### 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.**

Upon successful completion of this unit, the student will be able to:

- define the conditions necessary for a rigid body to be in equilibrium;
- define torque remembering that it is a vector physical quantity; and
- solve statics problems.

### 8.1: Conditions for Equilibrium

Read sections 9.1, 9.2, and 9.3 of "Chapter 9: Statics and Torque". Look over the conceptual questions in each section to test your understanding.

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". Look over the conceptual questions in the section to test your understanding.

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". Look over the conceptual questions in each section to test your understanding.

Try solving these practice problems.

### Unit 8 Assessment

Take this assessment to see how well you understood this unit.

- This assessment
**does not count towards your grade**. It is just for practice! - You will see the correct answers when you submit your answers. Use this to help you study for the final exam!
- You can take this assessment as many times as you want, whenever you want.

- This assessment