Friction makes the observation of Newton's first law difficult because it is an unseen force that interferes with the law. Newton's first law states that an object in motion continues its motion unless acted on by an unbalanced force. Friction is a contact force that opposes motion; an object in motion in a real-world scenario eventually slows down due to friction, even though it seems to slow down by itself.
The common experience that a force needs to be applied to an object to keep it in motion is also due to friction. The force a person applies to a book to keep it sliding across a table, for example, balances out the force of friction that acts to slow the book down. Newton's first law still applies because all forces are balanced, even though it seems like a constant force is needed to keep the motion.
Friction does not only occur between solid objects. Fluids also exert friction on an object moving through them, leading to air resistance. An object in free fall continues to accelerate due to the force of gravity, but at a certain speed, the air resistance balances the force of gravity and the object no longer accelerates but only maintains its motion. This speed is known as the terminal speed of the object.