Q.A vehicle is moving along the positive $x$ direction. If sudden brakes are applied, then

Once an object begins sliding — i.e. once the applied force has exceeded the maximum static friction $\mu_sN$ — the opposing force from the surface is called kinetic (or sliding) friction, obeying the empirical relation $f_k=\mu_kN,$ with $\mu_k$ the coefficient of kinetic friction. Universally, $\mu_k<\mu_s$: it always takes somewhat less force to keep an object sliding than it took to start it sliding.

Comparing static and kinetic friction: static friction is a variable force, ranging from zero up to its maximum $\mu_sN$ to exactly match whatever is applied (as long as the object stays at rest); kinetic friction, once sliding starts, is essentially constant at $\mu_kN$, independent of the object's speed or of how much extra force is applied beyond what's needed to keep it moving. Both are, notably, independent of the visible area of contact — only the normal force $N$ and the nature of the two materials matter. To keep an object moving at exactly constant velocity once it is sliding, the net force must be zero, meaning the applied force must exactly balance kinetic friction — this is a common source of confusion, since it looks like "no force is acting", when in fact two forces (the applied push and kinetic friction) are present and simply cancel.

Everyday examples: a vehicle's wheels skidding under sudden braking experience kinetic friction opposing their forward slide relative to the road; and the origin of both static and kinetic friction, at the microscopic level, is the same — electromagnetic bonding between atoms at points of true (not merely apparent) surface contact.