Four separate experiments were performed to compare the friction of different objects with gravity. One experiment used an ice cube. Another experiment used a rock. The third used an eraser, and the last used a wood block. Each object is placed at
one end of a plastic tray. The end of the tray is lifted slowly. A metric ruler is used to measure the height of the raised end of
the tray at which each object begins to slide.
Identify one similarity between the four experiments