Projected Motion While in Air
Getting Started
The easiest way to start studying projectile motion is to study the path of the object being projected, in this case a figure skater. A skater could jump straight up in the air, with absolutely no horizontal displacement - just a regular vertical jump on skates. However, this is not done in competition, skaters actually have vertical and horizontal displacement during theirs jumps. Furthermore, skaters are usually trying for the biggest jumps possible, high and covering a long distance.
When jumping for distance and height, the trajectory of the
object is affected by the constant acceleration of gravity and
air resistance. In sports such as speed skating and ski jumping,
air resistance is critically important. Generally the faster an
object is moving, the more important the influence of air resistance.
In figure skating, the skater's generally are not moving fast
enough for air resistance to affect their jumps.
To follow the path of a skater's jump, it is important to identify the center of mass (COM) of the skater. This is critical, because a skater is moving his or her arms whilst in the air. If you follow the path of the left wrist, for example, you would see not only motion due to the skater's jump, but also motion due to the movement of the skater's arm. This confounds the problem. We want only to study the projectile motion part of the jump. This is done by identifying the center of mass, which is the point where the skater's mass is considered to be concentrated. It is also the point of the system whose motion is affected by gravity only. All the other parts of the body can be manipulated by the skater as he changes body position in air to complete his rotation.
While the center of mass is not a point which can be visually observed on a skater, it does reside in the trunk region. Thus, when looking at the path of a figure skating jump, it is convenient to follow the path of the hips as a representation of the path of the center of mass.
Look at the following jump, and follow the path of the hip.