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Downhill Race

Downhill Race
Which wheel rolls downhill the fastest?

Believe it or not, when objects roll downhill, it’s not their weight or size that determines their speed—it’s how their weight is distributed.

Downhill Race exhibit
A family plays with the Downhill Race exhibit.
 
What’s going on?

Many people expect that a heavier wheel will naturally roll downhill faster than a lighter one. But when an object rolls downhill, its speed depends not on the weight of the wheel, but on where the weight is located.

When weight is located far from the center of the wheel, the wheel is harder to get rolling. (A physicist would say it has more rotational inertia.) So even though they may weigh the same, a metal disk will accelerate down a hill faster than a metal hoop, because the hoop’s weight is farther from the center of rotation.

Likewise, sliding the adjustable brass weights on the wheels of this exhibit inward decreases their rotational inertia, allowing them to roll downhill faster.

Going further

Serious skateboarders choose the size of their wheels with rotational inertia in mind. Smaller wheels are easier to get spinning, and also make for a more maneuverable skateboard, which is why they’re the preferred choice of most street skaters.