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Magnet rule
(lemmy.world)
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this post was submitted on 11 Dec 2023
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I am a complete idiot and I would also like to know.
Newton's third law. Every action has an equal and opposite reaction.
Basically what that means is if the magnet is exerting some amount of force on the truck to pull it towards it, the truck is exerting the exact same amount of force on the magnet in the opposite direction to pull the magnet closer to the truck. This is why when you let go of two magnets they fly towards each other instead of one staying still (unless you hold it still) and the other flying towards it.
If the arm in the picture could bend, the magnet would just stick to the front bumper. If it couldn't move, the magnet would pull on the plate, but that would be cancelled out by the plate pulling on the magnet. They're trying to attract each other, but neither of them can move, so it just stays still.
Now, if the magnet was attached to a different truck, and that started moving, it would pull the truck along just fine. If the second truck was in neutral, it'd roll backwards and the two trucks would meet in the middle. But if you wanted them both to go, you'd have to turn on the engine in the second truck, and you've effectively just invented the world's least reliable tow hook. You can't cause a car to accelerate without some outside force (the second truck's engine) pulling it along.
Newton's third law also applies to gravity. When the Earth's gravitational pull makes something fall to the ground, that something actually exerts the same amount of force pulling the Earth towards it. The earth is several orders of magnitude heavier, though, so it doesn't move very much. Gravity from the moon pulling on water in the ocean does create the tides, though.
What if the magnet was of similar mass to the truck? Assuming it could be made to balance, maybe this actually might work to move the truck such that it meets the magnet in the middle. Then the arm could slide the magnet upwards to detach, reposition it, and repeat. Of course, there are much better ways to make a truck move, but maybe it could work?
This would work, but this is the point where you'd need to involve an external energy source. If you move the truck-magnet-arm system such that the whole system reaches a steady state and the truck has "moved" (relative to an outside observer), to continue any motion, you need to disturb the steady state to reposition the arm and create a new resting point. Doing so would require energy. From a battery, or an engine, or whatever, but you need energy from an external source. And so we're right back to where we started: instead of all this jazz, why not just spend energy to spin the wheels instead?
Yup 👍
That could work, yes. You would have to unstick the magnet from the bumper every time -- or alternately just leave it attached and use it as a tow hook -- but it would work
Because the mass is shared. You could get it to move the truck by moving a giant magnet forward but so long as it’s attached to the truck you’re only ever getting out the movement you put in to the magnet.
Agreed. It could work but it's not a perpetual motion machine and would require energy input to make it work.
The magnet pulls the truck to the right. The truck pulls the magnet to the left. They both have the same amount of pull. No one wins this tug-o-war.
The best you can hope for is for the truck to move slightly to the right and the magnet to move far to the left until they meet
Because the magnet is attached to the truck bed, so the bumper is being pulled forward and the bed pushed back, and the forces cancel out to 0.
Dumb follow up question. If there’s a way to reverse the direction the bed is being pushed, with gears or something, will that work? lol
You need a catapult in the back to fling the magnet ahead of you, you get pulled forward, pick up the magnet and fling it again. This technology has existed since the '20s but Big Internal Combustion Engine is repressing the tech.
Sure, but you'll need something to turn the gears.
Typically, we use a crank that converts a reciprocating motion into a rotation. Then we move a piston up and down by putting a little explosion at one end. Connect it to a tank of explosive fuel and badaboom, perpetual motion, as long as you have explosive fuel in the tank of course.
No, because in the end, something needs to keep moving the magnet forward to keep the truck moving forward.
This thought processes is exactly how scammers on YouTube make perpetual motion seem possible, but by definition, the forces always have to cancel out.
Newtons laws of motion, most clearly the third.
"If two bodies exert forces on each other, these forces have the same magnitude but opposite directions."
In simple terms: metal is just as much attracted to magnet as magnet is to metal, this is fundamental and doesn't change under any circumstances.
So the force driving the car forward is exactly equal to the force driving the car backward, and you can't have one without the other.
The truck will be attracted forward towards the magnet, but the magnet will also be attracted backward towards the truck with equal force. The backward force will be transfered through the arm holding the magnet and the net force on the whole system will be 0. If there was no arm holding the truck and the magnet apart, the truck could move forward slightly and the magnet could move backward to meet it. (The magnet would move much more than the truck since they would still have equal force applying to them, and the truck's mass is much higher than the magnet's.)
And then to continue moving this way you'd have to unstick and move away the magnet, which would require just the same energy it takes to move the truck this far away
Imagine that instead of a magnet there's a spring there. Why is it more obvious now that it doesn't work? How is a magnet different than a spring?
Funny thing is that this isn’t technically true. If the fan is strong enough, the current will hit the sail and reflect backward and to the side, creating some very inefficient thrust. Any fan strong enough to achieve this would do better to remove the sail entirely and just point the fan backwards, but it would technically work.
And by the same principle, you could shine a powerful laser towards a very reflective sail and actually move, with the laser as a part of the boat.
I too always found this very funny. Confuses the hell out of people who just understood why the OP would not work.
Full disclosure, I’m an idiot, so I could be misunderstanding something, but im pretty sure that this is based on that principle.
I believe you've invented an air boat.
See the following reference material: https://www.imdb.com/title/tt0074564/
If the fan blew across an airfoil aligned perpendicular to the axis of the boat, it can definitely generate thrust. The top of the airfoil will develop a lower pressure than the bottom, and the difference in pressure causes force that moves the boat. It's just way more efficient to use a fan to push the boat.