11. Cleats or spikes on shoes make friction because they provide a surface. 12. Rolling friction is than sliding friction. 13. Ball bearings purpose is to create friction. 14. In order for a train to move on the tracks there must be friction between the and the . 15. Heat shields on a space craft are necessary when it reenters the earth’s atmosphere because at the high speed of entry air will . 16. Crickets use friction to . 17. Mucous on slugs is remarkable because it’s both and . 18. When air molecules into each other they cause friction.