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Homework Help: Could electromagnetism work in a catapult?

  1. Aug 26, 2015 #1
    Hi! im new to the forum and need a little of help in a science experiment
    I wanted to ask you which of the following catapult systems will be more accurate and efficient:

    1. Weight as the source of energy. The weight at the bottom of the beam moves back, the beam will go forward and the projectile at the top will be launched. a "trebuchet"

    2. The clasic catapult

    3. And I have the idea that perhaps a catapult with magnets might work, but I'm not sure if we would need very strong magnets

    Please any help is appreciated
  2. jcsd
  3. Aug 26, 2015 #2
  4. Aug 26, 2015 #3


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    I worked on the EMALs system for the US Navy some 16 years ago. The wikipedia article seems to suggest that it is all super simple and just the obvious thing to do. That certainly was not true 16 years ago, and I rather doubt that it is today.

    One of the big issues on shipboard is weight and space. To make a fair comparison, the entire systems should be compared, including space and weight of the energy source, the control system, and the actuator itself. The electronic controls for such a system are huge. The currents required are extremely large, and generating the variable frequency AC required takes some really large power electronics. These large currents lead into another problem area, heat generation. Recall that power goes as I^2*R, and when the currents are extremely large throughout the entire system, there will be a terrific amount of heat generated.

    Storing up the required energy burst is also a major problem. It can be done, but it is not simple and not cheap. It remains to be seen whether or not it can be done with sufficient reliability to go on shipboard.

    When I worked for the Navy, we used to joke about how the all electric warship would require a second ship, a tender, to carry all the power electronics and a very long extension cord between the two.
  5. Aug 26, 2015 #4


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    The OP asks about using magnets, not electricity.
    A magnet is not a source of energy. Yes, it will attract certain objects, accelerating them, but as soon as you try to use that to send the object as a projectile it acts the wrong way.
    There are two ways you could overcome that. If using an electromagnet you could arrange for it to switch off at the right instant. So this is more like the electromagnetic propulsion previous responders have discussed. Or you could have a projectile that is not itself attracted to the magnet held in a harness which is. I doubt that second approach would be very efficient. The force on the harness would depend on its size, but a large harness would mean a lot of the KE is wasted.
  6. Aug 26, 2015 #5


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    This is essentially what is done with the rail gun where the circuit is completed by a shoe that does not travel down range with the projectile.
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