Newton on Ice

1. Every body continues in its state of rest or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed on it.

2. Change of motion is proportional to, and in the direction of the straight line of, the motive force impressed.

3. To every force there is always opposed an equal reaction force.

These are Newton's laws, the famous statements of the fundamental principles that govern large bodies in motion. They are also the bane of the student who is first introduced to them. They can be the bane of Alaskan drivers as well.

Let's look at a car stopped at an intersection. The force of gravity is acting on the car, causing it to apply a downward force equal to its weight on the road. The road applies a reaction force upward on the car equal to the car's weight. The net force on the car is zero, and the car remains stationary. When the light changes and the driver steps on the gas, the wheels push backward on the road and the reaction force from the road pushes forward on the car. The forward force on the car is not balanced by any counterforce, so the car moves forward.

Once the car is moving, friction becomes important. When a car is moving at a steady speed, the forward force from the road is just balanced by the backward force from the friction of the air. Since the forces are balanced, Newton's first law comes into action, and the car moves in a straight line at a uniform speed. Stopping is the reverse of starting: the wheels push forward on the road, the road pushes backward on the car, and the change in motion acts against the motion already present.

When the front wheels turn, the part of the car's forward motion that is in the direction the wheels are pointed applies little or no horizontal force on the road, but the part that is crossways to the turned wheels does. The reaction force from the road pushes the front of the car sideways, and the car turns.
All of the above, however, assumes that the wheels can apply a horizontal force to a horizontal pavement, and in order to do this friction is essential. Because the friction between ice and rubber is low (especially at temperatures around freezing), this is not always true in Alaska. Attempts to start, stop or turn suddenly often result in the wheels losing most of their ability to exert force on the pavement, and Newton's first law takes over: the car continues in a state of rest or (more or less) uniform motion in a straight line until it finds something (a snowbank if the driver is lucky, a telephone pole or another vehicle if not) on which it can exert a force. The obstacle exerts an equal and opposite force on the car, thus halting the motion.

If the driver is wearing a seat belt, the car will transmit the reaction force to the driver, who will then stop with the car. If not, Newton's first law states that the driver and any other unbelted occupants of the car will continue in a state of uniform motion, even though the car has stopped, until the forces they exert on the steering wheel or windshield produce reaction forces to halt their motion.

So, for those of you waiting to see if Alaska enacts a law requiring seat belts to be used: Nature enacted it first. A modern corollary of Newton's Laws might be, "buckle up".

Update July 1996: Yes, Alaska did enact a seat belt law. The physics above, however, is as valid as ever.