This paper describes the use of a smartphone's sensors to investigate the motion of a torsion pendulum to demonstrate energy conservation. The smartphone was placed on and attached to a metal disk hanging by a wire. The oscillation of the disk was measured using the smartphone's sensors to simultaneously record angular position, angular speed and angular acceleration. These experimental data were processed to demonstrate mechanical energy conservation during oscillation. We expect that this smartphone-based experiment will be useful for physics teachers and can make students better understand an oscillating disk's relationship with energy conservation.