so far as the stone is concerned no work is done. So a man may hold a heavy weight in his hand or on his shoulder, sustaining it with considerable effort against the force of gravity, and yet no work is done on the stone so long as it is not raised to a higher level. If the stone is carried in a horizontal plane, no work is done on the stone; while if it is carried down hill or lowered vertically, negative work is done on the stone. That is, since the stone possesses less potential energy at the foot of the hill than at the top (the difference being equal to the weight of the stone multiplied by the difference of altitude), the stone has lost energy, and this energy lost by the stone has been communicated to the man, who has had work done upon him by the stone, albeit he may have lugged it down the hill or lowered it from an elevated position with considerable effort.
When a car is propelled by an electric motor deriving its current from a storage battery carried on board the car, the energy of the car consists of three parts: (A) Mechanical potential energy due to the mass of the car being at some elevation above the surface of the earth. (B) Kinetic energy, due to the motion of the car as a whole and of its parts with respect to one another and the heat of the car. (C) Chemical potential energy stored up in the battery. When the car is running up grade, energy is being expended not only in overcoming friction, but also in lifting the car against the force of gravity. In doing this, energy is transferred from C to A. When the car descends again to its former level the energy stored up in A is given up, less energy is therefore required from the battery to propel the car, and the battery is accordingly in so much spared. If the grade be steep, the motor may actually be driven as a dynamo, and the current which is thereby generated may be stored up in the battery. In this case energy is transferred from A to C, and at the bottom of the hill the energy C may be greater than that at the top. The battery has done negative work on the car coming down the hill: that is, the car has done work on the battery and stored up energy.
The same considerations apply to the animal body. If a man carries himself up a hill, he is doing work upon his body in so elevating it against the force of gravity, and if he weighs 150 pounds and ascends an altitude of 10,000 feet, he has done 1,500,000 foot-pounds of work upon his body. This represents the quantity of energy which has been transferred from his tissues to his body as a mass; from chemical potential energy to mechanical potential energy. The tissues correspond to the storage battery, the muscles to the motor and the man's weight to that of the car. So when the man walks down the mountain again he does negative work, lowering his body (like lowering the car), involving the transfer of potential energy from his body as a mass to his tissues. Just what form the energy takes as it is so transferred is not altogether clear, but the distinction between the potential energy of the body as a mass, due to its elevation above the surface of the earth, and the potential and kinetic energy resident in the tissues of the body, is one of fundamental importance and should be kept clearly in view.
We may consider the man to be a complex machine, weighing, say, 150 pounds and having a quantity of potential and kinetic energy stored up within his body, which store of energy is drawn upon whenever external work is to be done, and which, besides, is being constantly expended in keeping the body warm and performing the internal work of the body. The energy of the body, like that of the electric car, then, consists of three portions, viz.: (A) Mechanical potential energy of the body as a whole, due to its position with respect to the earth. This is zero when it is at the earth's surface, or say the sea level, and increases as it rises above the sea level. (B) Kinetic energy, due to the heat of the body and to the motion of the body as a whole and of its several
