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In Praise of Friction The Media Slips Again


I recently posted a complaint about physics misconceptions promulgated by the media. To be fair, I need to report a good job describing the physics of a situation when it appears.

In an article posted on Dec. 27, 2006 by the op/ed staff of the North County Times (near San Diego), and titled A Physics Lesson, the authors do a very nice job of describing the role of friction in driving:

Your tires rely on friction to speed up, turn or stop. On a dry day, there's usually plenty of friction when the rubber hits the road. When it rains, the weight of your car must push water out of the way for the tires to reach the road. The faster you drive, the greater amount of water your tires must push aside. If that water gets trapped between the asphalt and the tires, you'll lose control of your car -- you'll be hydroplaning. The lesson here is that when the roads are wet, you can't drive as fast as you would on a normal day. Even if the rain is light, slow down at least five to 10 mph.

When I teach a first-semester course in Physics, I typically begin the first day trying to get students to identify forces acting on them as they do basic things. My favorite example is on walking. I ask the following question: if you go from standing still to walking at a steady pace, you accelerated. According to Newton there must be an unbalanced force acting on you in the direction of your acceleration. What is this force?

For those who have never taken a physics course, answers typically range from "my legs", "my feet", "my muscles" to even "my mind." The answer is always a surprise to most students: the force pushing you forward, in effect causing your acceleration, is the force of friction between the floor and your shoes. To understand this, consider that walking on very sheer ice, i.e. a nearly frictionless surface, will get you nowhere!

Later in the course, when circular motion is analyzed, the role of friction in providing the centripetal force needed for turning on a flat curve is then discussed.

So the North County Times do a nice service by stating that "our tires rely on friction to speed up, turn or stop." Indeed, most will tend to identify friction as only a stopping force.

Unfortunately, the authors make an error towards the end of their article.

Rain makes it harder to stop, too. Consider Newton's First Law of Motion: An object in motion tends to stay in motion unless acted upon by another force. That other force, hopefully, is your brakes. The faster you're driving, the longer it's going to take to slow down and stop. When the roads are wet, it takes even more time because there's less friction. Give yourself space -- the lost art of Follow Distance -- when turning, stopping and trailing other drivers. If you're reading bumper stickers in the rain, you're too close.

The error here is mixing in the idea of the force due to the brakes. The force of the brakes do not slow down the car! This is explained by a basic Newtonian fact: for a car to slow down there must be an unbalanced force that opposes the car’s motion. The force of the brakes are not applied in a direction opposite to your motion. Depending on the type of brake (disk brake vs. drum brake), the force is perpendicular to your motion (the brake pad pressing against the wheel in disc brakes), or the force is applied radially outward (brake shoes pressing outward against a brake drum). (See HowStuffWorks for an excellent tutorial on automobile braking.)

The ONLY force that will slow the car down then is the friction between the car and the road. If you still believe that the brakes slow the car down, consider the fact that, even with the best braking system in the world, if there’s no friction the car will slide even if the wheels are completely locked due to the brakes.

So what do the brakes actually do? Surely I can’t be saying that the brakes aren’t necessary to slow the car down! The brakes DO slow down and (hopefully) stop the rotation of the wheels by exerting a torque on the wheels in a direction opposite to the angular velocity of the wheels! Friction plays a huge role here - but it is the friction between the brake pad and disk, or brake shoe and brake drum. More important, it is not the force, but the torque produced by the force that is slowing down the wheel. Water can reduce this friction and hence torque to almost nothing, which is why stopping after driving through a deep puddle is often very iffy.

Some might see my complaint as merely semantics: so the brake force slow down the wheels’ rotation - what’s the big deal in saying that the brake force slows down the car? The problem is in learning how to apply Newton’s Laws successfully, and specifically the 2nd law, one must identify the forces that are acting on a particular object, or system of objects, and analyze these forces in terms of a coordinate system that is used to describe the motion. For rotational motion, it is torques that must be identified and analyzed. It is essential then to be clear: brakes only act on wheels via frictional torques, the road only acts on tires via frictional forces.

So I applaud the North County times for their treatment of friction between the tires and the road, but must call them on their incorrect reference to the brake force.

The article will serve as a good example (or even test question) down the road….