Key points for understanding:

-The resultant force on the object is the vector sum of all forces on an object.

-Acceleration is a rate of change of velocity.

-Resultant force causes acceleration. Changes in resultant force cause changes in acceleration.

Answer:

When the object is first dropped, the only force acting on it is weight (w). So, there is large downwards acceleration, increasing its velocity.

As velocity increases, there is more drag (d) opposing the direction of motion due to air resistance. This leads to a smaller resultant force, so there is smaller acceleration.

Eventually, the forces of drag and weight become equal, hence there is no resultant force. No resultant force means no more acceleration is possible, so you reach terminal velocity.

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