3. The frictional force See动画库/力学夹/204功的计 算举例(1) The total work done by frictional force is not zero in a round trip. Ch 12 Energy II: Potential energyCh.12 Energy II: Potential energy 3. The frictional force The total work done by frictional force is not zero in a round trip. See 动画库/力学夹/2-04功的计 算举例 (1)