1. Tutorial: Flat Friction
Engineering 25
Tutorial: Flat Friction
Bruce Mayer, PE
Licensed Electrical & Mechanical Engineer

2. Spring-n-Wts on Ramp
Two blocks A and B have a weight of 10 lb and 6 lb, respectively. They are resting on the incline for which the coefficients of static friction are µA = 15% and µB = 25%. Determine the incline angle for which both blocks begin to slide. Also find the required stretch or compression in the connecting spring for this to occur. The spring has a stiffness of k = 2 lb/ft.
H13e P8-40

3. Conceptual Analysis
Since Block-A has the lower µS value, it will Slide FIRST, thereby PULLING Block-B Down the ramp with it
Thus expect the Dual-Block Slip-Angle
Take the FBD of both Blocks noting that the Spring Force, FS, will be equal & opposite for both blocks

4. Conceptual Analysis
Note that Motion IMPENDS for BOTH Static-Position Blocks
For Block-A The Block Weight Generates 𝐹 𝑓𝐴 = 𝜇 𝑆𝐴 𝑁 𝐴 Which NOT sufficient to stop Block-A from sliding
Only the SPRING Pull maintains Block-A in the Static Position
For Block-B The Block Wt Does NOT Generate 𝐹 𝑓𝐵 = 𝜇 𝑆𝐵 𝑁 𝐵
ONLY the SPRING Pull puts Block-B in the condition of Impending Motion