is often simply called "power" when the context makes it clear. The instantaneous power is then the limiting value of the average power as the time interval approaches zero. In the case of constant power , the amount of work performed during a period of duration is given by: In the context of energy conversion, it is more customary to use the symbol rather than . Mechanical power Power in mechanical systems is the combination of forces and movement. In particular, power is the product of a force on an object and the object's velocity, or the product of a torque on a shaft and the shaft's angular velocity. Mechanical power is also described as the time derivative of work. In mechanics, the work done by a force on an object that travels along a curve is given by the line integral: where defines the path and is the velocity along this path. If the force is derivable from a potential (conservative), then applying the gradient theorem (and remembering that force is the negative of the gradient of the potential energy) yields: where and are the beginning and end of the path along which the work was done. The power at any point along the curve is the time derivative: In one dimension, this can be simplified to: In rotational systems, power is the product of the torque and angular velocity , where measured in radians per second. The represents scalar product. In fluid power systems such as hydraulic actuators, power is given by where is pressure in pascals, or N/m2 and is volumetric flow rate in m3/s in SI units. Mechanical advantage If a mechanical system has no losses, then the input power must equal the output power. This provides a simple formula for the mechanical advantage of the system. Let the input power to a device be a force acting on a point that moves with velocity and the output power be a force acts on a point that moves with velocity . If there are no losses in the system, then and the mechanical advantage of the system (output force per input force) is given by The similar relationship is obtained for rotating systems, where and are the torque and angular velocity of the input and and are the torque and angular velocity of the output. If there are no losses in the system, then which yields