Simplify the following text for a Grade 14 native English-speaking person.
Original: These equations arise from applying Newton 's second law to fluid motion , together with the assumption that the fluid stress is the sum of a diffusing viscous term ( proportional to the gradient of velocity ) , plus a pressure term .
Simplified: The equations result from applying Newton 's second law to fluid motion , with the belief that the fluid stress is the sum of a diffusing viscous term ( in relation to the gradient of velocity ) , plus a pressure term .