Koz wrote:
RowToTri wrote:
Sorry to keep replying to you! I want to do a little thought experiment about power (or not) to keep a bike and rider stationary in a 30 mph wind: Without a force applied, the bike will roll backwards. If the force is applied by a rigid structure and nothing is moving in order to generate the force, then yes, power is 0. But if it is held in place by a human applying force to the pedals, power is definitely required, even though the bike's power meter would read 0 because cadence is 0. Things are moving - your heart is pumping, electrons are flowing through neurons, muscle fibers are contracting...
Think about it as if it were an electric motor, rather than a human, connected to the crank. Just enough current is supplied to the motor to keep it from turning backwards. The power in that case is easy to measure - voltage * current.
Comment? Disagree? Agree?
Tom A wrote:
Force is being applied to the pedal, but since it isn't moving, then by definition no power is being "used"...but, in the case of a muscle supplying that force, electro-chemical energy will be "spent"...but, as you say, a rigid link could also easily do the same job.In your electrical example, you are correct that there would be a measurable voltage and current and, therefore, electrical power would be going somewhere. However, by definition, as Tom points out, there is no motion and, therefore, no net mechanical power (electrical energy was not converted into kinetic energy and is one bad ass trackstand if the rider could stay that steady!). The electrical power, rather than being converted to mechanical power, is being converted into the torque holding the crank in place for some duration of time (as well as some amount of residual heat).
Equivalently, bringing this back to physiology, the rider doing that bad ass trackstand is burning calories doing it, but no mechanical work is being done (by definition), since none of those calories are being converted into kinetic energy of the machine. All of those calories are going into the holding torque for the duration the the bike is kept from rolling backwards (as well as some amount of residual heat...and various biochemical reactions).
ETA: Interestingly, with a few other parts of the electrical system appropriately specified, if you just connected the motor coil leads together, you could likely achieve the same static condition with no voltage applied because of the fact that no mechanical power is being generated.
Right - I get all of that. If you draw a boundary around a system that ends at the surface of the pedal no power is required. But what if you define the system more broadly? If you go back far enough in the chain, in both the motor and the human element - can you not say mechanical power is required? Back at the power station, a turbine is being turned with steam to create the electrical current required to hold the bike steady. Inside your body, all manner of things are contracting, expanding and pumping to generate the force your feet are applying to the pedals (but all of that is much harder to measure than in the power station). The human experiences this in just the same way he experiences applying mechanical power to the pedals. Even in the case of a solar powered motor, the sun is accelerating particles in its fusion reaction.
Sorry for taking this thread so far off-topic....
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Ed O'Malley
www.VeloVetta.com
Founder of VeloVetta Cycling Shoes
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