Describe the different forces upon a falling object with reference to Newton's first law.

A falling object will reach terminal velocity when the force draging it down (ie its weight due to gravity) is equal to the air resistance opposing the motion upwards. When the two forces are equal the object will experience a constant velocity downwards and thus no acceleration. This is in accordance with Newton's second law that states that an object will remain in its current state of motion unless acted upon by another force.