Final answer:
Friction decreases after an object starts moving because static friction gives way to kinetic friction, which is lower due to reduced surface contact and interlocking.
Step-by-step explanation:
Friction generally decreases after an object starts moving because static friction, which prevents motion, is usually higher than kinetic friction, which occurs once motion has begun. Static friction is the force that must be overcome to start an object sliding across a surface. Once an object is in motion, there are fewer points of contact and less interlocking between the surfaces, which is why kinetic friction is typically less than static friction. To understand this, consider the example of pushing a heavy crate. Initially, the static friction force is high and matches your pushing force. When you push hard enough, the crate ‘breaks free’ and starts moving, transitioning to kinetic friction, which is lower, allowing the crate to move more easily.
This concept is crucial in surface physics and can be seen in action on an air hockey table. With the air turned off, friction between the puck and the table slows it quickly. But when the air is on, lifting the puck slightly and reducing contact points, the puck glides smoothly with minimal friction, simulating a near-frictionless surface. If the resisting force of friction could be entirely removed, as postulated by Galileo, an object would maintain its motion indefinitely, demonstrating the significant role friction plays in slowing down motion.