Analyzing the physics of dance can add fundamentally to a dancer’s skill. Although dancers seldom see themselves totally in physical terms—as body mass moving through space under the influence of well-known forces and obeying physical laws—neither can they afford to ignore the physics of movement. For example, no matter how much a dancer wishes to leap off the floor and then start turning, the law of conservation of angular momentum absolutely prevents such a movement.
Some movements involving primarily vertical or horizontal motions of the body as a whole, in which rotations can be ignored, can be studied using simple equations of linear motion in three dimensions. However, rotational motions require more complex approaches that involve analyses of the way the body’s mass is distributed, the axes of rotation involved in different types of movement, and the sources of the forces that produce the rotational movement.
17. The primary purpose of the passage is to
(A) initiate a debate over two approaches to analyzing a field of study
(B) describe how one field of knowledge can be applied to another field
(C) point out the contradictions between two distinct theories
(D) define and elaborate on an accepted scientific principle（B）
(E) discuss the application of a new theory within a new setting
18. The author mentions all of the following as contributing to an understanding of the physics of dance EXCEPT:
(A) the law of conservation of angular momentum
(B) analyses of the way in which the body’s mass is distributed
(C) equations of linear motion in three dimensions
(D) analyses of the sources that produce rotational motions（E）
(E) the technical terms for movements such as leaps and turns
19. The author implies that dancers can become more skilled by doing which of the following?
(A) Ignoring rotational movements
(B) Understanding the forces that permit various movements
(C) Solving simple linear equations
(D) Learning the technical terms utilized by choreographers（B）
(E) Circumventing the law of conservation of angular momentum
20. Analysis of which of the following would require the kind of complex approach described in lines 14-19?
(A) A long leap across space
(B) A short jump upward with a return to the same place
(C) A sustained and controlled turn in place
(D) Short, rapid steps forward and then backward without turning（C）
(E) Quick sidesteps in a diagonal line