Friction-Blocks-Surfaces-Rapid-PH10572F-P-These-friction-blocks-&-surfaces-can-be-used-to-illustrate-the-concept-that-friction-depends-on-the-nature-&-smoothness-of-materials-P-P-The-kit-consists-of-a-glass-mirror-with-sandpaper-affixed-to-one-side-plus-two-wooden-blocks-one-plain-&-one-with-a-mirror-cemented-to-one-face-Both-static-&-sliding-frictional-forces-between-different-surfaces-can-be-compared-&-contrasted-using-this-kit-P-Rapid-Friction-Blocks-Surfaces