Conceptual Physics (12th Edition)

Published by Addison-Wesley
ISBN 10: 0321909100
ISBN 13: 978-0-32190-910-7

Chapter 6 - Think and Discuss - Page 107-108: 100


Call the astronauts A, B, and C, labeling them from left to right, and suppose they start at rest relative to a nearby space station. When A throws B, they move away from each other at equal speeds by conservation of momentum. Say that A has velocity -V, and B has velocity +V. When C catches B, they move to the right at velocity V/2. (see Figure 6.14 on page 99). When C then throws B toward the left, she recoils at speed V, in her own reference frame. Remember, the astronauts are equally strong. Her velocity relative to the nearby space station is V + V /2 = 1.5V, to the right. Meanwhile, the velocity of B is the V/2 she initially had while coming toward C, plus the - V from the second throw, resulting in a velocity of -V/2, directed to the left. This is slower than astronaut A, still moving at -V by Newton's first law, so B will never catch up to A. There will be no more throws, the game has ended, and someone had better rescue these astronauts!
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