Statics
Statics is the part of mechanics that studies objects in balance, or equilibrium. In engineering, statics is described as "the study of rigid bodies in equilibrium": a rigid body is an object treated as if it does not bend, stretch or twist when forces act on it.[1] Real objects always bend a little, so this is a simplification, but it still gives useful answers. Engineers might, for example, treat a whole high-rise building as one body when designing its foundations, and later check each column and beam separately.[1]
What equilibrium means
An object in equilibrium is not accelerating. It is either standing still or moving in a straight line at a steady speed.[1] Physicists call the first case static equilibrium (motionless) and the second dynamic equilibrium (constant velocity).[2]
The two conditions
Statics rests on two conditions. The first comes from Newton's second law: if a body is in equilibrium, all the forces acting on it must add up to zero.[1] Because forces have a direction as well as a size, they must be added as vectors.[1]
Balanced forces are not enough on their own. If two equal and opposite pushes are applied at different points on an ice hockey stick lying on ice, the stick can start to spin even though the forces cancel out.[2] So the second condition is that the total turning effect of the forces, called torque, must also be zero.[3] Torque depends on how strong a force is, where it is applied and in which direction it acts. This is why a door is easier to open by pushing far from its hinges.[3]
An everyday example
A see-saw shows both conditions at work. When two children balance on a see-saw, the lighter child must sit farther from the pivot so that the two turning effects are equal, and the pivot pushes up with a force equal to the total weight of the children.[3] Engineers use the same ideas to work out unknown forces in structures: if a body is known to be in equilibrium, the forces on it must add up to zero, and that fact can be used to find the forces that make it so.[1]
Related subjects
In a later course, called strength of materials or mechanics of materials, students drop the rigid-body simplification and study how materials stretch and bend.[1]
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Baker, Daniel W.; Haynes, William (2020). "3.1 Equilibrium". Engineering Statics: Open and Interactive. Retrieved 20 October 2026.
Engineering statics is the study of rigid bodies in equilibrium
- ↑ 2.0 2.1 OpenStax. "9.1 The First Condition for Equilibrium". College Physics 2e. Rice University. Retrieved 20 October 2026.
for both static equilibrium (motionless), and dynamic equilibrium (constant velocity)
- ↑ 3.0 3.1 3.2 OpenStax. "9.2 The Second Condition for Equilibrium". College Physics 2e. Rice University. Retrieved 20 October 2026.
the second condition necessary to achieve equilibrium is that the net external torque on a system must be zero