Starting with Aristotle, for nearly 20 centuries, were all convinced that the movement with constant speed needs to maintain in effect from the outside. It seemed to be confirmed by our everyday experience. For example, the car will go faster. more powerful than its engine. If it is off, then the car sometime stop. The same can be said about the bike, boat.
In fact, isolated (not interacting with anything else) body always moves at a constant speed. To say that the body moves by inertia. Only the intervention of another body changes its velocity.
In normal conditions there is always resistance from air, land and water. There is friction: the resistance during the movement of one object on the surface of another. Therefore, to maintain the vehicle speed constant, you need to make an effort. If friction were not, the vehicle speed would not change.
The friction force is of an electromagnetic nature. This means that the cause of friction is the interaction of the particles that make up matter: when two bodies are in contact, the particles of which they consist, are attracted to each other, and for moving one body relative to another is required to make an effort.
Another cause of friction is the roughness of the surfaces. The protrusions on the surfaces touch and cling to each other and prevent movement of the body. That's why for movement on a smooth surface you want to apply a smaller force than for driving on rough. If you take two identical ball, put one on the floor of linoleum, and the second on the ground and push with the same force, the longer it will take to move the ball that rolls on the linoleum floor; the ball on the asphalt stops faster..
Friction forces arise from contact. They always act along the surface and is directed against the movement of the body.
Between the two solids friction is of three types: resting, rolling, and sliding. Rolling friction is much less than when sliding. Therefore, the invention of the wheel was the greatest achievement of man. Humanity has given a new impetus to development.