All great inventions.

But for me, the most important of all

was the wheel, mainly because it led to things like this.

Downhill cycling.

Why use two wheels when one makes you look twice as cool?

But before you even think about pulling off a downhill wheelie,

be aware that even on two wheels,

it could still be perilous.

What makes downhill cycling is such a terrifying pursuit

is that the bike isn't only powered by the cyclist's legs,

but also by our old friend gravity.

When a cyclist goes up a hill, he gradually gains

gravitational potential energy.

When he descends, this gravitational potential energy

can be converted over a very short period of time

into a terrifying amount of kinetic energy.

If the rider falls, that kinetic energy might be converted

into heat via friction, or if they stop suddenly,

dissipated via deformation.

So energy is never destroyed.

It's merely transferred into something else

like kinetic energy, heat, or even the sound

of an impact, which is often accompanied

by the sound of screaming.

To investigate, we sent out some of our more

reckless researchers.

This guy has made a deal with gravity

and built up a lot of kinetic energy.

So much so that he's able to pass cars.

Except that one.

The cyclist is unable to transfer

enough kinetic energy into heat the friction at its brakes.

When he hits the car, his remaining kinetic energy

is transferred into sound, heat, and dissipated a deformation,

also known as bouncing off.

Don't worry.

He was OK.

All right.

Let's try a gentler pace.

This is more like it.

A chance to take in the scenery.

And his friend's bottom.

He appears to make no attempt to reduce his kinetic energy.

And his linear momentum becomes angular momentum

as he rotates around the axle of the front wheel.

And maybe not the last.

Perhaps it's safer to go downhill on three

wheels with these drift trikes.

Or maybe not.

It's just as well our drift trike gang are

heading home on four wheels.

Maybe safer just to walk.