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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.**

At the same time, the floor pushes back up on the object to counter the objects force. The object is in vertical equilibrium

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**Vertical Equilibrium Equilibrium is when two forces are in balance.**

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. Examples of normal forces are: floor hand lift thrust

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**Can an object in equilibrium be moving? **

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? Yes – think of being in an elevator. It also can be moving horizontally > > > > >

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

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**The two types of friction are :**

Static – stationary friction 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.

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**The amount of friction depends upon:**

Nature of the surfaces in contact Force that presses the two surfaces together 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**

MEW tomorrow

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**The amount of friction depends upon:**

Remember: 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”

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μ Coefficient of Friction “μ“ Pronounced Mew & spelled mu 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)

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**The formula for the coefficient of friction:**

μ = f / N Where f = the frictional force generated N = the Normal force (= to weight)

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**(in order of increasing resistance)**

The three μ’s (in order of increasing resistance) μs = coefficient of static friction μk = coefficient of kinetic friction μr = coefficient of rolling friction

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**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. 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. The lubricant between two surfaces does three things: Reduces friction Reduces wear Reduces heat

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

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**The formula for Drag Resistance is: Rd = F/v Where:**

Rd = 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

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Streamlining Streamlining reduces Drag Streamlining reduces turbulence

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**Complete problems 2, 3,4, 8 & 9 - pg 22/23**

Complete “lets Review” on top of page 22 (units) Complete “Student Exercises” on page 17

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