Friction

Another force is the force of friction. Friction is a force between an object and the ground, which always acts to oppose the motion of the object. For example, after a skater pushes against the ice and the ground reaction force causes him or her to glide forward, friction acts between the blade and the ice to slow the skater down. We know this, because if there were no friction the skater would continue gliding forever. This would be an example of Newton's 1st law--every object remains at rest or in at a constant velocity until acted upon by a non-zero net force. Since the skater slowly glides to a halt after pushing against the ice, we know that a force is opposing his or her motion causing him or her to slow down. The magnitude of the force of friction depends on the weight of the object and the surface. You may not think of ice as having much friction. In fact, the friction depends greatly on the temperature and how many other skaters have already skated on the ice, roughing up the surface. For example, very cold ice is not as slippery as warmer ice. A scenario of two skaters, one on very cold ice and one on warmer ice has been developed to illustrate this concept.