Bob Doyle

Bob Doyle

In teaching physics each summer at FSU, I have found a number of physics principles that may be useful to my readers.

First, consider the braking distances for cars at different speeds. Here the kinetic energy of a car is key. Kinetic energy is ½ x mass x speed squared.

This energy must be overcome by your car’s brakes, which supply a force opposite to the motion of your vehicle. You must include reaction time, during which the car continues to move at constant speed before you actually apply your brakes.

One British study assumes the reaction time to be 0.7 seconds, typical for an alert driver. At 20 mph, a car would travel about 20 feet during the reaction time and would take 20 feet to stop. At 40 mph, a car travels nearly 40 feet in reaction and 80 feet while braking. At 60 mph, a car would travel 59 feet in reaction and 181 feet in braking.

You will notice that the reaction travel is proportional to the speed while the braking distance is proportional to the speed squared. The braking distance reflects the kinetic energy of the car.

So as one goes faster, the spacing between cars should increase greatly to avoid rear-ending the car ahead of you or hitting a barrier. A good rule is several car lengths for each multiple of 10 mph.

Second, consider making a turn onto a road that runs at right angle or 90 degrees to the road you are traveling on.

During this turn, you would actually be moving along a circular radius so that in one quarter of that circle, you would have made your turn onto the intersecting road. When anything moves in a circle, constantly changing the direction of motion, an inward (towards the center of circle) force must be exerted.

This is called centripetal (center seeking) force. This force’s formula is F. Centrip. = Mass x speed sq./radius.

The only force that can supply this centripetal force is the static friction between the car’s four tires and the road. The static friction (which is required for forward motion as well) depends on the weight of the car times the coefficient of static friction.

This static friction depends critically on the coefficient of static friction fiction which can vary for 0,1 to 1.0. A typical road may have a coefficient of 0.8, a little bit less than the car’s weight,

But when the road is icy or wet, the coefficient drops and with it the static friction (which you need to turn).

Your only safe option is to slow down as you go into a turn. If you don’t, your car won’t make the turn and you may end up in a ditch, demolish a house, or road side structure. Those road signs cautioning you to slow down as you go into a turn or bend are meant for your safety.

Third, electrical fires inside a house or businesses have caused much destruction and even killed people. Too many appliances can overload the wires and heat them so the insulation burns off. Then if there are combustible materials nearby, a fire can start.

A well-designed structure will have circuit breakers that will flip out, stopping the current in a room or part of a house. Sometimes, when the wiring is inadequate and devices are left on, and no one is around, fires can start.

I recall a fitness center on Route 40 over a decade ago that burned down due to inadequate wiring. It occurred in the middle of the night when no one was around.

THE SKIES FOR FIRST WEEK OF SEPTEMBER: Dawn begins at 5:45 a.m., with sunrise an hour later, mid day is at 1:14 p.m. (when sun is highest), sunset is at 7:43 p.m. and dusk ends at 8:45 p.m. (when stars become visible).

Daily sunlight is about 13 hours, These sun times are approximate and vary depending on the terrain, trees and buildings. Each day, we have about two minutes less sunlight than the previous day.

Jupiter and Saturn shine steadily in the early evening southern sky. The evening moon will grow from a skinny crescent early in the week to half full for the coming weekend, appearing near Jupiter.

Bob Doyle, professor emeritus at Frostburg State University, invites any readers comments and questions. E-mail him at He is available as a speaker on his column topics.

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