P0IITU04 - Friction

P0IITU04 - Friction - balaji&balaji FRICTION 1)...

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Unformatted text preview: balaji&balaji FRICTION 1) Introduction 2) Static friction Solved Problems 3) Kinetic friction Solved Problems 4) Exercises 5) Assessment test Introduction When one surface slides over another or has a tendency to, a force parallel to the interface acts on the two surfaces which resists the sliding. This force is called friction. This force is due to the roughness of the surfaces, the tiny bumps and valleys in a surface that prevent one surface from smoothly sliding over the other and inter molecular attractive forces between the surfaces. These are forces between the molecules of one body and those of another, so friction is ultimately electromagnetic in nature. The normal reaction is also the result of all the electromagnetic forces between the molecules at the surface of one body with those of another. In fact, if one body exerts a force F on another , the component perpendicular to the surface of contact is called the normal reaction and the component parallel to the surfaces is called friction. “Friction opposes motion” You have heard that many times. You have to be careful how you understand that. Lets say you are standing still, and then you start to walk. You have accelerated in the forward direction. What is the force that accelerated you forwards? If you are tempted to say, “ My foot!” , keep in mind that we need an external force to accelerate an object. An object cannot act on itself. Your feet can push on the rest of your body and accelerate it, but if I considered your entire body as one system, then we need an external force to accelerate its center of mass. There is only one external force in the horizontal direction: friction. hence, it must be acting in the forward direction. How can that be? Lets consider another example. A car accelerates forward. What is the force that provided this acceleration? If you are tempted to say, “Why, the engine!”, imagine the same car out in space, drifting along. You hit the accelerator, the wheels spin, you go nowhere. Your engine is not much good. Put the same car on a frictionless surface ( a lake of ice ), the wheels spin, you go nowhere. Put the car on a road with plenty of friction, then watch it go. Friction is the external force that accelerates the car forward. It must then point in the forward direction. How can that be? To understand these two examples, you must look not at the objects as a whole, but at the two surfaces where the friction acts: When you walk, you place your foot on the ground and push backwards. The foot tends to slide backward. The ground exerts an opposing frictional force, in the forward direction. Friction opposes the relative sliding of two surfaces in contact....
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This note was uploaded on 10/05/2011 for the course PHY 203 taught by Professor Grimaldi during the Spring '11 term at St. Mary NE.

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P0IITU04 - Friction - balaji&balaji FRICTION 1)...

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