However, when a moving object hit a soft material, such as sand
, it is obvious that the energy is not conserved because the sand seems to absorb the movement.
This puzzle remained unsolved until it was shown that moving objects have energy that can be changed into other forms (pp. 24-25).
Thus, when a moving place" object hits sand the energy is not lost, but changed into a form such as heat. All forms of energy are either kinetic or potential energy (pp.14-15)