The ping pong ball water cannon

TL;DR

This paper describes an educational physics experiment where a water-filled container with a floating ping-pong ball ejects the ball at high speed upon impact, illustrating fluid dynamics principles through student-led investigation.

Contribution

It introduces a novel teaching project demonstrating complex fluid impact phenomena and provides detailed experimental analysis and explanation of the high-velocity ejection mechanism.

Findings

Free flight is essential for the phenomenon.

Momentum transfer matches theoretical predictions.

Wettable surfaces and stirring increase momentum transfer.

Abstract

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.