We will only cover selected types of problems in this chapter.

page 13-2 Equations for Equilibrium

Equilibrium: The value of the rotational acceleration is zero. The value of the acceleration of the center of mass of the object is zero.

To meet this conditions the sum of the torques is zero and the sum of the forces in each coordinate direction is zero.

We will identify two types of torque: clockwise and counterclockwise. You will assign a negative sign to one of those directions.

Balancing Weights

Make a drawing of a meter stick that has a mass of 200 grams. Attach 100 grams at x = 25 cm. Attach a string at x = 70 cm. The string applies an upward force. Calculate the value of the tension in the string. Calculate the mass required at x=90 cm such that there is equilibrium.

page 13-4 Gravitational Force Acting at the Center of Mass When you draw the forces acting on an object, draw the mg vector starting at the center of mass. What direction should the mg vector be drawn?

page 13-5 Technique of Solving Equilibrium Problems 1. Write down the net force equations in each direction.

2. Select an axis of rotation for the torque calculation such that one or more of the unknown forces has zero lever arm.

3. Solve for one unknown.

4. Solve for any remaining unknowns.

Skip the Wheel and Curb problem.

page 13-7 Skip the rod in a frictionless bowl problem.

page 13-8 Ladder

Make a sketch of the following situation: A 10 kg ladder is 7 meters long.

It is placed against a frictionless wall such that the base of the ladder is

1.5 meters away from the wall. A 70 kg painter is standing on the ladder

5 meters up from the base of the ladder. Calculate the force of the wall on the ladder. Calculate the horizontal and vertical components of the ground on the base of the ladder. Calculate the required force of friction.

Will the required force of friction be greater or less when the painter is further towards the top of the ladder?

page 13-9 Skip the bridge problem.

page 13-10 Skip the rope problem.

page 13-11 Lifting Weights and Muscle Injuries

Work Exercise 9.

Work Exercise 11.