1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why wouldn't this work?

  1. Aug 22, 2010 #1
    My friend says this would work and although i am not a huge science dude i have no doubt that this would not work, can some1 explain the technical stuff i don't know?

    10576ds.jpg
     
  2. jcsd
  3. Aug 22, 2010 #2

    Danger

    User Avatar
    Gold Member

    Welcome to PF, TheK.
    Very simply, the device is pulling against itself. The only possible movement from this arrangement is that the magnet could pull the boom in toward the front of the truck. (And that, by the way, would take a very powerful magnet.)
     
  4. Aug 22, 2010 #3
    If you make the same drawing, but replace the magnet and the metal plate with a rubber band stretched between the two points, you have physically the same situation.

    You could argue using newtons third law, which states that every action has an opposite reaction of the same magnitude.
    The metal plate will be pulled forwards, but the magnet will be pulled backwards with the same magnitude, and since both objects are attached to the truck, the net force on the truck is zero. However if the force is large enough, the arm holding the magnet would likely deform or break.
     
  5. Aug 22, 2010 #4

    Pengwuino

    User Avatar
    Gold Member

    Imagine if instead of a truck, it were you... made out of metal. If you got that magnet and put your arm out and pointed it towards yourself, your body would feel attracted to the magnet, but the magnet would also feel attracted towards your body. Thus, both would move towards each other and eventually attach.

    I know there's a VERY clear analogy to this that would make it very obvious why this won't work but I can't think of it at the moment.
     
  6. Aug 23, 2010 #5
    You know the story of Baron Munchausen, lift himself up by his ponytail from the quicksand. Does not work :)
     
  7. Aug 23, 2010 #6
    However if you had an electromagnet and turned it on attracting the iron and then turned off the magnet when the iron was flying at the magnet, after the iron hits the magnet and bounces back, it will move forward a little. Because the change in momentum is greater when it hits an bounces back, then when the iron originally pushed off the truck .
     
  8. Aug 23, 2010 #7
    That would not work either. At any point in time when the metal piece has a certain (horizontal) momentum, the truck will have exactly the same, but opposite, horizontal momentum.
     
  9. Aug 23, 2010 #8

    Office_Shredder

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You still have the problem where the electromagnet was pulled towards the iron as well. So maybe you ended up moving forward a little bit, but it's at the expense of your electromagnet being directly next to your iron plate. Either you need to reposition it or you've stopped moving.

    I would also be surprised if the huge power expenditure required to drag a truck like that is anywhere close to efficient (keep in mind that electromagnets are powered by electricity)
     
  10. Aug 23, 2010 #9
    Yes it would , it would be like standing in a railroad car and throwing baseballs at the front of the car. When you throw the baseball you might move back a little but when it hits the front of the car and bounces off the impulse you deliver is greater and it will move forward a little bit. It's not very efficient i'm just saying it will happen .
     
  11. Aug 23, 2010 #10
    Are you trolling us?
    That is elementary. It would only work to the extent that you could make use of the friction against the ground etc. A clearer example would be that you could stand on a rug and move across a floor by jumping and kicking it sideways repeatedly. Without friction and similar influences you would get nowhere (apart from the option of throwing the rug in the opposite direction and not catching it)

    Personally I reckon the clearest illustration would be along the lines of the Munchausen example. Replace the magnet with a rope and turn the whole kaboodle on its side so that it is like lifting yourself by your proverbial bootstraps. That has the advantage that K could try it for himself and see why it doesn't work. Having convinced himself, he could invite his friend to demonstrate the contrary either horizontally or vertically. :tongue2:
     
  12. Aug 23, 2010 #11
    If I were sitting on a donkey, and if I held a carrot on a stick in front of the donkey, the donkey moves forward. Isn't this the same?
     
  13. Aug 23, 2010 #12

    Danger

    User Avatar
    Gold Member

    No. That would be the same as driving the truck with its wheels.
     
  14. Aug 23, 2010 #13
    Ask your friend to lift himself up. He can't. (Although he can lift someone heavier than himself)

    [But if he jumps up, and asks -"how is it possible then" then don't ask me! The question of why people are able to jump has always troubled me! :rolleyes:)
     
  15. Aug 23, 2010 #14
    In the magnet situation, the magnet is pulling the truck, but also the truck is pulling the magnet back equally, and as a result their center of masses stays immobile.
    In the donkey situation, the carrot is pulling the donkey but the donkey is not pulling the carrot back, so you end up with a net impulse gain ;)
     
  16. Aug 23, 2010 #15

    Danger

    User Avatar
    Gold Member

    Not quite. The donkey is walking toward the carrot because it's too stupid to realize that it can't be reached.
     
  17. Aug 23, 2010 #16

    DaveC426913

    User Avatar
    Gold Member

    TheK1ll577, consider this:

    If I hold a magnet near a large metal object, and let it go, it will jump to the metal object, right? I'm just establishing that, when it comes to the attraction between a magnet and a metal object, it works both ways, right? This isn't Wile E. Coyote here, a magnet will not remain stationary as it pulls a rocketship toward it. Magnets move. Agreed?

    Now, look at the diagram. You have a magnet attracted to a large metal plate. Why would we assume that it would be the truck that would try to move toward the magnet? It is just as valid to assume that the magnet should move toward the truck.

    In fact, if your friend were right, one might expect the truck to actually move backwards, pushed by the magnet, which is pulling towards the plate.

    This is every bit as valid as the original scenario (where the truck is pulled forward). Take a moment to convince yourself of this.

    Now you have two situations which are identical yet exactly opposite each other. They can't both be true!
    And if they're not both true, yet one can't be true without the other also being true (since they're identical) - the only option left is that they're both false.


    (Or, simply put, the magnet pulls on the truck, tugging it forward; the truck pulls on the magnet, tugging it backward. Assuming the arm is rigid, they cancel out. The truck/magnet does not move.)
     
    Last edited: Aug 23, 2010
  18. Aug 23, 2010 #17
    you are good!
     
  19. Aug 23, 2010 #18

    You totally won me 50 bucks dude, thank you everyone.
     
  20. Aug 23, 2010 #19

    DaveC426913

    User Avatar
    Gold Member

    Cool. Have a ball spending what's left of it after you get my consulting bill.
     
  21. Aug 23, 2010 #20

    Danger

    User Avatar
    Gold Member

    Your bill? Who was the first respondent to this question? :grumpy:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why wouldn't this work?
  1. Why is this not work? (Replies: 11)

Loading...