(а) (b) Fnet (N) 500- 400 - Dancer begins 300 |bending knees./ 200- Dancer leaves ground. 100 - 0- t (s) 0.10 0.30 / 0.50 0.70 - 100 - -200 -300 FIGURE P9.84 -400 -
Understanding the details of timing and forces in motion can improve the performance of athletes, including dancers. Consider the forces involved in a ballet jump called a sauté demi plié. P9.84a shows the sequence of moves in the jump. The dancer starts upright, then quickly bends her knees, moving downward. After she reaches the bottom of this dip, she extends her legs, pushing herself upward. After this upward push, she leaves the ground, beginning a short period of time in the air. P9.84b is a slightly idealized graph of the net force on a 42 kg dancer executing this move.
The sauté demi plié begins with a phase in which the net force on the dancer is negative. During this phase of the jump,
A. The normal force of the floor on her is zero.
B. The normal force of the floor on her is less than her weight but greater than zero.
C. The normal force of the floor on her is equal to her weight.
D. The normal force of the floor on her is greater than her weight.
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