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Resistance in Mechanical Systems Unit 4 – Subunit 1

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When an object rests on a floor, the weight of the object is a force that pushes down on the floor. When an object rests on a floor, the weight of the object is a force that pushes down on the floor. At the same time, the floor pushes back up on the object to counter the objects force. At the same time, the floor pushes back up on the object to counter the objects force. The object is in vertical equilibrium The object is in vertical equilibrium

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Vertical Equilibrium Equilibrium is when two forces are in balance. Equilibrium is when two forces are in balance. Vertical equilibrium is when the weight of an object is countered by an opposing force. Vertical equilibrium is when the weight of an object is countered by an opposing force.

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The force that pushes back (or up) is called the Normal Force (N) because it is perpendicular to the surfaces of contact. The force that pushes back (or up) is called the Normal Force (N) because it is perpendicular to the surfaces of contact. Examples of normal forces are: Examples of normal forces are:floorhandliftthrust

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Remember Remember –When an object rests on a floor, the weight of the object is a force that pushes down on the floor. –At the same time, the floor pushes back up on the object to counter its force. –The object is in vertical equilibrium Question: Can an object in equilibrium be moving? Can an object in equilibrium be moving? Yes – think of being in an elevator. Yes – think of being in an elevator. It also can be moving horizontally > > > > > It also can be moving horizontally > > > > >

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Friction Friction is a force that opposes the force that causes motion. Friction is a force that opposes the force that causes motion. Friction is caused by the roughness of a surface Friction is caused by the roughness of a surface

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The two types of friction are : The two types of friction are : Static – stationary friction this is the initial friction on a non moving object with a force applied. this is the initial friction on a non moving object with a force applied. Kinetic – moving friction This is the friction that is generated as the object moves. This is the friction that is generated as the object moves.

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The amount of friction depends upon: The amount of friction depends upon: –Nature of the surfaces in contact –Force that presses the two surfaces together Friction does not depend upon: Friction does not depend upon: –The rate at which the objects are moving (unless its stationary vs moving) –Size of the contact area

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The Coefficient of Sliding Friction MEWtomorrow

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Remember: The amount of friction depends upon: The amount of friction depends upon: –Nature of the surfaces in contact –Force that presses the two surfaces together The ratio of the frictional force to the normal force is called the Coefficient of Friction The ratio of the frictional force to the normal force is called the Coefficient of Friction

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μ Coefficient of Friction μ Coefficient of Friction μ Pronounced Mew & spelled mu Pronounced Mew & spelled mu μ represents the measure of the frictional characteristic between the surfaces of materials μ represents the measure of the frictional characteristic between the surfaces of materials The higher the value of μ, the more frictional force is present when sliding occurs (or movement) The higher the value of μ, the more frictional force is present when sliding occurs (or movement)

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The formula for the coefficient of friction: The formula for the coefficient of friction: μ = f / N μ = f / N Where f = the frictional force generated N = the Normal force (= to weight)

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The three μs (in order of increasing resistance) (in order of increasing resistance) 1. μ s = coefficient of static friction 2. μ k = coefficient of kinetic friction 3. μ r = coefficient of rolling friction

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See example 4-A and 4-B on page 11 See example 4-A and 4-B on page 11 Complete problems 1,2 and 6,7,8 on pages 22 & 23. These problems deal with mechanical resistance. Complete problems 1,2 and 6,7,8 on pages 22 & 23. These problems deal with mechanical resistance. Tomorrow: Tomorrow: –Effects of lubricants –Rolling friction –Drag in fluid systems

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Lubricants in Mechanical Systems Lubricants protect sliding surfaces by providing a thin layer of fluid between them. Lubricants protect sliding surfaces by providing a thin layer of fluid between them. The lubricant between two surfaces does three things: The lubricant between two surfaces does three things: 1. Reduces friction 2. Reduces wear 3. Reduces heat

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Drag Drag is a mechanical resistance produced when an object moves through a fluid. Drag is a mechanical resistance produced when an object moves through a fluid. A fluid can be either a liquid or a gas A fluid can be either a liquid or a gas

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The formula for Drag Resistance is: The formula for Drag Resistance is: R d = F/v Where: R d = drag resistance R d = drag resistance F = drag force V = speed (velocity)

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Assuming constant velocity, an airplane will be in both horizontal and vertical equilibrium as it moves through the air Assuming constant velocity, an airplane will be in both horizontal and vertical equilibrium as it moves through the air

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Streamlining Streamlining reduces Drag Streamlining reduces Drag Streamlining reduces turbulence Streamlining reduces turbulence

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Complete problems 2, 3,4, 8 & 9 - pg 22/23 Complete problems 2, 3,4, 8 & 9 - pg 22/23 Complete lets Review on top of page 22 (units) Complete lets Review on top of page 22 (units) Complete Student Exercises on page 17 Complete Student Exercises on page 17

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