Why is it harder to start moving a heavy object than to keep it sliding?
When an object is stationary, it is held in place by static friction. At a microscopic level, all surfaces have tiny, jagged ridges called asperities. When two surfaces touch, these ridges interlock and create a physical grip that prevents movement. The strength of this grip depends on the normal force, which is the weight of the object pressing the surfaces together. To begin moving the object, enough force must be applied to overcome this initial interlocking of the ridges.
How does friction change once an object is in motion?
Once the object begins to move, it experiences kinetic friction, which is generally weaker than static friction. Instead of being deeply interlocked, the surfaces now glide over the peaks of the microscopic ridges. At this stage, the resistance comes from molecular bonds that constantly form and break as the surfaces pass one another. Because the surfaces are no longer fully jammed together, it requires less continuous force to maintain the motion than it did to break the initial stationary grip.
Why do surfaces get hot when they rub together?
Friction involves a significant amount of microscopic activity that requires energy. As the surfaces move, the constant smashing of ridges and the breaking of molecular bonds convert kinetic energy into thermal energy. This process is why rubbing two objects together causes their temperature to rise. The energy used to overcome the resistance of friction does not disappear but is instead released as heat into the surrounding environment.