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Physics question? Why doesn't this work? Answered

So included I have the original "troll physics magnet car" that supposedly would use magnets for propulsion. I know that it doesn't work since the front magnet pulls the car and the car pulls the front magnet, so the energy is evened out. But if you put a stronger magnet on the front, then the smaller magnet on the car wouldn't have enough strength to equalize the larger magnet, so why wouldn't the car move forward? 

What if you applied the same idea to a generator? (look at the picture with the circles and rectangles) Wouldn't the bar magnets continue to repel each other and make the generator wheel would spin forever? If the wheel was balanced and was given a little push to get things going, I don't see what the problem would be.

I'm not planning on building this machine, so trial and error is out of the question for me. 
Please help!


The word "force" is a concept invented by physicists, most notably Isaac Newton, to explain why things move. E.g. Why does this apple fall downwards? Why does this car pull to one side of the road? Why does an electron in a cathode ray tube move the way it does?   The usual explanation is that there is some "force" pushing, or pulling, the object in that direction.

A force is always though to exist between two objects.  If some object A is experiencing a force, then there must be some other object B causing that force.  That is to say, object A experiences a force because it is being pushed or pulled by object B.  Also you can always look at things from the perspective of object B.  The force on B due to A, pushes, or pulls, in the exact opposite of the direction of the force on A due to B.  Note that you cannot really say which object is responsible for the force, which object is causing the force, because they both are. The force is the result of an interaction between A and B.

This notion of reciprocity in the definition of "force" goes back to Newton's third law of motion, here:

Newton was one of the first to put this observation into words, and the idea is fundamental to the way force is defined and understood by physicists and engineers today.

For example for your, uh,  magnet-car there, if you call those two magnets A and B, there is a force FAB on magnet A, pulling the car forwards, and a force FBA on magnet B pulling the car backwards. Because FAB = - FBA, the sum of these two is zero, and the net force on the car is zero.

Another way to look at force, is to say that force is the way by which objects transfer momentum, and momentum is one of those things that seems to be conserved.

Objects do not just start moving without a transfer of momentum, although sometimes it kind of looks like they do.  According to legend, Newton himself was inspired by an apple falling from a tree, and that is an event that kind of looks like momentum creation: Previously the apple was still and attached to the tree, but then all of a sudden, the apple is moving and falling towards the ground. 

However if you look at the big picture, containing both the apple and the Earth itself, it is just the two of them pulling each other together, as if they were attached by an giant invisible rubber band, and the total momentum, of the apple and the Earth together, remains constant.

As a mental exercise, you can always imagine these force interactions between objects, whatever the physics problem, as consisting of imaginary compressed springs (for repelling forces), or imaginary rubber bands (for attractive forces).  For example you can use that trick with your magnet-car.  Replace the pair of magnets with a giant stretched rubber band, and then ponder which way this rubber band is going to pull your car.

To experience force, there should be relative motion between the source and the object; like to push a car, we should be stationary, or else if we push a car that is in motion, it would experience less or no force.

In case of motor or generator, the rotor only moves because of stationary magnetic field produced by stationary magnets.

Thank you for your question.

Its like this:

Yes, the magnetic forces get balanced.
And if you use a more powerful magnet at the front, it should work... BUT, you're forgetting, Magnetism is not the only factor here.

You got the internal forces, holding the ROD & the cart together, & the also ROD intact, & frictional forces above & below.

You see, in ideal case, the forces will get balanced, & there would be no motion involved.
But, if you use a super magnet, the result would be Extreme internal tension in the Rods, & stationary condition still,, or even Breaking of the Rods.

That's it. Just two possibilities. And the cause of both being Internal Tensions.

Try lifting yourself off the ground by putting your hands under your feet and pulling (It's like that)


If your magnet was aimed at OTHER VEHICLES then it would work.


For the cars, think of it this way.

The magnet on the left wants to go right
The magnet on the right wants to go left
Since the arm connects both magnets the whole vehicle wants to go left and right at the same time and will therefore not move.

Also, attraction between magnets is caused by the sum of the magnets and not individually so having one strong and one weak is the same as having two "medium".

For the generator answer this:
If north repels north why would the disk move to get each exterior north next to the next interior north?
The only movement you create will be to a point where each exterior north gets as far as possible from the interior norths and then the disk will stop.

The problem through the ages has been that many of these perpetual ideas look good - or at least reasonable - However the bottom line is nothing for nothing so they are doomed to failure.

You generat6or for example the N poles also repel the on coming North pole so it's not going to work.

If forces didn't balance in nature we would see things flying apart and stability would be hard to achieve (it is anyway)


6 years ago

BTW you could easily model this premise with an ordinary  LEGO set ...


The magnet car doesn't work because of Newton's second law. The forces balance. Period.

For your "generator," go through the examples here. You've basically reinvented the wheel.


I used to preview never machines for Bill Lear.

Sorry MB you are not even preview worthy here.