  When a blunt body impacts an air-water interface, large hydrodynamic forces
often arise, a phenomenon many of us have unfortunately experienced in a failed
dive or "belly flop." Beyond assessing risk to biological divers, an
understanding and methods for remediation of such slamming forces are critical
to the design of numerous engineered naval and aerospace structures. Herein we
systematically investigate the role of impactor elasticity on the resultant
structural loads in perhaps the simplest possible scenario: the water entry of
a simple harmonic oscillator. Contrary to conventional intuition, we find that
"softening" the impactor does not always reduce the peak impact force, but may
also increase the force as compared to a fully rigid counterpart. Through our
combined experimental and theoretical investigation, we demonstrate that the
transition from force reduction to force amplification is delineated by a
critical "hydroelastic" factor that relates the hydrodynamic and elastic
timescales of the problem.
