Originally Posted by
gregf83
Sorry, you'll need to review your high school physics. Acceleration of the bike and rider is a function by the net force acting on the bike. In the case of pedalling on the flats at a constant speed the force you apply to the pedals is balanced by the forces resisting forward motion, i.e. wind resistance, rolling resistance etc. At constant speed, by definition, there is no acceleration. a = dv/dt. No change in velocity means no acceleration.
Now as I stated earlier there are actually very small accelerations going on with each pedal stroke, since we don't pedal with constant torque, but those accelerations are minimal and not relevant to this discussion.
High school physics is good for demonstrating the principles because you can leave out all those bothersome details that get in the way of understanding the principles. In the high school physics world, you can get on the bike, give the pedals a mighty shove and the bike will move at a constant speed until it is acted upon by an outside force. Since you've removed the outside force, there is no velocity change and no acceleration in any direction. I have no problem with that explanation nor that model. It's simple to explain but maybe a little difficult for most to understand.
But in the real world those bothersome details have real consequences. Let's look at what you said above: you agree that you are applying force to the bike to balance the forces resisting forward motion, so do I. Once balanced you can maintain a constant speed but what do you have to do to maintain that constant speed? Unlike the high school physic model, you have to constantly apply force to the pedals to maintain that balance and the constant speed. What happens when you stop applying that force to the pedals? You can't just set the bike on its way and expect it to roll forever because the bike is being acted upon by outside forces changing it's speed and momentum. The bike will slow down, i.e. experience a negative acceleration because of the forces acting against the bike and rider. The
net velocity change of the
system is zero while you are applying a force to counteract the negative forces but because you have to constantly apply force to the pedals to maintain that balance, the rider is having to constantly accelerate the bicycle. Otherwise the bike would have to stop moving.