Air resistance and non-uniform acceleration
Non-uniform acceleration is acceleration that changes with time.
Many real situations involve non-uniform acceleration. This can be because of air resistance.
Air resistance is the force on a moving object by the air. Air resistance always works to slow the object down. The faster an object is moving, the greater the air resistance.
An object falling through the air has a non-uniform acceleration. The acceleration due to gravity stays the same. The deceleration due to air resistance increases as the object speeds up.
The overall acceleration of the object decreases with time.
We often simplify real situations by ignoring the effects of air resistance.
When a pencil falls from the table to the ground, air resistance is normally ignored.