A
hoverfly is happily maintaining a fixed position about 10 ft above
the ground when an elephant charges out of the bush and collides
with it. The fly bounces elastically off the forehead of the
elephant. If the initial speed of the elephant is 20 mph, what is
the speed of the fly after the collision? Hint: The mass of the fly
is significantly smaller than the elephant, so that the mass of the
fly is approximately 0.