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College Physics

Introduction to Statics and Torque

College PhysicsIntroduction to Statics and Torque

Layered rock formation.
Figure 9.1 On a short time scale, rocks like these in Australia’s Kings Canyon are static, or motionless relative to the Earth. (credit:

Chapter Outline

9.1 The First Condition for Equilibrium
  • State the first condition of equilibrium.
  • Explain static equilibrium.
  • Explain dynamic equilibrium.
9.2 The Second Condition for Equilibrium
  • State the second condition that is necessary to achieve equilibrium.
  • Explain torque and the factors on which it depends.
  • Describe the role of torque in rotational mechanics.
9.3 Stability
  • State the types of equilibrium.
  • Describe stable and unstable equilibriums.
  • Describe neutral equilibrium.
9.4 Applications of Statics, Including Problem-Solving Strategies
  • Discuss the applications of Statics in real life.
  • State and discuss various problem-solving strategies in Statics.
9.5 Simple Machines
  • Describe different simple machines.
  • Calculate the mechanical advantage.
9.6 Forces and Torques in Muscles and Joints
  • Explain the forces exerted by muscles.
  • State how a bad posture causes back strain.
  • Discuss the benefits of skeletal muscles attached close to joints.
  • Discuss various complexities in the real system of muscles, bones, and joints.

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. Furthermore, their acceleration is zero because they remain motionless. That means they also have something in common with a car moving at a constant velocity, because anything with a constant velocity also has an acceleration of zero. Now, the important part—Newton’s second law states that net F=maF=ma size 12{F= ital "ma"} {}, and so the net external force is zero for all stationary objects and for all objects moving at constant velocity. There are forces acting, but they are balanced. That is, they are in equilibrium.


Statics is the study of forces in equilibrium, a large group of situations that makes up a special case of Newton’s second law. We have already considered a few such situations; in this chapter, we cover the topic more thoroughly, including consideration of such possible effects as the rotation and deformation of an object by the forces acting on it.

How can we guarantee that a body is in equilibrium and what can we learn from systems that are in equilibrium? There are actually two conditions that must be satisfied to achieve equilibrium. These conditions are the topics of the first two sections of this chapter.

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