Can someone please clarify this concept?
The forward driving force is provided by the friction exerted by the ground on the rear wheel. It is the action – reaction pair to the friction force of the wheel pushing backwards on the ground
Retarding forces acting on the bicycle include air resistance (drag) and friction on the wheels (especially when the brakes are applied).
So what does friction do? How is part of the driving force and the retarding force? So if the bicycle is travelling where there is no air resistance, it won’t ever stop? Because the retarding force is trying to stop it, but instead it actually helps it to move, but as it moves the friction on the wheels try to stop it, and then the cycle repeats itself?
As I understand it depends on what the wheels are trying to do;
If you're pedalling and trying to turn the wheels, consider the contact point of each wheel with the ground. It's trying to push the ground back because that's the way the wheel is turning when you try to pedal, and the reaction force pushes the entire bike/rider system forward. In addition to this there's a little bit of 'rolling resistance' which is a force opposing the propulsion due to the surface but for most surfaces and certainly most VCE physics questions this can be ignored.
If you're braking it's a slightly different story. Brakes apply a torque to the wheels to
stop them from rotating rather than a torque to
make them rotate which is the case above. As the wheels are slowing down, the situation is reversed. The wheel pushes against the ground the
other way at the point of contact with the surface, and the surface pushes back and you slow down.
At all times, there's going to be air drag which is proportional to your velocity.
Hope that helps!