Lighter gases rise. Helium balloons float away. Why doesn’t the atmosphere fractionate, with heavier gases closer to the ground and lighter gases at higher altitudes? (Thanks to Sebastian Kuhn of Old Dominion University for suggesting the question.)

How many major league fastballs need to hit a 1 sq. meter target each second to produce an average force equal to the force of 1 atmosphere on 1 sq. meter? (Thanks to B.H. Suits of Michigan Technical University for suggesting the question.)

Look for the answers online at tpt.aapt.org. Question suggestions are always welcome! For more Fermi questions and answers, see Guesstimation 2.0: Solving Today’s Problems on the Back of a Napkin, by Lawrence Weinstein (Princeton University Press, 2012).

AAPT members receive access to The Physics Teacher and the American Journal of Physics as a member benefit. To learn more about this member benefit and becoming an AAPT member, visit the Joining AAPT page.