Transcription
All right, it seems like every few years this problem makes its way around the internet. Uh, so today I want to talk about what's actually wrong with this problem and why it doesn't work the way some people think it does.
You see, the situation is we've got a truck with a steel plate welded on front of it, and there's some beam on the other end of that beam; there's a magnet. The idea is that if the magnet can pull on the steel plate, then the truck is going to get pulled forward, and this whole thing would be like free propulsion. Who wouldn't want that? The problem is, this doesn't work, and let me explain why.
See, nobody can argue that this magnet right here is in fact going to pull this steel plate forward. But the problem is something called Newton's third law. You see, Newton's third law says for every action force, there's an equal and opposite reaction force. Or look at a simple example here: if you were to take a rubber band and stretch it, no matter how hard you pull on one side of the rubber band, there's always going to be the same force over on the other side of the rubber band, even if the rubber band's speeding up.
The situation with the magnet and the truck really isn't all that different. Yeah, the magnet is going to pull on the truck and the steel plate to the right, but because of Newton's third law, the magnet is going to be pulled back to the left. Now, if you could set this situation up with just the truck and the magnet, and there was no connecting arm between the two, then yeah, if you were to just hold this magnet in place, that would pull the truck forward. But really, it's this connecting beam between the two that makes this whole scenario not quite work.
You see, looking at these three different objects individually, or separating them from one another, yes, the magnet is going to pull the truck forward. But because of Newton's third law, the magnet is going to be pulled backwards. So something's got to hold this magnet in place; that means the beam is going to have to be acting forward on the magnet, and again, that's where Newton's third law pops up. If the beam is pushing forward on the magnet, that means the magnet is pushing backwards on the beam. Well, we don't want this beam moving backward, so that means something's going to have to push forward on the beam. Well, the only thing that's holding the beam in place is the truck itself, and so the truck is going to have to push forward on this beam, and again, Newton's third law is popping up here. If the truck is pushing forward on the beam, that means the beam is pushing back on the truck.
So look at what's happened here: yes, the magnet is pulling forward on the truck, but the beam is pushing backward on the truck, meaning the total force on the truck is zero. So even though we've got a magnet out in front of the truck which is pulling the truck forward, the beam ultimately is going to be pushing backwards on the truck, meaning the truck doesn't go anywhere. You see, everything here is connected as one rigid system, and there are no forces that are acting external to the system, meaning there's nothing from the outside world pushing this truck forward. So all we have are forces that are sort of acting internally within our little truck-beam-magnet system.
Now I know some of you might not actually believe what's going on here with this, so seriously, go sit in your car and push forward on the steering wheel. If you push forward on the steering wheel, that car isn't actually going to move anywhere. So really, how is this magnet pulling forward on the car any different?
All right, so hopefully this cleared up some issues with this problem. If you have any more questions or you're still not totally convinced as to why this won't work, just leave your comments below. And on that note, that's all for now.