I'll pick a few nits, but Russ's treatise is well worth the read.

Momentum and kinetic energy are related, but not the same. Momentum is mass times velocity.

Kinetic energy is mass times velocity squared, which is the same as momentum times velocity.

Momentum is measured in foot-pounds-mass whereas kinetic energy is measured in foot-pounds-force.

Mass is a measure of resistance to acceleration (from Newton's first law: bodies in motion tend to

stay in motion, bodies at rest tend to stay at rest unless acted upon by an external force).

To get mass into force, you multiply pounds mass times the acceleration of gravity (32 ft/sec/sec).

1 pound of force is the amount of force required to accelerate one pound of mass so that

for each second its velocity changes by 32 ft/sec.; hence, 32 ft/sec/sec or 32 feet per

second squared.

A bus that weighs 32,000 pounds, traveling at 50 ft/sec (a little over 40 mph), the momentum

is 1,600,000 foot-pounds (mass). The kinetic energy is 80,000,000 foot-pounds (force).

The amount of work required to stop a bus is equal to the kinetic energy which must be

converted to heat through friction (including air resistance which can be significant at higher

speeds). Thus 80 million foot-pounds of work must be done. That is equivalent to the

amount of work required to lift the bus 80,000,000 divided by 32,000 feet into the air,

which is 2500 feet (almost a half-mile).

On the other hand, the amount of work to stop the same bus at 10 ft/sec (about 6.5 mph)

is only 3,200,000 foot-pounds, equal to lifting the bus 100 feet. Thus a 5-to-1 increase in

speed increases the energy required to stop (kinetic energy in the bus that must be

dissipated) by 25 times. Kick the speed up to 70 mph, and the energy to stop goes up

100-to-1.

That's why they say "speed kills". It's the kinetic energy that must be converted into something

else that can wreak huge damage in an accident where the vehicle comes to a dead stop in five

feet or less instead of 500 feet or more. That allows you to bend a lot of metal.

A 35,000-pound bus at 60 mph hits a 3000 pound car, head-on, coming the other way at the

same time. The bus slows down about 5 mph. The driver of the car has what I call "an abrupt

experience".

A jack-knife occurs when the centerline of a tractor and the centerline of the trailer are not

parallel or coincident with each other. If a trucker tries an abrupt stop on ice, for example, and

turns the steering ever so slightly, the two halves are no longer going in *exactly* the same

direction. The slight angle causes the force from the kinetic energy in the trailer to have

a lateral component (velocity is a vector, having both magnitude (amount) and direction) along

with the longitudinal component that is still parallel to the axis of the tractor. This longitudinal

force tries to push the fifth-wheel sideways. If the available friction between the driver tires

on the tractor and the pavement is less than the lateral force, the tires slide sideways (or

creep by scouring thanks to the pliability of the tire tread), increasing the angle until it eventually

takes over and most of the trailer force is pushing the drivers sideways. That's bad -- very bad.

Clarke