Statics #02 | The Free Body Diagram: Isolate the Body, Turn Every Contact into a Force

Instructor: Dr. Süleyman Burak ÇELİK

A crate rests on a smooth 30 degree ramp, held by a rope parallel to the ramp. How hard does the rope pull? The numbers come next lesson, but in problems like this most mistakes are not made in the equations. They are made in one drawing before any equation: the free body diagram. In this lesson we learn to make that drawing correctly. In this lesson: - What a free body diagram is: cut the body loose, and every connection you cut becomes a force - Weight: W = m times g, computed step by step: 50 times 9 = 450, 50 times 0.81 = 40.5, total 490.5 N - The catalog of contacts: rope (tension T, pulls only), smooth surface (normal force N, perpendicular), rough surface (N plus friction f), spring (F = k times s), applied force P - The most common trap: the normal force is not always equal to the weight - The four-step recipe: isolate, weight, contacts, labels and axes - The check: number of forces = 1 plus the number of contacts; never draw a motion arrow - Full worked drawing: W, N and T for the crate on the ramp, and why N makes 30 degrees with the vertical - Two quick examples: a lamp on two cables, a crate pushed on a rough floor; five classic mistakes Chapters: 0:00 Intro 0:03 One drawing before any equation 0:53 Cut the body loose: computing the weight 2:21 The catalog of contacts 3:49 The four-step recipe 4:56 Worked example: crate on a ramp 6:45 Two more diagrams and five classic mistakes 8:11 Summary and what comes next Next lesson: equilibrium of a particle; on a body at rest the forces add up to zero, and we find the rope tension and the normal force with numbers. Statics (Engineering Mechanics) series, lesson 2, for civil, mechanical and mechatronics engineering students.