Why would 34,000 pounds of explosive black powder being shipped by sea from Brazil with the state of Vermont as the final destination not continue to be shipped by sea all the way to a port close to Vermont in New England [front page, June 3]? Was an unloading and reloading stop in Newport News, Va., necessary?

There ought to be a law requiring hazardous material to stay at sea and take the shortest land route possible to get to where it is going. Certainly, it was ludicrous for that load to have to go through eight states and ever come anywhere near I-95.

THERESA GREENE REED

Rockville

Engineers may know, as reported by Mozharul Hoque in his letter on June 8, "that when you turn, you develop centrifugal force."

Physicists, however, know that centrifugal force is a fictional force, useful only when working in a rotating frame of reference. It does not exist.

What is illustrated by the unfortunate incident at the Springfield interchange is the application of Newton's first law of motion. This law states that a body at rest or in uniform motion remains at rest or moving uniformly unless acted upon by an external force. In other words, when a speeding truck enters a curve the road must be able to exert sufficient transverse (centripetal) force upon it to cause it to travel in the circular path.

This transverse force is a combination of frictional forces between the tires and the road and normal forces. The normal force can contribute to the centripetal force only if the roadway is banked. In the absence of sufficient centripetal force the truck, keeping closer to its original path, runs off the road.

The truck rolled over. The centripetal force, acting on the wheels, applies a torque to the truck around its long axis. Gravity, acting on the mass of the truck, applies a counter torque. When the torque due to the centripetal force required to keep the truck on the curved road exceeds that due to gravity, the truck will roll over.

JAMES H. FAHS

Fairfax