AviationBanter - View Single Post

**Ernest Christley**

wrote:

The laws of physics say once an object is in motion it takes no energy
to maintain the velocity UNLESS there is some other force at work
that would cause the velocity to decrease.

Since at a constant speed, the a in F=ma is zero, the force is zero
no matter the mass.

Once at speed in a car (or airplane or rocket ship) the only energy
needed to maintain speed is that equal to any drag forces that
would otherwise slow the car down.

Have you looked at the current crop of high mileage cars?

They all have very aerodynamic profiles to get the air drag down.

They also have very narrow, hard tires. Unfortunately, the DOT has laws
against solid rubber tires or they could be made even harder.

Your analysis would be mostly correct if we were talking about trains.
I've stood beside a loaded one and watched it deform the tracks. A car
on the road is like a machine rolling across a mattress. Extra weight
pushes the tire down into the mattress and increases the drag. The
energy is going into deforming the tires and heating them. Ask any over
the road trucker what happens when you're hauling 40-tons and you don't
keep your tire pressure up. Tends to light up the night.