Think of something like a table or a couch . It has four legs that touch the floor and form abase of support(a polygon form by an objective ’s touch points with the dry land ) and as long as the tabular array or couch ’s center of gravity ( the mean fix 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 unchanging , or static when at relaxation .
A bike , 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 bag of support is a bank line segment . A good base of sustenance needs at least three contact point with the ground , so bicycle are hard to keep just when they ’re still . Bikes are , however , dynamicallystable , or stable when moving frontward , because direction allows a passenger to move the bike ’s point of support around under the center of gravity and keep it balanced , often with steering adjustment small enough that the rider may not even recognise they ’re making them . It ’s sort of like stand on one foot . If you do n’t hop-skip around a picayune , and you initiate decrease sideway , you ca n’t recover and you fall over . If you do hop , though , you’re able to move your foot around to keep your center of gravity above it and keep your counterpoise .
A bike has two features that help this dynamical constancy immensely : its steering wheel . Spinning wheels have angulate momentum , and when you ’re sitting on a wheel , you and it and its wheels make up a system that obeys the principle of preservation of angular impulse . Unless torque , or twisting force , is employ from outside the organization to change the wheels ' angular impulse , that momentum and the direction of the momentum remain constant . In a nutshell , once the wheels line up a certain mode , they want to stay lined up like that . It ’s easy for you to move them , but severely for an outdoor force to do the same , and so the wheel is easy to keep balanced but does n’t tumble easily . A non - moving bike has bike that are n’t spinning and zero angulate impulse , which makes it very easy for outside torsion to vary the wheels ' direction , making the bike hard to balance .
Even when appease relatively motionless , though , a passenger can balance a bicycle with some attempt . By steering the front wheel to one side or the other and moving frontwards and backwards slimly , a passenger can keep the line between the cycle ’s two contact points with the priming under the cycle and rider ’s combined mall of sobriety . you could see this cathartic lesson in activity whenever a wheeler is stopped at a red light.(Image at left from Wikipedia userAndrewDressel . )
That was a scant and mellisonant way to answer Brian ’s direct question , but there ’s a lot more to bicycle physics . If you ’re so inclined , Wikipedia ’s page onbicycle and motorcycle dynamicsis a good starting head to get a line about some of the other internal and external military unit , motions and dynamics that are need in a unproblematic cycle ride .
