Running is often used to stay in shape. running can be related back to newtons 3rd law. the force a runner puts back onto the ground is what the ground pushes back allowing the runner to be propelled forward. the more force applied on the ground the farther or faster someone will go according to newtons second law -- Fnet = MA. the more force applied on the ground by the runner (who has a constant mass) the higher his or her (lets not be sexist now) will go. running can also be related to friction. In a frictionless world running would not be possible because the would be nothing for the shoes of the runner to push back against. People would only be able to run in place which while would be very cool wouldn't be as invigorating.