### Unit 8: Statics and Torque

What might desks, bridges, buildings, trees, and mountains have in common - at least in the eyes of a physicist? The answer is that they are ordinarily motionless relative to the Earth. Thus, their acceleration in the Earth frame of reference is zero. Newton's second law states that net F = ma,so the net external force is zero on all stationary objects and for all objects moving at constant velocity. There are forces acting, but they are balanced. That is, the forces are in equilibrium.

**Completing this unit should take you approximately 8 hours.**

### 8.1: Conditions for Equilibrium

Read sections 9.1, 9.2, and 9.3 of "Chapter 9: Statics and Torque" (pages 289 to 298). Look over the corresponding conceptual questions on page 310 to test your understanding of the sections.

Try solving these practice problems.

This video discusses torque as a cross product of the lever arm and the force vector and illustrates how the direction of the torque is related to the directions of the lever arm and the force.

In this animation, you will vary the angle between the two vectors and observe the effect on the cross product.

Work on solving problems 1 and 2 from SAC118. The solutions are available, but make a serious attempt to solve the problems before looking at the solutions.

### 8.2: Applications of Statics

Read section 9.4 of "Chapter 9: Statics and Torque" (pages 298 to 301). Look over the corresponding conceptual questions on page 310 to test your understanding of the sections.

Work on solving problems 1 through 5 from SAC121. The solutions are available, but make a serious attempt to solve the problems before looking at the solutions.

### 8.3: Simple Machines

Read sections 9.5 and 9.6 of "Chapter 9: Statics and Torque" (pages 301 to 309). Look over the corresponding conceptual questions on pages 310 and 311 to test your understanding of the sections.

Try solving these practice problems.

### Unit 8 Assessment

Please take this assessment to check your understanding of the materials presented in this unit.

**Notes:****There is no minimum required score to pass this assessment, and your score on this assessment**__will not__factor into your overall course grade.**This assessment is designed to prepare you for the Final Exam that will determine your course grade. Upon submission of your assessment you will be provided with the correct answers and/or other feedback meant to help in your understanding of the topics being assessed.****You may attempt this assessment as many times as needed, whenever you would like.**