  The course "Phy Ex" was created by Yves Couder in the Paris VII university to
teach experimental physics through projects. In this article, we present this
teaching method through a particular project that took place in the autumn
semester 2019: the ping-pong ball water cannon. In this experiment, a glass
containing water and a floating table tennis ball is dropped from some height
to the ground. Following the impact, the ball is ejected vertically upwards at
speeds that can be several times the impact speed. We report the student team's
initial dimensional and order-of-magnitude analysis, and describe the
successive experimental setups that showed (1) that free flight is essential
for the phenomenon to occur, (2) that the order of magnitude of the ball
ejection momentum is correctly predicted by a momentum balance based on
integrating the pressure impulse during impact and (3) that making the ball
surface more wettable, or stirring the liquid, drastically increases the
momentum transfer. The proposed explanation, confirmed by direct high-speed
video observations, is that the immersion depth of the ball increases during
free fall due to capillary forces or vortex depression-in the absence of
buoyancy-and that the enormous excess pressure on the bottom of the ball during
impact drives the ball up towards its buoyancy equilibrium. The transfered
momentum is sufficient to expel the ball at high velocity, very similar to the
formation of liquid jets in collapsing cavities in liquids.
