Name: Class: Date: WHAT IS FRICTION? Friction Friction is the force that opposes the motion of two surfaces that rub together. Friction acts in a direction opposite to the direction of the moving object. Friction will eventually cause a moving object to come to a stop. What Causes Friction? A surface that seems very smooth, like polished metal, is actually is rough when you look at it close enough. If you examine the surface of the metal under a powerful microscope, you will see tiny bumps. When two surfaces are pushed together, sticking happens where the bumps touch one another. These places are called microwelds. Micro means “very small”. Friction is caused by the microwelds that form when two surfaces are touching. To move one surface over another, a net force must be applied to break the microwelds. The amount of friction between two surfaces depends on two things: (i) the types of surfaces touching each other and (ii) the amount of force pushing the two surfaces together. The rougher the surface, the more microwelds there are and the more “sticky” the surface is. The greater the force pushing the two surfaces together, the stronger the microwelds since more of the bumps are touching each other. There are four types of friction: static friction, sliding friction, rolling friction and fluid friction. Type Description Example Static friction Acts on objects that are not moving A table resting on the floor Sliding friction Occurs when 2 solid surfaces slide across one another Moving a box across the floor Rolling friction When an object rolls across a surface A tire rolling on a road Fluid friction When an object moves through a fluid (liquid or gas) A boat moving through water Air Resistance Air resistance is a special type of fluid friction that occurs when an object falls through the air. When you try to walk against a very strong wind, you can feel air resistance acting on you. It also occurs on skydivers falling through the air. Air resistance is not the same for all objects. The greater the surface area of an object, the greater the air resistance. Parachutes work because their very large surface area maximizes air resistance to slow down a skydiver. Air resistance also increases with velocity. As the velocity of a falling object increases, air resistance increases until it is equal with the force of gravity. When the two forces are balanced, the falling object stops accelerating and the velocity remains the same. This greatest velocity that can be achieved by a falling object is called terminal velocity. REVIEW - WHAT IS FRICTION? 1. What is the definition of friction? 2. Friction acts in a direction _______________ to the object’s direction of motion. 3. The strength of the force of friction depends on what two factors? 4. Describe how microwelds explains how there can be friction between two seemingly smooth surfaces. Match each type of friction with its definition by writing the letter of the correct definition in the right column in the space beside the term in the left column. Type of Friction Definition ____ Fluid friction A. The kind of friction that results when a wheel turns on a surface. ____ Static friction B. The kind of friction that results when you rub sandpaper against wood. ____ Rolling friction C. The kind of friction that exists between oil and a door hinge, for example. ____ Sliding friction D. The kind of friction that slows a falling object as it moves through the air. ____ Air resistance E. The kind of friction that occurs even when two objects are not moving. 5. What two factors can increase the force of air resistance for an object? 6. What is terminal velocity? What can be said about the forces on the object at terminal velocity?