When a dancer begins to move, there is also physics involved in the process. In order for a motionless dancer to begin to move, or accelerate, they must move one leg forward while exerting a backward force on the foot remaining in place. This can easily be demonstrated by having students pay attention to the way they begin to walk from a still position. At the same time a foot moves forward, a backward force is applied to the foot remaining still. This process is repeated over and over and allows us to walk. In the case of a dancer, the process can be even more dramatic by increasing the force with each motion and gaining momentum. Motion in a Curved Path The term acceleration not only refers to a change in speed, but also a change in direction of motion. One such change in direction often used in ballet is motion in a circular path. As a dancer travels around the stage in a circular path, there is a force on the body as the dancer pushes off the floor which is directed toward the center of the circle. This force causes a constant change in motion which allows the dancer to travel on the curved path. The dancer will also lean toward the center of their circular path as they travel. This lean will prevent the dancer from leaving the circular path. Jumps All jumps involve vertical acceleration and forces. In order for a dancer to jump, they must exert a force downward, against the floor, which is greater than their weight. To increase the force created by a dancer, many vertical jumps begin from the plié position, or from a position with bent legs. The motion leading into a jump is important also. A jump from a plié at rest position is less effective then a jump moving into and out of the plié in one smooth motion. Jumps can also be performed with horizontal motion. These moves produce trajectories, or paths of motion through space. Many forces come into play here including friction and gravity. Once the dancer loses contact with the floor, the dancer’s center of gravity will follow a parabolic trajectory, or curved path, that is determined by the conditions of motion at the beginning of the jump. The dancer may change the shape and configuration of the body while in flight, but the path will remain the same. The path will be a combination of a motion rising with decreasing speed, an instant with no vertical speed and finally an accelerated downward motion. This is shown in the diagram. The arc shown by the black line is the parabolic path of the jump. You can also see that the dancer only jumped two feet vertically, but traveled over seven feet horizontally. The effectiveness of jumps also depends on the strength, height and weight of the dancers performing them. For example, in vertical jumps with a partner, a shorter dancer might have a harder time jumping high enough to fill the time if the music is of a slow tempo. However, a taller dancer’s attempt might not be as impressive since the audience sees the height of a jump as compared to the dancers overall height. In the Classroom Projectile motion: Leaping dancers are cannonballs! Demonstration: Stand in front of the classroom tossing a soft ball (tennis, beach) across the room. Have students describe the shape of the path of the ball- a parabola. Use this demonstration to discuss the path of projectiles and the relationship between free flying objects in motion and gravity. Were it not for the force of gravity, when we jump up, we would keep moving up in a straight path. When dancers leap, they are following the same type of path, a projectile. One other characteristic of a projectile is that the vertical velocity that the object moves approaches zero at the top of the projectile path and then accelerates once it/ he/ she begins to descend. The PLIE ( the bend that takes place before the jump) is critical in applying a force against the floor that is greater than the dancers own body weight. The FORCE generated by the soles of the feet against the floor is not enough. Try jumping from a straight leg. The PLIE must be enough to create sufficient acceleration to cause the dancer’s body to become airborne. Too great a bend can make the jump less productive. Another factor affecting the dancer’s jump is the ankle’s range of motion and for the ballet dancer use of the turned out position also diminishes the force a bit. All jumps have a vertical acceleration, but leaps also must have horizontal acceleration. This is acquired through FRICTION.