If we divide the total tractive power by the gross rating tonnage, we will have the available power in pounds per ton. If we subtract from this the tractive resistances, including grade resistance and the resistance due to curvature, the remainder will be the force which is available for acceleration. If these resistances are more than the available power per ton, the difference will be negative and will indicate a retarding force.

On the same principle. as used in computing gravity resistance (§ 117), we may say that a given accelerative force per ton will be able to alter the potential head (or velocity head) by a proportional amount in a given distance. If P = the accelerating force per ton, then P:2000::(h2-h1):s, in which h2 and h1 are the two velocity heads and s the distance. If we say that h=0.03511 V2 (see §124), and substitute for V12 and V22 in Eq. 4 (§120) the values h1/.03511 and h2/.03511, we will have P= 2000/8 (h2-h1), or s= 2000/P (h2-h1), .(14) which is the same as the above equation. If our accelerating force is, say, 10 pounds per ton, then the distance required for a change of one foot of velocity head will be 2000 10 or 200 feet.