Friction is a force which affects the motion of materials. Whenever one object is in contact with another, a force of friction acts in the opposite direction to the motion. Friction is often caused by the irregularities in the two surfaces which are touching, because the two materials must slide or slip over these slight imperfections. It takes more force to make the materials move past one another when the surfaces are rough than when the surfaces are smooth. The amount of friction is related to the types of materials which are in contact and how much of each of the surfaces are pressed together.
Friction does not just occur between solids but it can occur between ANY materials that are in contact. For example, a solid moving through a gas or liquid also creates resistance. Air resistance is the name for the force that occurs when a fluid like air (air flows, so it is a fluid!) contacts something moving through that air. Air resistance, like other forms of friction, is considered to be a nonconservative force, because energy is lost to the rest of the Universe or lost into another form.
When an object falls through a medium that resists its fall and reaches a constant speed, the speed is called terminal velocity. For a falling object, the upward force of the air resistance must equal the downward force of gravity on the falling object, since we remember that a constant velocity implies an acceleration of zero. While the magnitude of terminal velocity will depend upon things like an object’s mass, shape, and size, for this experiment the magnitude of the terminal velocity is specifically related to the upward resistant force of the air on the falling object.
Falling coffee filters generate a reasonable amount of air resistance. In this experiment we will vary the mass of a falling coffee filter and observe the filter’s resulting terminal velocity.