It’s all about the friction
By S. K. Smith
Pump those brakes! Why? It’s right kind of friction.
I came of age in Montana. Since I was a kid, I have seen it snow every month of the year, except July. And it has snowed in July in some places in Montana, I hear. Driving in snow, ice, and slush indeed presents its challenges, especially when trying to stop safely.
I recall my older brother driving my classmate and me home from school one nasty winter day. He stopped on the icy road to drop off my friend. Smash! In an instant, we all experienced Newton’s first law of motion: Every object moves in a straight line unless acted upon by an outside force. The car behind us moved in straight line all right as it slid into our back bumper, breaking both of its front headlights.
When I turned fifteen, I took driver’s education. Our teacher impressed on us tactics for driving during winter conditions. Slamming on the brakes would most likely precipitate skidding, which was not a good thing. Pumping the brakes was our best ally to literally get a better grip on the road. It wasn’t until I entered college that I fully understood the physics behind this winter driving wisdom.
Our freshman college course concentrated on the laws of motion. The most prevalent natural force in the universe is gravity. Another pervasive macroscopic force is friction. And there are different kinds of friction – static and kinetic.
One of our “real fun” physics labs was to measure coefficients of friction (commonly denoted as µ) for µ (static) and µ (kinetic). Our lab instructor directed us to put metal bricks on ramps. These bricks were tethered to a line stretched over a pulley and attached to counter weights, which would tend to pull the brick up the ramp. Then we would pile on counter weights to observe the balance of forces acting on the brick: The force of gravity on the brick and friction was equal and opposite to the force of counter weights. (Check out, if you wish, the following applet for a demo: http://lectureonline.cl.msu.edu/~mmp/kap4/cd095a.htm)
We needed to note the counter weight (force) on the brick the moment it started sliding up the ramp. This measurement allowed us to calculate the “static” coefficient of friction, µ (static). It got even trickier when the brick started moving. We had to adjust the counter weights and ensure the brick was moving at a constant velocity. The new value of the counter weights allowed us to calculate the “kinetic” coefficient of friction, µ (kinetic).
The point of the lab was to show that the “static” and the “kinetic” coefficients of friction for the same materials were different. Indeed, the static value was greater than the kinetic. Hence, we usually had to nudge the brick to get it moving to overcome static friction.
What does this have to do with winter driving? Everything.
If the driver slams on the brakes and the tires lock, the immobile tires are sliding on the surface beneath it. Hence the friction is “kinetic” since the two surfaces are moving relative to each other. If the driver pumps the brakes to keep the tires rolling, the point of contact of the tires to the surface is at rest relative to one another. Therefore, the friction is “static.”
Here are some measured values for the coefficients of static and kinetic friction of tires on different surfaces (courtesy of http://ffden-2.phys.uaf.edu/211_fall2002.web.dir/Ben_Townsend/StaticandKineticFriction.htm):
Surfaces µ (static) µ (kinetic)
Tire on concrete 1.00 0.80
Tire on wet road 0.60 0.40
Tire on snow 0.30 0.20
The table above shows that rolling tires, gripping the road with static friction, have greater friction than locked tires, sliding on the road with kinetic friction, whether the surfaces are dry, slushy, or snowy. Therefore, rolling tires have the best chance of stopping the vehicle safely. In fact, many cars today have computerized anti-lock braking systems (ABS), which will sense the rotation of the tires and prevent the wheels from locking while braking.
Coefficients of friction may not be the most exciting topic, but their application is very practical. As for my driver’s education teacher, his advice was backed by the sound physics - literally where the rubber meets the road.
So pump those brakes (or get a smart car to do it for you) and be safe!
© January 10, 2009 S. K. Smith