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Q & A: magnet drive?

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Most recent answer: 05/03/2009
Q:
Would this work to propel a spaceship? First, you take 2 electromagnets, one smaller than the other, and lay them with one flat side facing the other magnet's flat side in a tube. You would place them so that, when electricity is run around them, the two facing flat sides have the same polarity. You will have the bigger magnet be stationary, and the smaller magnet free to move. When you activate the magnets by running electricity around them, shouldn't the bigger magnet push the smaller magnet away? Then, you could have it hit the end of the tube the magnets are in and that will push the space ship forward. From there you can have the force of the magnets repelling each other, with the smaller one being repelled more because it is smaller, push the ship. Or, you can switch the bigger magnet's polarity to attract the smaller one and then switch it again to repel it to continue hitting and pushing the spaceship forward.
- Peter Wysocki (age 16)
Darien, Illinois, USA
A:
The problem is that the forces on the magnets are equal and opposite. The little magnet may be pushing your spaceship but the big magnet will be pulling the other way with equal force. Since total momentum is conserved, the only way to give the spaceship forward  momentum is to dump some backward momentum on something else. If you expelled a spray of magnetized material, that could work, although I guess the efficiency would be low.

Mike W.

(published on 05/08/2008)

Follow-Up #1: magnet propulsion

Q:
In your response to the question above you haven't included the fact that the magnetic field is transmitted with photons that travel at the speed of light. What if the polarity of the magnets are changed with a frequency proportional to the distance between them. This could ensure that the rear magnet is always attracted to the front magnet and the front magnet is always repelled by the rear magnet. I do realize this breaks Newton's 3rd Law - but it does so using special relativity, so this must be a special case.
- Nick (age 33)
Adelaide, Australia
A:
The law of conservation of momentum applies in special relativity just as much as in Newton's physics. Last we heard, they aren't offering any exemptions for special cases.

Mike W.

(published on 04/28/2009)

Follow-Up #2: magnet drive?

Q:
It seemed like you were thinkers rather than followers on this site. Looks like I was mistaken (or I just haven't explained it clearly). To simplify the situation, imagine a spaceship made of wood (so it doesn't interfere with the magnet drive) that is 300000km long (takes light 1 second to travel from one end to the other). On one end is a bar magnet with a guy holding it and the same for the rear end. Each guy knows he has to flip the magnet over every one second. When the first guy flips the magnet (at the rear of the ship), it is facing north and changes to south. Meanwhile, 1 second later at the front of the ship, the guy has his magnet facing south (towards the end of the ship) so the two magnets attract and then he flips it so that it will still attract to the flip that occurred on the other end. Meanwhile, the signal from the front magnet is travelling to the rear of the ship except it is seeing like poles so always repels. This is not a practical ship design, there are many ways of engineering it to be feasible. Now either, the two magnets are communicating to each other faster than the speed of light, or Newton's 3rd law doesn't apply, and the ship will move. I've never received a reasonable explanation for why this doesn't work. Saying just trust us that special relativity and Newton's 3rd law are both correct doesn't seem like a reasonable explanation either.
- Nick (age 33)
Adelaide, Australia
A:
As I understand it, you've got two magnets, Front and Back, which are each rotated 180 once per second. I'll go along with you temporarily in ignoring the EM waves they emit, and consider only the direct static dipole forces.

Write the orientations of each magnet at the i'th second as Fi and Bi, either + or -.
Each magnet sees the other's orientation one second in the past, so you've set up a force which is FiBi-1- BiFi-1.  If you try to write down any sequence for the orientations of the two magnets at each second, you'll find that the sum of the forces still averages to zero. Your rotating says Fi=-Fi-1 and Bi=-Bi-1 so FiBi-1- BiFi-1= FiBi-1- Bi-1Fi= 0.

It's simpler to just remember that momentum is conserved.

Mike W.

(published on 05/03/2009)

Follow-up on this answer.