Free Body Diagram of Forces on a Sliding Object

For any given object:

The coefficient of static friction is higher than the coefficient of kinetic friction

This means that:

More force is required to initially get a stationary object moving than to keep it moving afterwards

Once the large static friction force has been overcome and the object is in motion:

The smaller coefficient of kinetic friction takes over, and the resistant force of friction decreases

This is why moving heavy objects usually involves:

One extra big push at the beginning, but then a somewhat smaller force for the rest of the trip

One more important difference between static and kinetic friction is that:

Static Friction only arises in response to an attempt to move an object along a surface

Like the normal force that holds up objects with weight:

It responds to an applied force by matching and counteracting it

The formula is actually used to determine the maximum of:

A variable value for static friction, beyond which it breaks and an object will begin to move

Both kinds of friction forces are directly proportional to:

The normal force of the surface

If you push very hard towards the surface, you may find you are not even able to overcome:

The force of static friction and slide your hand without decreasing the normal force first

When a person leans against a wall:

He/she allows a portion of the weight to push against the wall, which pushes back with an equal and opposite normal force

This normal force contributes to a corresponding static friction:

That prevents the person from sliding down the wall to the floor

Most frequently, the normal force arises on flat horizontal surface:

As a response to the weight of an object pressing it down

As a result, the normal force is equal to an object’s weight much of the time, so:

That the forces of friction are equal to the coefficients of friction multiplied by the objects weight