Final answer:
Friction is the force that opposes motion between two contacting surfaces, causing objects to slow down or resist starting to move. It's an essential concept in Newtonian physics, balancing applied forces to maintain states of motion or rest as per the first law of motion.
Step-by-step explanation:
The force that always acts opposite to the motion of an object and thereby prevents or slows down motion is known as friction. Friction is a resistive force that occurs when two surfaces are in contact; it is the external influence that resists the relative motion or the tendency of such motion between the surfaces in contact. As explained through Newton's first law, without a net external force, such as friction, an object would maintain its state of motion - either at rest or moving with a constant velocity.
Friction not only acts on moving objects, causing them to slow down and eventually stop but also resists the initiation of motion, as is the case with static friction, which opposes the motion of two systems in contact that are not already in motion relative to one another. When an object moves at a constant velocity, the force applied to it (for instance, by a person pushing this object) is counterbalanced by an equal magnitude of frictional force, but in the opposite direction, effectively making the net force equal to zero.