roblewis56 - 2015-04-20
As part of Cal Day Professor Joel Fajans of the Physics Department describes the physics involved in riding a bike. How, in order to turn left you must turn right and that angular momentum only plays a minor part. He tells us what trail is and how it's the most important thing in determining how a bike behaves. He also discusses safety and why you should wear a helmet. The 2014 Academic Ranking of World Universities ranked Berkeley Physics the #1 physics depertment worldwide, beating Princeton, MIT, Harvard and Cal Tech. Ride your bike to Berkeley and study with the best.
What a charming teacher!
Centrifugal force and centripital force are two different things. One applies outward the other inward.
"Roll on You Bears!"
Thank you so much Professor Fajans! I've referred to your lecture many, many times in the past. In fact, I even got on NPR's Science Friday to challenge one guest's assertion that the gyroscopic effect is responsible for a bike's steering. All thanks to your lecture about 10 years ago!
Go Bears!
PS I took classes in this very lecture hall. And I lucked into having a lecture end one day when a group of people were gathering outside the doors and obviously waiting excitedly to get in for the next lecture. I asked what all the hubbub was about and someone said, "Professor Lee from China is here to present his breakthrough work on cold fusion!" I stayed. Turns out his work wasn't as revolutionary as expected and cold fusion is still "about ten years away!"
Great explanation.
I disagree somewhat. Although a centrifugal force (although fictional) certainly exists and affects balance when riding a bike, it is only part of all that's happening in keeping a bike upright. For instance, a rider can keep upright while on a track stand, maintaining balance by making minute body adjustments, much like standing on one's feet or an inverted pendulum, to keep his center of gravity aligned while waiting for the opponent to make the first move. Track cyclists can do this for a reasonable length of time with little to no displacement. Although a fixed-gear bike helps, the same feat can be attained with a regular bike, although some bike displacement might be needed. Another instance is while riding on rollers. There is no translational displacement when riding rollers, so centrifugal force cannot affect balance. It must, therefore, be attained again by the rider's skill in keeping his/her center of gravity aligned and perhaps benefiting from angular momentum.
I'll challenge your assertion.
A track stand doesn't utilize a bicycle's geometric stability characteristics to stay up. All a track stand is is moving the bike back and forth laterally to prevent a lateral fall (or to the side). I can track stand all day long. And ride backwards reasonably well - even on a tandem. (I also ride unicycle, but that's another matter completely.)
A track stand requires the front wheel to be angled at a very sharp degree. I haven't measured it, but it's VERY steep. I'd even go as far as saying the track-stand angle is too steep to be able to ride. No way you can ride "normally" at such a sharp angle. Now you could circle probably, but not "ride" in the traditional sense.
Also, track standing doesn't involve countersteering at all. The front wheel is simply turned so far to the side that forward action on the pedal moves the bike more sideways underneath you than forward. I trackstand pointed left (convenient for two reasons: 1. I'm left-handed and dominant and 2. Left means UP the crown of a typical US road, so the bike can coast "right" by letting off pedal pressure) so when I start to fall left, I pedal forward, moving the bike to the left underneath me. When I start to fall right, I let off pedal pressure and the bike rolls backwards and to the right, catching my fall to the right. (Obviously not a fixed gear, so I cannot "push" the bike backwards, thus the aid of a hill. That said, I can actually "push" myself back. I do this with the brakes. I hold the brakes, then push my body back, then release the brakes. My body's momentum pulls the bike back. Voila! Track stands on level ground.) This is how a track stand works and is separate from normal bicycle riding dynamics.
And to your second point, rollers rely on bicycle frame geometry just as the road does; there's no difference. What makes rollers so challenging is the tiny and curved contact surface. Riding them takes practice, but with it, you can ride no hands just like on the street.
I don't mean to be a jerk. Actually, I'd like to thank you for your post because it made me analyze some riding technique I hadn't really thought about in such detail.
Is steering your car into a skid similar?
Crinched at 20:08 that he would accidentally stick his finger in that spinning wheel.
I'll be darned, Van Driessen is a real person.....
What about riding without hands
Ok nvm lol
What about it? The bicycle's geometry causes "steer toward the fall," so you don't need hands to ride. This explains why you can "ghost ride" a bike down a hill. It rights itself due to proper bicycle geometry.
12.33 better you would go out the door and not return.
Why is there a small child in the audience? Is California so fucking crazy that you're allowed to bring your kids into class now?
It's a public lecture. These are extremely common in academia, across the entire world.
Typical deluded physics teacher. Reality has no part in his discourse nor does he allow it from the audience.
explain
Guoenyi - 2018-04-05
Good explainations! Good fun too!