Chapter 12, Problem 2CQ

If you launch a projectile upward with a high enough speed, its kinetic energy is sufficient to allow it to escape the earth’s gravity—it will go up and not come back down. Given enough time, hydrogen and helium gas atoms in the earth’s atmosphere will escape, so these elements are not present in our atmosphere. Explain why hydrogen and helium atoms have the necessary speed to escape but why other elements, such as oxygen and nitrogen, do not.