Lift is a force perpendicular to the chord of the wing profile directed towards the exterior upper surface of the wing. If there is a diversion in the air flow, or if the air, originally at rest, is accelerated, then a force has been applied to it. Newtonian physics stipulates that for every action there is an opposite reaction of equal force. Thus to generate lift the wing must have an action on the air which generates a reaction called the lift. This lift is equal to the modification in the momentum of the air that is diverted downwards. Momentum is the product of mass and speed. The lift of a wing is therefore proportional to the amount of air diverted downwards multiplied by the vertical speed of the air.