Balancing on a Bicycle: The Phenomena

iStock/Izf
iStock/Izf

Reader Brian writes in to ask "Why can you stay on a bicycle when moving, but not when it's standing still?"

Think of something like a table or a couch. It has four legs that touch the floor and form a base of support (a polygon formed by an object's contact points with the ground) and as long as the table or couch's center of gravity (the mean location of the gravitational force acting on an object, or the effective point at which gravity acts) is above this base of support, it'll be statically stable, or stable when at rest.

A bicycle, on the other hand, is statically unstable because it only has two contact points with the ground (whatever portion of the front and back wheels happen to be touching the ground) and its base of support is a line segment. A good base of support needs at least three contact points with the ground, so bikes are hard to keep upright when they're still. Bikes are, however, dynamically stable, or stable when moving forward, because steering allows a rider to move the bike's points of support around under the center of gravity and keep it balanced, often with steering adjustments small enough that the rider may not even realize they're making them. It's sort of like standing on one foot. If you don't hop around a little, and you start falling sideways, you can't recover and you fall over. If you do hop, though, you can move your foot around to keep your center of gravity above it and keep your balance.

A bike has two features that help this dynamic stability immensely: its wheels. Spinning wheels have angular momentum, and when you're sitting on a bike, you and it and its wheels make up a system that obeys the principle of conservation of angular momentum. Unless torque, or twisting force, is applied from outside the system to change the wheels' angular momentum, that momentum and the direction of the momentum remain constant. In a nutshell, once the wheels line up a certain way, they want to stay lined up like that. It's easy for you to move them, but hard for an outside force to do the same, and so the bike is easy to keep balanced but doesn't topple easily. A non-moving bike has wheels that aren't spinning and zero angular momentum, which makes it very easy for external torque to change the wheels' direction, making the bike harder to balance.

Even when staying relatively motionless, though, a rider can balance a bike with some effort. By steering the front wheel to one side or the other and moving forward and backward slightly, a rider can keep the line between the bike's two contact points with the ground under the bike and rider's combined center of gravity. You can see this physics lesson in action whenever a cyclist is stopped at a red light.
(Image at left from Wikipedia user AndrewDressel.)

That was a short and sweet way to answer Brian's direct question, but there's a lot more to bicycle physics. If you're so inclined, Wikipedia's page on bicycle and motorcycle dynamics is a good starting point to learn about some of the other internal and external forces, motions and dynamics that are involved in a simple bike ride.

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Andrea Piacquadio / Pexels.com
Andrea Piacquadio / Pexels.com

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Researchers Discover New Details In Vermeer’s Girl With a Pearl Earring

Johannes Vermeer's Girl With a Pearl Earring, circa 1665.
Johannes Vermeer's Girl With a Pearl Earring, circa 1665.
Art Media/Print Collector/Getty Images

In 2018, the Mauritshuis gallery in The Hague, Netherlands, gathered an international team of researchers to take part in its “Girl in the Spotlight” project, which aimed to unlock the secrets of Johannes Vermeer’s famed Girl With a Pearl Earring, circa 1665.

Their recently published findings reveal many intriguing details about Vermeer’s artistic process and the artwork itself, though the identity of the painting’s enigmatic subject remains a mystery. Using X-rays and other advanced imaging techniques, the researchers discovered Vermeer depicted the girl in front of a faint green curtain—not an empty dark background—and even painted eyelashes on her eyes.

As The Guardian reports, scholars in the past have cited both the lack of eyelashes and the blank background as support for the theory that Vermeer was painting a conceptual, idealized image of a girl, so these newfound features could be evidence that an actual person posed for him in a specific setting. And, according to head researcher Abbie Vandivere, it’s not entirely a bad thing that we still don’t know who that person is.

“It is good that some mysteries remain and everyone can speculate about her. It allows people their own personal interpretation of the girl; everyone feels their own connection with the way she meets your eyes,” she told The Guardian. “The fact that she is still a mystery keeps people coming back and keeps her exciting and fresh.”

While we’re all pondering the puzzling origin of one of the most captivating models in art history, there are plenty of other fascinating revelations from the Mauritshuis investigation to talk about, too. For one, the Dutch artist evidently spared no expense in bringing Girl With a Pearl Earring to life: the raw materials he used to create various colors in the painting came from just about everywhere, including England, Mexico, Central America, and maybe even Asia or the West Indies. Ultramarine, a blue pigment derived from lapis lazuli (an export of what’s now Afghanistan), which Vermeer used for the girl’s headscarf and jacket, was more valuable than gold at the time.

The study also shed light on Vermeer’s painting methods. He began with broad brush strokes of brown and black paint, layering the girl on top of the background, and then made slight adjustments to her ear, the back of her neck, and the top of her scarf.

If “Girl in the Spotlight” has proven anything, it’s that there’s always more to discover about a work of art—and that’s just what the Mauritshuis intends to do.

“Please know that this is not the end point of our research, but an intermediate station,” Mauritshuis director Martine Gosselink said in a press release. “The collaborations are growing, and so is the desire to find out more.”

As you wait for more information to come to light, here are 15 fascinating facts about Girl With a Pearl Earring.

[h/t The Guardian]