That's a nice explanation, but can we take it further. I'm in a particularly empirically zeal-ous mood, so in the spirit of this blog, let's do a **calculation** and work out if this explanation fits the data.

**The first thing we need to know is what momentum is**. Momentum is a measure of how much stuff an object has, and how fast that stuff is moving. Something that's fast and heavy has a lot of momentum. Something that's light and slow has very little momentum. Mathematically, momentum is just the product of an object's mass and its velocity.

**momentum = mass x velocity**

Or, as physicists like to write it:

Here, *p* is just shorthand for momentum.. don't ask me why. *m* is mass, and *v* is velocity.

**The second thing we need to know is that whenever an object changes its momentum, it experiences a force.** If you think about it, this is pretty intuitive. For example, if you throw a tennis ball at a wall, its *momentum changed* from positive (forward) to negative (backward) when it bounces off the wall, because the wall slammed in to it (that's the force). If you were to slap the table in front of you, the momentum of your hand would go from something to nothing. The pain you'd feel is the direct result of the force that brought about this change in momentum.

Now, in this bead-chain, just as Steve described, the inner part of chain is traveling upwards, and then suddenly, at the top, its being pulled downwards. As each beads turn the bend, its change in momentum causes a little upwards *kick* of a force. There are many beads in the chain, and so there's a constant stream of little upward kicks, as the beads go around the bend. And it turns out that these kicks provide just enough of an upwards force to balance out the weight of the suspended part of the chain. That's why the chain seems to hover in mid-air - it's because the changing momentum of the chain provides a force that keeps it up.

**This isn't magic.** It's the same physics that's behind this crazy water-powered jetpack. In the jetpack, a constant stream of water suddenly changes direction at a bend in the pipe, providing a steady stream of upwards kicks. This is what keeps Derek hovering in the air in that video, and it's the same physics that keeps this chain afloat.

So how can we make this quantitative? Well, Newton figured out exactly how force is related to a change in momentum. What he taught us is that the force on an object equals the change in momentum divided by the time over which the momentum changes. Or,

**force = change in momentum/duration**

So if we know the change in momentum of the chain, we can work out the force.

Now, picture a tiny piece of chain that's moving up. Its length is delta x, and it's moving up at a speed v. If we call m the mass per unit length of chain, then the momentum of this upwards moving chunk of the chain is: