Hacker News Reader

Comment by ars | Hacker News Reader

If you plan to make one, make the radius smaller, but the velocity higher.

The energy stored in this goes by the radius squared. But the centripetal force on the object at the end goes by the radius.

So a smaller faster one is less dangerous, and also requires a less powerful motor (or the motor you have can give you a higher force).

Although the bearings are probably a bit harder to handle when it's faster, the whole thing will weigh less.

On the con side, a smaller one will have a larger difference in force between the top and bottom of the lava lamp, making your experiment less accurate.

View on HN

Energy may go by radius squared, but the rpms change to match.

For visualization: the ice-skater who spins slowly & pulls arms in. By spinning faster, no energy is lost, as can be seen by extending the arms. However, a slower-rotating device can more easily be predicted and avoided / hand-stopped if needed.

The energy is the same. A slower moving larger centrifuge can tear your arm off with the same ease a smaller, faster one.

Totally agree. But a smaller, faster one is less predictable, so you can't bleed of small amounts of energy as easily / safely.

Which would you rather try to stop, with your bare hands, assuming identical energy? A 1-inch pipe spinning at 10,000 RPM, or a large tire spinning at 10?

If the energy is identical, why would there be any difference?

Remember that with the larger slower RPM one, the radius is higher so the linear velocity is high too, even if the RPMs are slow.

One is more predictable than the other. In that video, I can point out when the arm will come around again with relative ease, and can very easily be accurate to below a tenth of a second.

Now picture it going several times faster in terms of RPM, and tell me when it'd be safe to stick your arm in its path.

Lets scale it up. Picture the earth. Near the equator, the linear velocity is almost 1000mph. Scale it down to the size of the centrifuge in the video, while keeping linear velocity the same. I can easily point out when half of the earth is facing the sun. Could you point out when the centrifuge is pointing north?

If you are planning to play chicken with a centrifuge, sure.

But I am more concerned with what happens if you do actually get hit. Or if it comes apart, or falls down.

And if you do get hit, or it breaks, a smaller faster one is MUCH (exponentially much) safer.

The force goes up by the RPM squared, but it goes down only linearly by radius. You don't need to speed it up very much to counteract the smaller size.

On the other hand the energy goes up by radius squared. Meaning the smaller size helps a LOT in terms of destructive power.

I've been too long out of physics, so I'll take your word on it. I wasn't sure how quickly size effected energy. Though I'll still take my chances with something I can predict if I have to interact with it.

One question though: is energy->radius^2 based on angular or linear velocity?

Angular.

So yes, when you increase the radius the linear velocity goes up even though the angular one doesn't change.

Faster RPM also increases velocity of would-be shrapnel. Either you prepare for the risk of a slowly-rotating, large rotor/axis assembly failing catastrophically, or a faster-rotating, smaller assembly shedding poorly-fastened bits from the ends of its rotor.

Actually no.

The velocity of the shrapnel tracks the energy stored in the device. Meaning a larger radius has faster shrapnel.

Note: velocity as used in my earlier message is circular velocity, (AKA RPM), not linear velocity.

The velocity of the shrapnel is the same as the linear velocity of the part it detached from. If it detaches from a faster moving surface, it will fly faster.

Sorry for fudging may response with a term for angular velocity, instead of linear velocity. I was writing from the assumption that the primary point of a smaller centrifuge would be to safely get higher centrifugal forces at the ends of the rotor, which would require that the ends of the shorter rotor move at greater linear velocity.

Reply on news.ycombinator.com