Nothing above absolute zero is fully rigid. You're going to transfer the energy quickly, you'll just break any "arm" that's light enough to accelerate with the

Nothing above absolute zero is fully rigid. You're going to transfer the energy quickly, you'll just break any "arm" that's light enough to accelerate with the energy available. You could maybe use an 'already broken' arm (rope, like a whip) but then it's not a trebuchet.