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The law of inertia, also known as Newton's first law of motion, basically says that an object will continue doing what it is already doing, unless an external force is applied to the object.
What this means, simply, is that once you start moving, you should continue moving at the same rate forever. However, even on a level surface you are encountering several types of forces that try to slow your bike down. These include wind, air resistance, friction within the wheels of the bike, and probably most important, the friction between the bike tires and the road. So you must continue to apply enough force to equal the various friction forces acting on your bike so that you move at a constant speed. If you stop pedaling you will slow down, and if you pedal faster you can accelerate. All according to Newton's first law of motion.
The answer posted above is quite correct but not complete. We must continue to pedal a bike to keep it moving at steady velocity even on a plane surface because of the need to overcome the frictional resistance, which includes the resistance offered by the atmospheric air, as well as the resistance offered to the movement of the various parts of the bike. But all these relate more to the laws of friction than of inertia.
It is important to note that laws of inertia are applicable only to motion in a straight line. However, many parts of the moving bike have circular motion. Also some of the forces are used to change the shape of the parts like rim of wheel and tyres. This also uses up some of the kinetic energy of the moving bike, which must be replenished by pedaling.
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