Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo
College Physics 2e

Section Summary

College Physics 2eSection Summary

9.1 The First Condition for Equilibrium

  • Statics is the study of forces in equilibrium.
  • Two conditions must be met to achieve equilibrium, which is defined to be motion without linear or rotational acceleration.
  • The first condition necessary to achieve equilibrium is that the net external force on the system must be zero, so that netF=0netF=0.

9.2 The Second Condition for Equilibrium

  • The second condition assures those torques are also balanced. Torque is the rotational equivalent of a force in producing a rotation and is defined to be
    τ = rF sin θ τ = rF sin θ

    where ττ is torque, rr is the distance from the pivot point to the point where the force is applied, FF is the magnitude of the force, and θθ is the angle between FF and the vector directed from the point where the force acts to the pivot point. The perpendicular lever arm rr is defined to be

    r = r sin θ r = r sin θ

    so that

  • The perpendicular lever arm rr is the shortest distance from the pivot point to the line along which FF acts. The SI unit for torque is newton-meter (N·m)(N·m). The second condition necessary to achieve equilibrium is that the net external torque on a system must be zero:
    net τ = 0 net τ = 0

    By convention, counterclockwise torques are positive, and clockwise torques are negative.

9.3 Stability

  • A system is said to be in stable equilibrium if, when displaced from equilibrium, it experiences a net force or torque in a direction opposite the direction of the displacement.
  • A system is in unstable equilibrium if, when displaced from equilibrium, it experiences a net force or torque in the same direction as the displacement from equilibrium.
  • A system is in neutral equilibrium if its equilibrium is independent of displacements from its original position.

9.4 Applications of Statics, Including Problem-Solving Strategies

  • Statics can be applied to a variety of situations, ranging from raising a drawbridge to bad posture and back strain. We have discussed the problem-solving strategies specifically useful for statics. Statics is a special case of Newton’s laws, both the general problem-solving strategies and the special strategies for Newton’s laws, discussed in Problem-Solving Strategies, still apply.

9.5 Simple Machines

  • Simple machines are devices that can be used to multiply or augment a force that we apply – often at the expense of a distance through which we have to apply the force.
  • The ratio of output to input forces for any simple machine is called its mechanical advantage
  • A few simple machines are the lever, nail puller, wheelbarrow, crank, etc.

9.6 Forces and Torques in Muscles and Joints

  • Statics plays an important part in understanding everyday strains in our muscles and bones.
  • Many lever systems in the body have a mechanical advantage of significantly less than one, as many of our muscles are attached close to joints.
  • Someone with good posture stands or sits in such a way that the person's center of gravity lies directly above the pivot point in the hips, thereby avoiding back strain and damage to disks.
Order a print copy

As an Amazon Associate we earn from qualifying purchases.


This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution License and you must attribute OpenStax.

Attribution information Citation information

© Jan 19, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.