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Forces, Motion and Friction Friction Properties Kinetic and Static Friction Drag Force Terminal Speed Uniform Circular Motion pps by C Gliniewicz

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Friction is a force which occurs due to cohesion and adhesion between two surfaces in contact with one another. Adhesion between two different substances occurs when molecular bonds partially extend from one surface to the other, while cohesion occurs when objects made of the same substance pass over each other and molecules attract each other. Microscopic hills and valleys in the substances’ surfaces mesh and also hold the surfaces together. When one tries to slide the surfaces over each other, the adhesion, cohesion and uneven surfaces make it difficult to move the surfaces over each other. The force needed to move the objects is the friction force. Friction always acts parallel to the surfaces. Friction depends on the materials in contact and varies widely. Friction depends on the force pressing the surfaces together. Friction depends very little on the area of the surfaces in contact unless the area becomes very small. Static friction is the force which keeps the motionless on the surface when a force is first applied. Kinetic friction is the force on an object when it is sliding across a surface. pps by C Gliniewicz

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Static friction force is always equal to or greater than the kinetic friction force. One observes this when one gradually increases the force on a stationary object. The object will jerk when it first starts to move as static friction is overcome and the force is greater than the kinetic friction needed to keep it moving. If the object does not move, the static friction only equals the opposing force. If it were greater, the object would move in the opposite direction of the force. The maximum value of the static friction force is :Is the coefficient of friction and is the ratio of the friction force to the normal force. If the object is sliding across a surface, the magnitude of the friction force rapidly falls to the kinetic friction force, given by Most tables list both coefficients for pairs of substances in contact. The are only a few materials where the coefficient of static friction exceeds 1.0, the most common of which is rubber on rubber where, depending on the formulation, it can reach 1.4. pps by C Gliniewicz

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A fluid is any substance which flows, generally a gas or liquid. Whenever there is a difference in velocity of the object and the fluid, there is a drag force. In the equation above C is the drag coefficient which usually varies between 0.4 and 1.0. is the density of the fluid through the object is moving. A is the cross sectional area of the object moving through the fluid. v is the velocity of the object. As an object falls through air, it begins falling at 9.81 m/s², but drag begins to slow its acceleration. Note that we are using g as positive in the equation above. Since we know that weight acts downward giving an acceleration which is downward. Terminal velocity is reached when the acceleration reaches zero and the drag force is equal to the weight of the object. Substituting for the drag force, the equation for the terminal velocity can be found. pps by C Gliniewicz

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If one rewrites the drag equation to depend on velocity alone, one finds the following: If the time, t, is very large, then the second term approaches zero and terminal velocity is just g/k. Note that if the initial velocity is greater than g/k, the terminal velocity is the same. This occurs for a space craft returning to earth. pps by C Gliniewicz

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Centripetal acceleration causes a centripetal force. This is the equation one uses if, for instance, a car travels around a curve on a flat track because friction is the centripetal force. Most tracks, however, are banked to allow greater speed in the turns. This means that in addition to friction, there is the force acting down the incline. Since centripetal means “center seeking,” only the horizontal component of that force is centripetal. Also, the normal force acting on the car changes and the horizontal component of that force also is centripetal. If the car has a wing and ground effects acting on it, those force increase the friction force. Using just the wing and ground effects, a car travelling in excess of 201 mph could move along the ceiling of a building. Getting there and returning to the ground would be the hard part. pps by C Gliniewicz

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