Write a heading on a new page Statics Sketch the seesaw and the ladder in your book. The seesaw and the ladder are “rods” for the purpose of statics. Assume the wall and the floor are smooth, and the seesaw has zero mass. Mark in the forces acting on each “rod”. What do you need for them to be in equilibrium?

Statics A rod or ladder or beam is static only if: You know about resolving forces from M1. You know about taking moments of forces around points from M1, and again from the M2 unit on centre of mass. A rod or ladder or beam is static only if: The components in every direction of all the forces acting on it add up to zero; and The moments around every point of all the forces acting on it up to zero. But if the moments around any point add up to zero, then the moments around every point add up to zero

Every statics problem in the exam is about a rod, or something like it (a ladder), against a wall. It may be held up by a string or by a strut. It may lean against the wall or be hinged to it. It may lean on the ground. It may have an extra weight added to it at some point. But it’s always about a rod. hinge All the problems are really just variations on this one. Draw the picture above using a protractor. Mark in all the forces acting on the rod, and their horizonal and vertical components. Write the three equations:   

Worked example Page 134, example 2 Best to take moments around a point on the line of action of a unknown force, then you get an equation without that unknown force in it. It’s easier to solve for the other unknowns. Try it out Ex. 5B (p.136) Q. 1, 2, 3 Draw a neat diagram for every problem!

Summing up What two things have to be true for an object to be in static equilibrium? What points can we take moments round to see if they add to zero? Which points (to take moments round) often work best to give us a quick answer?

Limiting equilibrium Part of the answer to the puzzle about how anything manages to remain static is that the forces of reaction and friction where the objects touch walls or floors adjust so as to balance things out. Sometimes a question tells you that a rod, or a beam, or a ladder, propped against a wall is “on the point of slipping” or in “limiting equilibrium”. That means ou can equate the frictional force to μR, where R = normal reaction and μ = coefficient of friction.

Worked example Ex. 5D (p.143) Q. 1, 2, 3, 4 Page 142, example 6 Ex. 5D (p.143) Q. 1, 2, 3, 4 Draw a neat diagram for every problem!

Summing up What does limiting equilibrium mean? Why is it that if we have a question which tells us about the coefficient of friction, or asks us about it, then there must be limiting equilibrium somewhere in the question?