Elite runners compete in the Boston Marathon for a chance to score a "personal best," but Heartbreak Hill may interfere with their plans. Most of the race is flat or downhill, allowing runners to perform at top speed. Around mile 20, the steep Heartbreak Hill causes them to break their speed and hit the proverbial wall, a term used to describe glycogen depletion during marathon training. Uphill incline decreases speed, but other factors influence how much.
Downhill Acceleration
An uphill incline causes deceleration. In contrast, a downhill incline causes acceleration. The laws of physics state that the greater the incline, the faster an object will slide down it. If you drive along Interstate 70 from Denver, Colorado, toward the ski resorts, the road is flat for the first part of the trip. Suddenly, steep downhills appear along the road as you approach Summit County. This steep downhill causes your vehicle to accelerate.
Friction
Incline is not the only thing that affects speed. If the road is icy, it will lack friction and increase acceleration. A truck driver traveling along that same portion of Interstate 70 will notice signs along the side of the highway indicating "runaway truck" paths, which feature steep, uphill roads lined with gravel. The gravel creates extra friction. This friction, when combined with a steep uphill incline, will cause the truck to slow down and avoid crashing into other vehicles along the road.
Galileo
Galileo discovered that a ball rolling down one hill will pick up enough speed to roll up another. If it rolls without friction, it will roll to the same height it started from on the first hill. A ski turn, for example, creates friction against the snow and allows you to ski in control. Reducing your number of turns therefore increases your speed. The downhill ski race -- the fastest race in the Olympics -- minimizes turning to maximize speed. Ski racers also use ski wax to reduce friction and increase speed.
Practical Example
The rolling hills in a ski area require you to reduce friction on the downhill. Failure to do so means that you will have to climb up the second hill. Your instructor will tell you to assume a skier's tuck position, with your knees bent, your poles behind you and your lower back rounded to create a wind tunnel. This aerodynamic position minimizes wind drag, which is another form of friction. If you assume this position and go down without turning, you will probably make it to the top of the other hill.
Treadmills
Friction also explains why you can run faster on an inclined treadmill than you can on an outdoor hill. Outdoor runners run against wind and air, which is another form of friction. Researchers at the University of Brighton in the United Kingdom found that it is necessary to set the treadmill to a 1 percent incline to equal the energy expenditure of an outdoor running session. The added incline compensates for the lack of friction. The "Journal of Sports Sciences" published the study in 1996.
