A lot of people will be driving a lot of kilometres over the next few months, as Canadians rush frantically around the country to make the most of summer. Unfortunately, quite a few accidents will undoubtedly result, some of which could be prevented if people better understood the physical forces at work when they drive their cars.
The most important thing to remember is that a car weighs a lot, which, as Isaac Newton pointed out, means that once it’s moving in a certain direction, it will continue moving in that direction until quite a lot of force is applied to it. It has “inertia.”
Inertia means that your car cannot really “stop on a dime.” It will only come to a stop as quickly as its kinetic energy can be turned into heat by the friction of the brake pads on the brake disks, probably well over 60 metres.
The other important force at work when you drive is friction. When you turn the wheel, the friction between the front tires, now at an angle to the direction of motion, and the road becomes so great that the car takes the path of least resistance and begins to roll in the direction the wheels are pointing. Turn the wheel too sharply or at too high a speed, however, and the car’s inertia will overcome that friction, causing the tires to slide, squealing, across the road in the original direction of travel. That’s a skid.
Ice or oil on the road reduces the friction between your tires and the road, making a skid much more likely. Too much water can have the same effect, due to “hydroplaning.” Tire grooves direct water out from under the tires. But drive too quickly through water deeper than the tread, and the grooves can’t get rid of the water fast enough. Instead, the tires float–and you lose all traction and control. Hold the wheel steady and ease off the gas until the tires regain their grip.
However you get into a skid, you’ll probably instinctively do the right thing and turn the wheel in the direction the rear wheels are skidding. But sometimes, when you do that, you suddenly find yourself spinning in the opposite direction. When a car goes skids, its weight shifts onto the side leading into the skid, compressing the springs and shock absorbers. As the car straightens out and the tires regain their grip, those springs and shock absorbers de-compress, unloading their energy in the opposite direction. If you haven’t yet straightened the wheel, that jolt of energy may spin your car the other way. To avoid this, straighten the wheel as quickly as possible after regaining control.
Another way you may end up in a skid is through a panic stop. When you slam on the brakes (unless you have anti-lock braking) your wheels quit spinning, and your forward momentum is scrubbed away by the tires sliding across the road. While that’s happening, you can’t steer. Driving instructors say panic stops should be a last resort: it’s better to steer around trouble if you can. That means you have to hang up the cell phone, concentrate on what you’re doing, and always have a possible escape route in mind in case of trouble ahead.
It also means you have to stay far enough behind the car in front of you to give yourself time to react. Professional driving schools recommend the “three-second” rule. Choose a road sign or other stationary object. When the car in front of you passes it, start counting seconds. Three seconds later (more if conditions are bad), you should be passing the same landmark.
Only stop on the highway if there’s a dire emergency. At 95 kilometres an hour, a car closes with a stationary object at 26 metres per second, which doesn’t leave much time to react. Don’t stop at the bottom of a hill, just over the crest of one, or on a turn.
Finally, wear your seatbelt. A body in motion tends to remain in motion, remember? If your car hits something and stops, your body is going to try to continue moving forward at whatever speed the car was going. In the absence of a seatbelt, you’ll hit the dashboard or windshield at that speed. The consequences I leave to your imagination.
This summer, drive smarter. Remember, you may get away with breaking a few traffic laws when nobody’s looking–but nobody gets away with breaking the laws of physics.
