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**Lecture 38: Examples of Friction Analysis**

ENGI 1313 Mechanics I Lecture 38: Examples of Friction Analysis

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Lecture 38 Objective to illustrate the equilibrium analysis of rigid bodies subjected to dry friction force by example

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**Miscellaneous Notes Lecture 37 Notes Have Been Reposted**

Minor corrections Quiz and Mid-Term Marks Posted

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Example 38-01 The 800-lb concrete pipe is being lowered from the truck bed when it is in the position shown. If the coefficient of static friction at the points of support A and B is μs = 0.4, determine where it begins to slip first: at A or B, or both at A and B.

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**Example 38-01 (cont.) Three Analysis Cases Impending motion at A**

Impending motion at B Impending motion at A & B

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Example (cont.) Draw FBD y x FA FB NA W = 800lb NB

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**Example 38-01 (cont.) Assume Impending Motion at A & B Analysis y x FA**

FB NA W = 800lb NB

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Example (cont.) Analysis y x FA FB NA W = 800lb NB

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**Example 38-01 (cont.) Assumption What is Calculated Check Assumption**

not correct y x FA FB NA W = 800lb NB

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**Example 38-01 (cont.) Assume Impending Motion at A y x FA FB NA**

W = 800lb NB

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Example (cont.) Analysis y x FA FB NA W = 800lb NB

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Example (cont.) Analysis y x FA FB NA W = 800lb NB

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**Example 38-01 (cont.) Assumption What is Calculated Check Assumption**

Slip impending at A y x FA FB NA W = 800lb NB

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Example 38-02 The man having a weight of 150 lb pushes horizontally on the bottom of crate A, which is stacked on top of crate B. Each crate has a weight of 100 lb. If the coefficient of static friction between each crate is μs1 = 0.8 and between the bottom crate, his shoes, and the floor is μs2 = 0.3, determine if he can cause impending motion.

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**Example 38-02 Impending Motion? A/B Interface B/Floor Interface**

Man/Floor Interface Potential impending motion at all three interfaces (rare) Crates tip

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**Example 38-02 (cont.) Assume Impending Motion for Crate A FBD Crate A**

Analysis FA W = 100lb NA

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**Example 38-02 (cont.) Assume Impending Motion for Crate B FBD Crate B**

Analysis FB W = 200lb NB

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**Example 38-02 (cont.) Assume Tipping Minimum P FBD Crate A & B**

Analysis Minimum P FB W = 200lb NB

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**Example 38-02 (cont.) Force Exerted by Man? FBD Analysis**

W Pmin FM NM Impending motion

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**References Hibbeler (2007)**

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