Final answer:
Inertia is the reason a bike continues to move forward when the brakes are hit suddenly; it is a property of matter described in Newton's First Law of Motion. Friction is an external force that eventually stops the bike.
Step-by-step explanation:
When a bike rider hits the brakes and the bike still moves forward, this is an example of inertia. Inertia is the resistance of any physical object to a change in its state of motion or rest. Newton's First Law of Motion explains this phenomenon, often stating that an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Friction is the kind of force that eventually stops the bike due to its interaction with the road's surface. It is an external force and it is the force that opposes the motion of two surfaces sliding past each other.
Understanding these concepts is vital to grasp the principles of Physics, particularly dynamics, the study of motion and the forces that affect motion.