Work is measured in joules (J) in the SI system. A joule is defined as the amount of energy transferred when a force of one newton acts over a displacement of one meter in the direction of the force. In formula form, when the force is constant, the work W is W = F · d · cos(θ) where F is the force, d is the displacement, and θ is the angle between the force and the displacement. If the force is in the same direction as the motion, W is positive; if opposite, W is negative. Power is the rate at which work is done, measured in watts (W), with 1 W = 1 J/s.