Chapter 4, Problem 53PP

BIO Forces on a dancer’s body. Dancers experience large forces associated with the jumps they make. For example, when a dancer lands after a vertical jump, the force exerted on the head by the neck must exceed the head’s weight by enough to cause the head to slow down and come to rest. The head is about 9.4% of a typical person’s mass. Video analysis of a 65 kg dancer landing after a vertical jump shows that her head slows down from 4.0 m/s to rest in a time of 0.20 s.

50. What is the magnitude of the average force that her neck exerts on her head during the landing?

1. A.    0 N
2. B.     60 N
3. C.     120 N
4. D.    180 N

53. The forces on a dancer can be measured directly when a dancer performs a jump on a force plate that measures the force between her feet and the ground. Figure 4.41 is a graph of force versus time throughout a vertical jump performed on a force plate. What is happening at 0.4 s? The dancer is

Figure 4.41

Problem 53.

1. A.    bending her legs so that her body is accelerating downward.
2. B.     pushing her body up with her legs and is almost ready to leave the ground.
3. C.     in the air and at the top of her jump.
4. D.    landing and her feet have just touched the ground.