Another treadmill thread: am I more correct than a physics professor? I'm a freshman in college; I took AP Physics C: Mechanics last year in high school. Today my roommate and I got in an argument over whether or not it is easier to run on a treadmill than on flat ground. My friend argues that since the treadmill is moving underneath you, you don't need to exert as much horizontal force on it so it's easier. I argue that when you're running at constant velocity with ideal form, you don't exert any horizontal force on the ground (except for a slight force to counteract air resistance). My roommate said I was retarded and that when we run on the ground, we obviously exert a horizontal force to propel ourselves forward. He says it's biomechanics, and we can't just treat ourselves like blocks. He cited many sources, including a page made by a physics professor. I'm at least 99% confident that I'm right, and yet this guy with a Ph.D says you have to exert a force at a 45 degree angle to the ground with each stride. Am I right while this professor is wrong? If I am wrong, please explain why. Thanks.