FORCE AND POWER.

Progress, Volume II, Issue 2, 1 December 1906, Page 68

FORCE AND POWER.

If we may judge by the articles which from time to time appear in the motor columns of the nontechnical press, it is evident that the prevailing ideas regarding the meaning of the terms force, energy and power, are by no means exact. It is more than likely that many motorists are sufficiently interested in these matters as applied to automobiles to justify a short discussion thereon, with a view to eliminating any uncertainties that may exist in their minds. In one of these pseudo- technical articles it was stated that the prime object of the changespeed gear of a modern car is to render it more powerful on the hills at the expense of its velocity, and that more power is required to start the vehicle from rest than to maintain it in motion on its top gear. To further illustrate these remarks, the case of a locomotive starting from rest with a heavy train is referred to as analogous, since the locomotive is said to develop more power when starting than when maintaining a velocity of sixty miles an hour on the level. It is, in the first place, incorrect that the locomotive develops its maximum power when starting up with a heavy load. From indicator diagrams taken from the cylinders of locomotives, it is found that the maximum indicated horse power is obtained when travelling at sixty or seventy miles an hour with a heavy tram in the rear, notwithstanding the fact that the engine is greatly assisted by the kinetic energy of the whole tram, which at high speeds is considerable. The actual power developed at starting (even with maximum boiler pressure and cut-off in the cylinders at seventy-five per cent, of the stroke) is less than half that which is obtained when travelling at full speed, owing to the fact that the locomotive is not geared down, the same ratio between engine and wheels obtaining at all speeds. The fact to be noted is that the horizontal force or pull exerted by the engine on the tram is about three times as great at the period of starting as when 1 mining at full speed. Before going further, it would be well to define in as few words as possible, the exact meaning of the terms force, power, etc. Force, considered dynamically, is usually denned as that which changes or tends to change the state of rest or motion of any body. It is measured m pounds weight by engineers. Work is done when a force moves through space in the direction m which it acts. Work done is therefore the product of two factors — force in pounds weight and distance m feet — and is measured in foot pounds. Power is the rate at which work is done, and is measured by the amount of work an agent is capable of doing in unit time. The horse power, being the unit of power is the rate of working of 33,000 foot pounds in one minute. Thus, if 33 pounds are lifted through a vertical height of 1,000 feet in one minute, one horse power is developed. Energy is the capacity of an agent for doing work m virtue of its mass and the square of its velocity. It is therefore measured in foot pounds being the product of half the mass of the body and its velocity in feet per second squared. This form of energy is kinetic. It is quite evident, therefore, that the terms force and power are quite distinct, and they should never be confused. Force is a single measurable quantity, whereas power is a rate, and always involves time The measure of a force remains the same whether it does its work quickly or not. It would clearly be impossible for the locomotive to develop its 1,000 h.p. during the first twenty }ards of its journey, since its pistons are moving comparatively slowly though the pressure behind them is probably greater than at any other period The tractive force or horizontal pull on the drawbar of the engine is at its maximum because the total load on the pistons in steam pressure is at a maximum, but this force is not performing its woik fast enough to register maximum power. In the case of a modern express locomotive this tractive force when starting will measure about ten, or even twelve, tons, which will fall to about three tons when a speed of sixty miles an hour on the level is obtained, owing to the cutoff in the cylinders taking place at twenty per cent, of the stroke. Further, it should be mentioned that the total horse power that would be necessary to propel a train weighing 300 tons at sixty miles an hour on the level is something over 1,500. With modern express locomotives, it is rare for more than 1 000 h p to be available. The deficit is made up by' the kinetic energy stored in the whole tram, which as before stated, is its capacity for overcoming lesistance, or doing work by vntue of its mass and velocity .