Suggest why as the speed of the car is increased, the maximum motive force increases to a maximum at about 30m

Question

A car in motion is subject to anumber of forces. In particular, it usually experiences a motive force. I also experiences a drag force which is due to the effect of both friction and air drag.

Suggest why as the speed of the car is increased, the maximum motive force increases to a maximum at about 30ms^-1(for this car model) and then decreases after that speed.

Answer 1

First I need to guess at what you mean by motive force. The first answer puts it in the context of constant velocity and suggests it's just the force needed to overcome drag, but I'm guessing you're interested in the force that produces motion, such as that from a motor coupled to the road by gearing and wheels, at any velocity. If that's the case, it's not surprising that full-throttle motor torque and thus accelerating force passes through a maximum. (I'm assuming there's no gearshift so car speed is strictly proportional to engine speed.) At very low rpms, an engine produces very few firings in unit time and although cylinder pressure is high it is present only for a small portion of the rotation cycle, thus low torque. At very high rpms the engine can't breathe as well as it would like due to increased pressure loss at high flow rate, so the fuel-air charge is less dense and again torque falls off. Typical conventionally-aspirated engines have a torque peak at roughly 0.65 to 0.9 * maximum rpm. Diesel peaks are typically much lower.

Answer 2

It doesn't.
The motive force equals the drag at any particular constant velocity. As you have noted, the drag is composed of a (nearly constant) mechanical friction force added to the air drag. As an aeronautical engineer, I can assure you that the air drag equals a constant times the square of the air velocity. (To a high degree of accuracy).
Therefore, the motive force will always increase with speed, assuming the air speed equals the speed of the vehicle over the roadway, ie, there is no significant head or tail wind blowing on theday of the experiment.

Answer 3

probably this automobile has an inner combustion engine, if it is the case, the potential output of the engine (and for this reason the objective tension) will boost, to a element, with larger engine speed because of the fact there are greater potential strokes in a given quantity of time. even though it takes time to get the gas and air into the engine and time for the gas to burn, so because of the fact the fee will boost each and each potential stroke produces much less potential and after a definite element the better variety of strokes does not atone for the decreased potential of the guy strokes.