It only says "double the velocity" to make an apples to apples comparison with "double the weight". Its point is that increasing velocity has a larger effect than increasing weight, not whether or not that's always possible.
pj
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But that's why I'm saying it's been misleading. So many people believe that changing the velocity is more important than changing the weight. But that's not true in the pool stroke model. The energy that the player brings to the stroke is everything. That's the input. Velocity is the result.
The kinetic energy of the cue (ignoring losses so that we can understand the model) is a black box equivalent energy transfer mechanism. The velocity increase is based on the energy equation, so any given mass change will change the velocity according such that the energy is still the same. It's not true that changing the velocity leads to an exponential increase in energy. The energy in is going to be the energy out (in a perfect no-loss situation).
So how much more velocity can a lighter stick be swung? Only enough that the energy available can swing it based on the energy equation. It wouldn't be correct to increase the input energy just because you lightened the cue. A person only has so much energy to give. There's a max. So you back calculate the velocity based on the energy, not the other way around. Velocity is an output, not an input.