Momentum
posted by Lui on .
A 8.4 kg laundry bag is dropped from rest at an initial height of 7.00 m.
(a) What is the speed of Earth toward the bag just before the bag hits the ground? Use the value 5.98 1024 kg as the mass of Earth.
(b) Use your answer to part (a) to justify disregarding the motion of Earth when dealing with the motion of objects on Earth.

mgh =mv²/2.
v=sqrt (2•g•h) = sqrt(2•9.8•7) = 11.71 m/s
The speed of the bag respectively the Earth is equal to the speed of Earth respectively the bag.