An object moving through a gas or liquid experiences a force in direction opposite to its motion. Terminal velocity is achieved when the drag force is equal in magnitude but opposite in direction to the force propelling the object. Shown is a sphere in Stokes flow, at very low Reynolds number. F is force, d is drag, and g is gravity. Small arrows show direction of movement of fluid relative to sphere. Large arrows show direction and magnitude of equal and opposite forces on the sphere, which has stopped accelerating and is moving at terminal velocity.