Quote:
Originally Posted by California98Civic
If it is a law, then this comment will reveal my ignorance: if 7.6 lbs consumes 1bhp then how does my 106 hp engine propel my 2100 lb car? See what I mean? Clearly, I don't understand something here ....  |
It's better not to think about horsepower since "power" is a difficult concept to visualize and/or understand despite most people thinking they do. "Power" is the time derivative of energy (energy and torque are also, sometimes confusingly, measured in the same units; so, when we talk about horsepower in a car we're really talking about "the change in torque per unit time." One horsepower is equal to 550 lb-ft/s). What accelerates a car is, as teoman correctly points out, tractive force which is directly relatable to the rate of acceleration of the car, and what causes it to move. Reduce the mass and, with no change in tractive force (which isn't true in reality, as the available traction is proportional to the normal force acting on the tire which is equal to the load/weight on that tire), the magnitude of acceleration must increase. Rules of thumb like "7.6 lb = 1 hp" are generalities; you'll also hear drag racers use "100 lb reduction = 0.1 s reduction in quarter-mile time." But that depends on the car, the actual force produced over time, the actual mass of the car, where the mass was removed, the change in tire load because of the weight reduction, etc.