Why is it so hard to balance on a bicycle that's not moving, and easy on one that is?

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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These Sparrows Have Been Singing the Same Songs for 1500 Years
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Swamp sparrows are creatures of habit—so much so that they’ve been chirping out the same few tunes for more than 1500 years, Science magazine reports.

These findings, published in the journal Nature Communications, resulted from an analysis of the songs of 615 adult male swamp sparrows found in six different areas of the northeastern U.S. Researchers learned that young swamp sparrows pick up these songs from the adults around them and are able to mimic the notes with astounding accuracy.

Here’s what one of their songs sounds like:

“We were able to show that swamp sparrows very rarely make mistakes when they learn their songs, and they don't just learn songs at random; they pick up commoner songs rather than rarer songs,” Robert Lachlan, a biologist at London’s Queen Mary University and the study’s lead author, tells National Geographic.

Put differently, the birds don’t mimic every song their elders crank out. Instead, they memorize the ones they hear most often, and scientists say this form of “conformist bias” was previously thought to be a uniquely human behavior.

Using acoustic analysis software, researchers broke down each individual note of the sparrows’ songs—160 different syllables in total—and discovered that only 2 percent of sparrows deviated from the norm. They then used a statistical method to determine how the songs would have evolved over time. With recordings from 2009 and the 1970s, they were able to estimate that the oldest swamp sparrow songs date back 1537 years on average.

The swamp sparrow’s dedication to accuracy sets the species apart from other songbirds, according to researchers. “Among songbirds, it is clear that some species of birds learn precisely, such as swamp sparrows, while others rarely learn all parts of a demonstrator’s song precisely,” they write.

According to the Audubon Guide to North American Birds, swamp sparrows are similar to other sparrows, like the Lincoln’s sparrow, song sparrow, and chipping sparrow. They’re frequently found in marshes throughout the Northeast and Midwest, as well as much of Canada. They’re known for their piercing call notes and may respond to birders who make loud squeaking sounds in their habitat.

[h/t Science magazine]

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10 Scientific Benefits of Being a Dog Owner
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The bickering between cat people and dog people is ongoing and vicious, but in the end, we're all better off for loving a pet. But if anyone tries to poo-poo your pooch, know that there are some scientific reasons that they're man's best friend.

1. YOU GET SICK LESS OFTEN.

Dog snuggling on a bed with its person.
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If cleaning commercials are to be believed, humanity is in the midst of a war against germs—and we shouldn't stop until every single one is dead. In reality, the amount of disinfecting we do is making us sicker; since our bodies are exposed to a less diverse mix of germs, our entire microbiome is messed up. Fortunately, dogs are covered in germs! Having a dog in the house means more diverse bacteria enters the home and gets inside the occupants (one study found "dog-related biodiversity" is especially high on pillowcases). In turn, people with dogs seem to get ill less frequently and less severely than people—especially children—with cats or no pets.

2. YOU'RE MORE RESISTANT TO ALLERGIES.

Child and mother playing with a dog on a bed.
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While dog dander can be a trigger for people with allergies, growing up in a house with a dog makes children less likely to develop allergies over the course of their lives. And the benefits can start during gestation; a 2017 study published in the journal Microbiome found that a bacterial exchange happened between women who lived with pets (largely dogs) during pregnancy and their children, regardless of type of birth or whether the child was breastfed, and even if the pet was not in the home after the birth of the child. Those children tested had two bacteria, Ruminococcus and Oscillospira, that reduce the risk of common allergies, asthma, and obesity, and they were less likely to develop eczema.

3. YOU'LL HAVE BETTER HEART HEALTH.

Woman doing yoga with her dog.
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Everything about owning a dog seems to lend itself to better heart health. Just the act of petting a dog lowers heart rate and blood pressure. A 2017 Chinese study found a link between dog ownership and reduced risk of coronary artery disease, while other studies show pet owners have slightly lower cholesterol and are more likely to survive a heart attack.

4. YOU GET MORE EXERCISE.

Person running in field with a dog.
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While other pets have positive effects on your health as well, dogs have the added benefit of needing to be walked and played with numerous times a day. This means that many dog owners are getting 30 minutes of exercise a day, lowering their risk of cardiovascular disease.

5. YOU'LL BE HAPPIER.

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Dog owners are less likely to suffer from depression than non-pet owners. Even for those people who are clinically depressed, having a pet to take care of can help them out of a depressive episode. Since taking care of a dog requires a routine and forces you to stay at least a little active, dog owners are more likely to interact with others and have an increased sense of well-being while tending to their pet. The interaction with and love received from a dog can also help people stay positive. Even the mere act of looking at your pet increases the amount of oxytocin, the "feel good" chemical, in the brain.

6. YOU HAVE A MORE ACTIVE SOCIAL LIFE.

Large bulldog licking a laughing man.
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Not only does dog ownership indirectly tell others that you're trustworthy, your trusty companion can help facilitate friendships and social networks. A 2015 study published in PLOS One found that dogs can be both the catalyst for sparking new relationships and also the means for keeping social networks thriving. One study even showed that those with dogs also had closer and more supportive relationships with the people in their lives.

7. YOUR DOG MIGHT BE A CANCER DETECTOR.

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Your dog could save your life one day: It seems that our canine friends have the ability to smell cancer in the human body. Stories abound of owners whose dogs kept sniffing or licking a mole or lump on their body so they got it checked out, discovering it was cancerous. The anecdotal evidence has been backed up by scientific studies, and some dogs are now trained to detect cancer.

8. YOU'LL BE LESS STRESSED AT WORK.

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The benefits of bringing a dog to work are so increasingly obvious that more companies are catching on. Studies show that people who interact with a pet while working have lower stress levels throughout the day, while people who do not bring a pet see their stress levels increase over time. Dogs in the office also lead to people taking more breaks, to play with or walk the dog, which makes them more energized when they return to work. This, in turn, has been shown to lead to much greater productivity and job satisfaction.

9. YOU CAN FIND OUT MORE ABOUT YOUR PERSONALITY.

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The kind of dog you have says a lot about your personality. A study in England found a very clear correlation between people's personalities and what type of dogs they owned; for example, people who owned toy dogs tended to be more intelligent, while owners of utility dogs like Dalmatians and bulldogs were the most conscientious. Other studies have found that dog owners in general are more outgoing and friendly than cat owners.

10. YOUR KIDS WILL BE MORE EMPATHETIC.

A young boy having fun with his dog.
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Though one 2003 study found that there is no link between pet ownership and empathy in a group of children, a 2017 study of 1000 7-12 year olds found that pet attachment of any kind encouraged compassion and positive attitudes toward animals, which promoted better well-being for both the child and the pet. Children with dogs scored the highest for pet attachment, and the study notes that "dogs may help children to regulate their emotions because they can trigger and respond to a child's attachment related behavior." And, of course, only one pet will happily play fetch with a toddler.

A version of this story originally ran in 2015.

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