Physics of Curling

One of my favorite sports to watch during the winter Olympics is curling. Curling may seem simple and boring on the surface, but it is really a complex sport with much skill and strategy involved. There are two important principles of physics that influence curling. The first principle is friction. According to Newton's third law of motion, for every action there is an equal and opposite reaction. So, as the stone slides along the ice, it exerts a force on the ice, and the ice exerts a force back on the stone. Friction can be defined as the reaction of the ground to the forces exerted on it in the horizontal plane by the person or object (in curling, we are concerned with the stone). The friction force on the stone will act in the opposite direction in which the stone is moving, and will decrease the stones velocity, thus slowing it down. If the curlers want the stone to travel further, then they need to reduce the amount of friction. This is done by "sweeping" the ice in order to make it smoother and reduce friction. Sweeping is also done to make the stone "curl."

Another important principle of curling is collisions. One of the strategies in curling is to knock the other teams stone(s) out of the house (scoring area). This involves the collision of one or more stones. Collisions occur frequently in sports, such as hitting a baseball, tennis ball, volleyball, etc. Again, because of Newton's third law, when two objects collide, they exert equal and opposite forces on each other. The sum of the momentum of the two objects will be the same before and after the collision, therefore, there is a conservation of linear momentum. Momentum is defined as the mass of an object or system multiplied by its velocity. Most curling stones have a mass of around 20 kg. So, a stone with a mass of 20 kg and velocity of 7 m/s (meters per second) linear momentum (G) would be 140 kgm/s. When the stone that has been delivered collides with a stone in the house, a change in momentum will occur between the two stones. Because the mass of the stones will remain constant, the change in momentum will be related to the change in velocity of the two stones. Because of the conservation of linear momentum, if the stone in the house's velocity increases to 4 m/s (from 0 m/s before the collision because it was stationary) after the collision, than the stone that was delivered will have a decrease in velocity from 7 m/s to 3 m/s. Check out the video below for some good curling throws.

Top Curling throws

Southeast ACSM

This past Thursday, Dr. Hale and myself traveled to Greenville, S.C., for the annual meeting of the Southeast Chapter of the American College of Sports Medicine. This three day conference involved meetings and research presentations by faculty and students from across the southeast. I did an oral presentation Friday morning on lateral ankle sprains, and Dr. Hale had a couple of graduate students making presentations. Generally, ACSM is composed more of exercise physiologists than biomechanists, which was never more evident than at the student quiz bowl when they decided to skip the Anatomy category (this was not a popular move with the non exercise physiologists). However, the president of SEACSM, Dr. Grandjean, made a concerted effort to have more biomechanics presentations and tutorials at this year's meeting, and hopefully this trend will continue in the future. Regional conferences are generally more laid back than national conferences, and it is easier to network and discuss research and teaching with other professionals. As you can tell in the picture, we got about 3-4" of snow Friday evening, which was nice for me because we don't get a lot of snow in Mississippi. We plan on returning to the conference next year with a greater number of our students. We want to make Mississippi State a force at this regional meeting.

Dwight Freeny's Ankle Injury

One of the big stories leading up to the Super Bowl this weekend is the status of Dwight Freeny's ankle. Freeny, a defensive end, is one of the best players in the NFL. The media is not giving a very accurate description of Freeny's injury, calling it a "lower" ankle sprain, or "basketball" ankle sprain, so I thought I would try to help out. From watching the video of the injury, Freeny jumped up in the air, and his right foot lands on the foot of Jets quarterback Mark Sanchez. This forced his ankle into excessive inversion (pointing the bottom of the foot in) and plantar flexion (pointing the bottom of the foot down). This mechanism of injury will sprain the lateral ligaments of the ankle, which include the anterior talofibular ligament (ATFL), the posterior talofibular ligament (PTFL), and the calcaneofibular ligament (CFL). The lateral ankle sprain is the most common type of ankle sprain, accounting for about 85% of all ankle sprains. The media is also reporting that Freeny has a Grade III ligament sprain. A grade III sprain indicates a complete rupture of the ligament or ligaments. This would cause a great deal of pain and instability of the lateral ankle, making it difficult for him to push off and change directions. It typically takes several weeks or months to return from a grade III sprain, and may require surgery. I'm sure the Colts athletic trainers and doctors will be working with Freeny around the clock so that he can play Sunday, but I would say if the reports of the extent of his injury is true, he will have a hard time getting on the field.