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A long, long time ago, I remember reading some scifi book that involved catapults whose throwing arm had a counterweight at the other end, such that the arm was balanced. The counterweight was not hinged (like on actual trebuches). The arm and its ends were fixed and did not move in relation to each other. IIRC, it was powered by an electric motor.

The thing is, the book claimed the catapult had no recoil during launch thanks to its fixed counterweight. Is this physically correct?

My instincts say no because applying a torque to anything will produce a countertorque. I imagine the catapult will still jump or rock backwards during launch.

However, ever since taking a Stochastic Models class, I do not trust instincts anymore. Is the book right?

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    $\begingroup$ You are asking about your cloudy memory of a sci-fi weapon. Your understanding is correct - applying torque produces a counter-torque. What more can be said? Your question does not seem worthwhile, since your only reason for doubting your understanding is entirely subjective and not because of any apparent violation of the laws of physics (outside of sci-fi). $\endgroup$ – sammy gerbil Jul 17 '16 at 10:05
  • $\begingroup$ @sammygerbil I am asking if a balanced arm still has recoil. Please do not judge the question based on its origins from skepticism of a cheap scifi thing. Personally i think it would be great if everyone questioned the technobabble and other nonsense that composes most pop scifi these days. $\endgroup$ – DrZ214 Jul 17 '16 at 19:13
  • $\begingroup$ I am judging the question on the basis that you do not have a demonstrable reason for doubting your own instinct and what you have learnt in class. It would be another matter if you described the recoil-less mechanism and it looked credible. The only basis is your bad experience in Stochastic Models. This is a poor reason for doubting conservation of momentum; and it is an even poorer reason for posting a question here. $\endgroup$ – sammy gerbil Jul 17 '16 at 19:50
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The thing is, the book claimed the catapult had no recoil during launch thanks to its fixed counterweight. Is this physically correct?

Of course not.

The projectile has been given some momentum, so the launcher must recoil by conservation of momentum. You can't dodge that.

In this case the problem with the proposed mechanism is that after the (initially balanced) arm releases the payload it is no longer balanced and you have this counter weight moving backwards to deal with.

Which doesn't mean that there might not be an engineering reason for such an arrangement if the recoil could be dealt with on a longer time basis (same impulse, less force).

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