# Wait, Is That Backpack … Floating?

This looks crazy. I mean, it’s a cool invention—the idea of the HoverGlide pack is to reduce the awkward bouncing and jolting you get with ordinary backpacks. But watching it makes my brain freak out a little. It kind of messes with your sense of how things should move.

The trick, I think, is that the bag is suspended on some kind of elastic cord. When your body moves up, it increases the force pulling up on the pack. But since the force increases only gradually with the elastic suspension (compared to the quick jerk you’d get from a stiff harness), it takes time for the upward velocity of the load to increase. By the time it starts moving up, your body is already coming back down.

That's the short explanation. But there’s more going on here than meets the eye. Let’s unpack the physics of this thing!

Why Do Backpacks Bounce?

First of all, why is this even needed? If you’ve ever tried to hustle with a heavily laden pack (late for class again, hmm?), you know how it jostles and jolts, all out of rhythm with your motion. But why is that? Shouldn’t it just move up and down with you?

After all, once you leave the ground, there is a gravitational force on both you and the pack. Since the gravitational force on an object depends on its mass, and the acceleration depends on the same mass, objects with different masses should have the same acceleration. This is why a bowling ball and a baseball dropped from a height will hit the ground at the same time. It seems like the backpack and person should also have the same acceleration and "fall" together.

But here's the deal: The connection of the straps to your shoulders acts sort of like a spring. There’s some give in it. Things in real life aren't perfectly rigid. Everything bends and squishes a little bit when there is an applied force. In fact, we can model this squishiness as if it were an actual spring—which is nice, because springs are easy to model!

The key to springs is that the force they exert is proportional to how much they’re compressed or stretched. This is called Hooke's law, and the proportionality constant relating force and compression is called the spring constant, k. You can think of it as the stiffness of the spring.