Lecture 38: Examples of Friction Analysis

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Lecture 38: Examples of Friction Analysis
ENGI 1313 Mechanics I Lecture 38: Examples of Friction Analysis

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

Miscellaneous Notes Lecture 37 Notes Have Been Reposted
Minor corrections Quiz and Mid-Term Marks Posted

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.

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

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

Example 38-01 (cont.) Assume Impending Motion at A & B Analysis y x FA
FB NA W = 800lb NB

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

Example 38-01 (cont.) Assumption What is Calculated Check Assumption
 not correct y x FA FB NA W = 800lb NB

Example 38-01 (cont.) Assume Impending Motion at A y x FA FB NA
W = 800lb NB

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

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

Example 38-01 (cont.) Assumption What is Calculated Check Assumption
 Slip impending at A y x FA FB NA W = 800lb NB

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.

Example 38-02 Impending Motion? A/B Interface B/Floor Interface
Man/Floor Interface Potential impending motion at all three interfaces (rare) Crates tip

Example 38-02 (cont.) Assume Impending Motion for Crate A FBD Crate A
Analysis FA W = 100lb NA

Example 38-02 (cont.) Assume Impending Motion for Crate B FBD Crate B
Analysis FB W = 200lb NB

Example 38-02 (cont.) Assume Tipping Minimum P FBD Crate A & B
Analysis Minimum P FB W = 200lb NB

Example 38-02 (cont.) Force Exerted by Man? FBD Analysis
W Pmin FM NM  Impending motion

References Hibbeler (2007)