Answer
a. Because at first the bike is at rest.
b. To counteract the forces of friction & air resistance.
Work Step by Step
a. When you’re starting from rest, your force needs to accelerate the bike and yourself from v = 0 up to your final velocity, and there is also friction to contend with. Once you are moving at constant speed, you only need a smaller force to continuously account for friction and air resistance.
b. If there were no friction or air resistance, you wouldn’t need to. However, since these retarding forces do exist, you need to pedal to keep the net force equal to zero, which keeps you moving at constant velocity.
