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Seven identical birds are flying north in a flock at constant velocity v. A hunter shoots one bird, which instantly becomes motionless and falls straight to the ground. The remaining six birds continue flying north at the same original speed v. After the dead bird has landed, what is the velocity of the center of mass of all seven birds?
- Continues north at the original speed v, but is now located some distance behind the six flying birds.
- Continues north, but at 6/7 of the original speed.
- Continues north, but at 1/7 of the original speed.
- Stops with the dead bird.
Correct answer: Continues north, but at 6/7 of the original speed.
Solution
Once the dead bird is at rest on the ground, the total momentum of the seven-bird system is 6mv northward. Dividing by the total mass 7m gives v_CM = 6v/7 northward.
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