Chapter 8, Problem 78PP

BIO Momentum and the archerfish. Archerfish are tropical fish that hunt by shooting drops of water from their mouths at insects above the water’s surface to knock them into the water, where the fish can eat them. A 65 g fish at rest just on the water’s surface can expel a 0.30 g drop of water in a short burst of 5.0 ms. High-speed measurements show that the water has a speed of 2.5 m/s just after the archerfish expels it.

78. A fish shoots a drop of water at an insect that hovers on the water’s surface. Just before colliding with the insect, the drop is still moving at the speed it had when it left the fish’s mouth. In the collision, the drop sticks to the insect, and the speed of the insect and water just after the collision is measured to be 2.0 m/s. What is the insect’s mass?

1. A.    0.038 g
2. B.     0.075 g
3. C.     0.24 g
4. D.    0.38 g