another rock.  The rock you threw previously is stuck to the backboard so it still makes up part of the mass of the platform.  Thus throwing the second rock moves you the same distance as the first rock did, as does every succeeding rock, until you run out of rocks.

So your thought is, "If I increase the number of rocks I start with, I should increase the distance I can move the platform."   The problem is that adding more rocks adds to the total mass of rocks the platform holds(either in your pocket or stuck to the backboard) when you toss a rock.   In other words, if X becomes larger,   (X-1)M becomes larger and thus -V, the recoil velocity of the platform, is smaller.  The platform moves a smaller distance for each rock thrown between toss and capture by the backboard.  Adding more rocks won't increase the total distance the platform moves, as each individual rock moves the platform a shorter distance. If l is the displacement per rock thrown, and L the total displacement after all rocks are thrown, then L= Xl

And as pointed out above, an infinite number of rocks equal an infinite mass, which results is no movement when tossing any one rock.*

In this example, the total distance that the platform will move(L) will be the distance between your release point of the rock and the backboard, no matter how many rocks you carry.  This is the theoretical maximum based on zero mass for you and the platform.

In real life, the platform( including you) would not be of zero mass, and that would have to be taken into account.   This additional mass would decrease the total displacement of the platform to being less than the above mentioned distance.    The larger the total mass of rocks compared to platform mass determines how close to the theoretical maximum you will get.

* in such problems is is generally best to avoid infinities, as they tend to create incongruities.   As noted above, increasing X has no effect on the total platform displacement, as long as X is finite.   Make X infinite and it drops to zero.   There is a discontinuity between X being finite and infinite.

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Let’s say you only have one rock and the board is like a trampoline allowing for a perfect elastic collision assuming some energy loss due to heat and friction created on impact with the trampoline and you could throw the rock an infinite amount of times because the rock returns to each time you throw it, could you then generate momentum?

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1 hour ago, MPMin said:

Let’s say you only have one rock and the board is like a trampoline allowing for a perfect elastic collision assuming some energy loss due to heat and friction created on impact with the trampoline and you could throw the rock an infinite amount of times because the rock returns to each time you throw it, could you then generate momentum?

No, because catching it involves momentum transfer as well.

You can only generate propulsion if mass actually leaves the system.

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You can't change the centre of mass and energy of an isolated system with respect to it's reference frame...even temporarily.

It's as simple as that.

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3 hours ago, swansont said:

No﻿, because catching﻿ it involves momentum transfer as well﻿

You can only generate propulsion if mass actually leaves the system.

If the rock is returned to the thrower from the trampoline with less energy (because some was lost in heat and friction on impact with the trampoline) doesn’t that produce a difference between the forces producing the momentum in either direction?

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1 hour ago, MPMin said:

If the rock is returned to the thrower from the trampoline with less energy (because some was lost in heat and friction on impact with the trampoline) doesn’t that produce a difference between the forces producing the momentum in either direction?

No.  If there is energy "lost" in the impact with the trampoline, the ball may return at a lower speed, but less momentum is transferred to the trampoline also.  It still comes out as a wash.

The only way this could generate any net movement over time is if that Energy lost in the collision is radiated away asymmetrically, making a kind of photon rocket.

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8 minutes ago, Janus said:

No.  If there is energy "lost" in the impact with the trampoline, the ball may return at a lower speed, but less momentum is transferred to the trampoline also.  It still comes out as a wash.

The only way this could generate any net movement over time is if that Energy lost in the collision is radiated away asymmetrically, making a kind of photon rocket.

Yes i now see that the throwing and rebounding of the rock in a closed system conserves the mass thus preventing any momentum being generated.

What if instead of a rock being thrown its a pulse of magnetic field instead?

Lets say instead of having a person throwing a rock at one end and a trampoline at the other end of the platform in space, there are two electro magnets at either end in place of the person and the trampoline. If you were able to generate a short pulse of magnetic field from one end, (the pulse would have to be short enough to leave the first electromagnet before it arrived at the other electro magnet) travel towards the other electro magnet as a free floating pulse,  just as the pulse arrives at the other magnet, that magnet is turned in to repel the on coming magnetic pulse: would this generate momentum?

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Posted (edited)
59 minutes ago, MPMin said:

What﻿ if instead of a rock being thrown its a pulse of magnetic field ins﻿tead?﻿

Short answer: No; rocks or particles (such as photons) makes no difference. You cant generate acceleration using internal forces. Using impacts from massive bodies, magnetism or other makes no difference.
The equation $F=ma$ still applies. If you want acceleration $a>0$ that means Force $F>0$. Or the other way around, zero force equals zero acceleration, it does not matter if rocks, magnets or other methods are used.

Edited by Ghideon
Added the math, couldn't do that while using phone

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1 hour ago, Ghideon said:

Short answer: No; rocks or particles (such as photons) makes no difference. You cant generate acceleration using internal forces. Using impacts from massive bodies, magnetism or other makes no difference.
The equation F=ma still applies. If you want acceleration a>0 that means Force F>0 . Or the other way around, zero force equals zero acceleration, it does not matter if rocks, magnets or other methods are used.﻿

Even shorter: there is no way you can beat the law of conservation of momentum, whatever way you produce a force.

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1 minute ago, Eise said:

Even shorter:﻿ there is no way you can beat the law of conservation of momentum, whatever way you produce a force.

+1

Even shorter:No

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9 hours ago, MPMin said:

Yes i now see that the throwing and rebounding of the rock in a closed system conserves the mass thus preventing any momentum being generated.

What if instead of a rock being thrown its a pulse of magnetic field instead?

Lets say instead of having a person throwing a rock at one end and a trampoline at the other end of the platform in space, there are two electro magnets at either end in place of the person and the trampoline. If you were able to generate a short pulse of magnetic field from one end, (the pulse would have to be short enough to leave the first electromagnet before it arrived at the other electro magnet) travel towards the other electro magnet as a free floating pulse,  just as the pulse arrives at the other magnet, that magnet is turned in to repel the on coming magnetic pulse: would this generate momentum?

A "short magnetic pulse" is essentially an electromagnetic wave,  or in other terms, a burst of photons.    Electromagnetic waves/photons would not be repelled by the second electromagnet.   Again, unless some of those photons produced escape the platform in a non-symmetric way, you get no net motion of the platform. If more escape in one direction than the other, you have a photon rocket again.

You simply cannot "fool" the universe into violating the conservation of momentum, no matter what "tricks" you try to use.  As you make your schemes more and more complex all you are doing is making it more likely that you will lose track of all the momentum transfers along the way, something nature never does.

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!

Moderator Note

Photon question has been split

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Ive seen experiments on utube where wires carrying currents can repel or attract each other depending in which way the currents are going.

If I’m not mistaken, in a closed system in space the two forces from the wires would cancel each other out thus producing no momentum.

But what if an extremely short current was pulsed through one of the wires causing an extremely short magnetic field pulse that emanated outward from the first wire expanding outward as a growing ring in every direction from that wire and also heading toward the other wire, keeping in mind that the pulse had left the first wire no longer in contact with the first wire. If the second wire were to also have a current pulsed through it as the first magnetic pulse approached, why wouldn’t that second wire move?

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27 minutes ago, MPMin said:

Ive seen experiments on utube where wires carrying currents can repel or attract each other depending in which way the currents are going.

If I’m not mistaken, in a closed system in space the two forces from the wires would cancel each other out thus producing no momentum.

But what if an extremely short current was pulsed through one of the wires causing an extremely short magnetic field pulse that emanated outward from the first wire expanding outward as a growing ring in every direction from that wire and also heading toward the other wire, keeping in mind that the pulse had left the first wire no longer in contact with the first wire. If the second wire were to also have a current pulsed through it as the first magnetic pulse approached, why wouldn’t that second wire move?

(bold by me above)

If you send photons in all directions you have a different setup, it is not a closed system anymore? Does the second wire (=antenna) also send in all directions?

Note: no matter what idea you propose, conservation of momentum wins.

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Posted (edited)
1 hour ago, MPMin said:

If I’m not mistaken, in a closed system in space the two forces from the wires would cancel each other out thus producing no momentum.

But what if an extremely short current was pulsed through one of the wires causing an extremely short magnetic field pulse that emanated outward from the first wire expanding outward as a growing ring in every direction from that wire and also heading toward the other wire, keeping in mind that the pulse had left the first wire no longer in contact with the first wire. If the second wire were to also have a current pulsed through it as the first magnetic pulse approached, why wouldn’t that second wire move?

First, to repeat, there is no way you can beat the law of conservation of momentum. So whatever you think of, and also nicely expressed by Janus:;

15 hours ago, Janus said:

As you make your schemes more and more complex all you are doing is making it more likely that you will lose track of all the momentum transfers along the way, something nature never does.

So generally: you will never succeed. And to take the expression of swansont:

On 7/2/2019 at 4:55 PM, swansont said:

the center of mass will not have changed

This is even so for a rocket. Of course, the rocket can be accelerated, in the end,  that's what we make rockets for.

Take as starting point a rocket with its engine off. It will move with constant velocity, so in its own frame of reference it stands still. Now it turns it engines on, on its way to Alpha Centauri. The fascinating thing is: because of the law of conservation of momentum, it is guaranteed that the centre of mass keeps at this starting point. If you add the momenta of the rocket, and its combusted gases, and calculate where the centre of mass is, it is still at the place where the rocket turned its engine on (even if there is nothing physical at this place).

In your special case, it seems important to me that you specify if the wires are somehow fixed to each other (e.g. on a wooden board). Then only a temporary movement can exist. The system as a whole would only vibrate a short moment, but stay in place. If they are not fixed the wires would move, but that is similar to the situation with the rocket. The centre of mass still does not move.

Why do you think you can beat the the law of conservation of momentum? It is not based on analyses of all kind of situations, and oh wonder, we discover that momentum is conserved again and again. It is based on the most basic laws of forces and movements, that apply in all situations.

Do you also think you can exchange your left arm with your right arm by turning your body 180o around?

Edited by Eise

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In my mind the wires are fixed in parallel to each other. I’m also struggling to understand how shifting mass and matter are involved in terms of generating the propulsion. I know the objective is to ultimately move the entire mass and in traditional reaction propulsion systems rely on moving mass but I cant see how that applies to moving magnetic waves? I can understand shifting mass back and forth wont work because mass because cant be switched on and off but electromagnetic radiation can be switched on and off. By pulsing a current through a wire you bring a magnetic into existence that wasn’t there before without shifting mass to do so. I’m sure my error is clear to you all but i still don’t see it, please help me understand.

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4 minutes ago, MPMin said:

In my mind the wires are fixed in parallel to each other. I’m also struggling to understand how shifting mass and matter are involved in terms of generating the propulsion. I know the objective is to ultimately move the entire mass and in traditional reaction propulsion systems rely on moving mass but I cant see how that applies to moving magnetic waves? I can understand shifting mass back and forth wont work because mass because cant be switched on and off but electromagnetic radiation can be switched on and off. By pulsing a current through a wire you bring a magnetic into existence that wasn’t there before without shifting mass to do so. I’m sure my error is clear to you all but i still don’t see it, please help me understand.

I'll try two different ways:

A:

It might help if you think in terms of "force" rather than mechanical work, electromagnetism, rocks or photons. If you want to move (=accelerate) an object you must apply a force. If no force is applied the object will not accelerate, it will stay at rest (or continue to move at constant speed).
When a magnetic field is generated to repel some object where are the forces? For instance when a maglev train is lifted up (=vertical acceleration for a short while)  the earth is moved in the other direction. The center of mass (train/earth) is not moved. If you have fixed wires pushing at each other, how could there ever be an external force pushing the complete rig where both cables are mounted? Water jets, rocks, magnetic forces... they are all obeying the same principle.

B:

If you prefer to think of "small bursts of photons" or "small packets of energy"* in transit between locations you will need to take into account that photons carry momentum. Conservation of momentum applies.

Three cases:
1: If you sent a burst of photons in one direction you have a "rocket". Momentum of photons is balanced by momentum of the rocket.

2: If you collect the photons again at some other location of the rig you have no movement, you will not be able create a reactionless drive. Momentum gained from sending the photons are balanced by the momentum lost when receiving the photons.

3: If you collect photons sent by some other party you will have propulsion, for instance like a solar sail**.

*) Not very rigorous but hopefully it illustrates the point

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1 hour ago, MPMin said:

By pulsing a current through a wire you bring a magnetic into existence that wasn’t there before without shifting mass to do so.

To add a third take to Ghideon's two: this does involve shifting energy: from the battery (or other power source) and into the pulse. Energy and mass are equivalent (cf. E=mc2) so moving energy is equivalent to moving mass around.