“The rate of change of momentum of a body is equal to the net force applied to it.” This modern translation of Newton’s second law of motion came to my attention while leafing through Douglas Giancoli’s 1998 textbook titled (cleverly enough) “Physics.”
In particular, I was perusing chapter five on circular motion and gravitation. Everybody does that, right?
A picture of a car rounding a curve on page 119 jumped out at me. Driving is an example of applied physics. It is momentum and net force. Friction is the force that acts upon tires. Properly inflated tires with pliable rubber polymers provide optimal friction against the road, and make things like rounding a curve and stopping happen within expected norms. I’m not going to get into the physics of brakes in this article, except, maybe to say, “Duh!”
Banked curves on mountain roads and freeway on and off ramps are designed to counterbalance the centripetal acceleration forces that tend to make cars skid when going around curves. At a given speed the centripetal force can reduce the skid friction to zero.
In non-physics terminology, this means that banking corners improves steerability by counteracting the forces of friction that make the car want to continue straight when you want it to turn. This all works out very well for Newton until you change the equation by adding ice or rain or excessive speed or old or improperly inflated tires.
By the same logic, the friction of the tires on the road when the brakes are applied on a straight-away is contingent upon the conditions of the road and the tires. Imagine applying your brakes on a slick, rainy road, versus a loose gravel road, versus an asphalt road. Now imagine braking on those surfaces with good tires, and under or overinflated tires. Congratulations. You have just done applied physics!
Sometimes I get tired of listening to me stating the obvious. I feel ineffectual at changing our driving culture. I feel like nobody’s listening. This is particularly true when I continue to see what appears to be perfectly normal, rational people driving their cars, knee on the steering wheel, eyes on the phone, completely oblivious to the people around them whose lives are apparently forfeit to reason.
I feel like quitting. And then, I’m leafing through a physics textbook and Sir Isaac Newton is sitting on my right shoulder (the angel side) … and I think, here is a law the people can’t ignore! I can share this and all sentient driving beings will finally understand that cars don’t kill people — irresponsible drivers kill people.
And it’s not just cell phones and French fries. Responsible drivers also have to keep their vehicles in optimal condition. Tires are so very often an overlooked safety feature and people take them for granted until sometime after they are in dire shape.
To better understand the role tires play in gravity, friction and other Newtonian aspects of driving I spoke to former Pasadenan and retired NASCAR short track race car driver Mike Willette. Willette told me that tires are possibly the most important safety feature on a vehicle.
According to Willette, the portion of the tire that makes contact with the road, in racing terms, is called the “contact patch,” and for friction, more is better. “The only purpose of tread on tires is to move water away from the road to avoid hydroplaning when it’s raining” said Willette. That is why racing tires have no tread.
“But the thing is …” said Willette “the contact patch is reduced when tires are under or overinflated, and the friction of ‘rubber to the road’ is reduced as tires are repeatedly heated and cooled,” as happens when driving. “As tires go through this heating and cooling cycle,” Willette continued, “the polymers in the tires become hard and less pliant. That is why racing tires are changed so often.”
Willette also told me that Newton was right about a lot of things, and that tire manufacturers design tires based upon these simple principles of physics. They have done the proverbial and literal math for us based on Newton’s laws of motion. All we have to do is maintain the recommended tire inflation pressure and replace bald or aging tires to get right and stay right with the physics of our all-important tires.