Transfer between translative KE and rotational KE in a rigid body

I have been inspired by some sci-fi cannons that seem to operate by initially spinning up a projectile inside the cannon, and then suddenly firing the projectile out at high speed. Now, I am wondering whether it is possible for such a cannon to perform practically.

So, from an energy point of view, it appears that this hypothetical cannon is able to do work on this projectile by spinning it up using a torque, and this work increases the rotational KE.

Then, there is some unknown process that cause the transfer of rotational KE into translative KE, causing the projectile to fly off with a particular velocity.

It would be appear that the spinning-up function of the cannon would be an interesting way of having the KE of the cannonball stored up and ready for firing. However, even though translative KE and rotational KE are both KE, the same type of energy, it looks like extra work needs to be done to transfer between both types of KE. To do this, by applying the principle of superposition, it looks like you would need to combine the effects of undoing the spin with a reversed torque, and then applying a force, in the direction of the cannon firing, in a short space of time. Does this apparent backtracking of doing work not seem inefficient, making the spinning-up of the projectile a waste of resources? Are there any real, practical mechanisms that work especially well in converting rotational KE into translative KE?

In short, can there be any practicality in a cannon that initially spins up it's projectiles?

• Is it not like a cyclotron?en.wikipedia.org/wiki/Cyclotron
– Paul
Jan 5, 2015 at 2:13
• i think there won't be any reverse torque. The "hammer throw" is a similar kind of example. Jan 5, 2015 at 6:20
• Is that really an adequate example? Isn't the hammer throw merely conserving the linear velocity the 'hammer' had at the time of release, and not actually converting any of the rotational energy into kinetic energy? Jan 5, 2015 at 7:08

In your cannon firing mechanism, it would be easy to construct a something to accelerate the cannon ball, but there is still the fact that the cannon ball's centre of mass is at rest with respect to the cannon. If you use gunpowder or any other conventional firing mechanism, It would be almost irrelevant whether the cannon ball was spinning or not, because it would be accelerated to a speed depending on the force applied. Infact, this is true for electromagnetic or any other forces (Which you might see in a sci-fi.)I say almost because a spinning cannon ball would deliver more energy on coming to complete rest. There are a few simple ways to convert Kinetic energy of rotation into that of translation. They are:

1. Friction: This is undoubtedly the easiest way. We just let the bottom of our spinning projectile touch the bottom of the cannon barrel and let it roll out. It is obviously completely impractical, and I just listed it for the sake of being complete.

2. Springs or strings: This is just an idea. Rotational energy, by using strings could conceivably be used to compress a spring. This would then proceed to launch the very same projectile (or a different one). You could also an elastic string system for the same purpose.

3.Grooves: We could build compatible grooves on the projectile and the barrel of the cannon. The cannon ball will need to be remade into a missile like shape for best efficiency. After the projectile is loaded into the barrel, in the very same way that a screw is driven forward on rotating it, beacause of grooves, the projectile will be launched forward. This is far more practical than the other two options.

Remember that even in a futuristic cannon, one would need to use Either contract forces or electro-static, magnetic force. (Unless you want to shoot spinning subatomic particles, in which case you could use the Weak Nuclear Force). If we use magnetic force:

You could hypothetically have a massive sphere of molten metal rotating so fast that it generates a magnetic field (Like the Earth's core does). If we then wrap wires around the barrel of the cannon, and they carry sufficient current, the molten mass would be accelerated. That would be a pretty deadly weapon.

The only practical application I can think of, is that rotating pointed projectile like missiles are more stabilised aerodynamically than non-rotating ones.