When we talk about balance in statics, it’s really important to know the difference between static equilibrium and dynamic equilibrium. They have different features and rules.

Static Equilibrium

• What It Is: A body is in static equilibrium when it stays still. This means that its speed is zero.

• How It Works: For something to be in static equilibrium, two main rules must be followed:

  1. Balanced Forces: All the forces acting on the body must add up to zero. This can be shown as: $\Sigma F = 0$

  2. Balanced Rotations: The sum of all moments (or twists) around any point must also be zero: $\Sigma M = 0$

Dynamic Equilibrium

• What It Is: A body is in dynamic equilibrium if it moves at a steady speed. This means the forces acting on it balance each other out, even though it is moving.

• How It Works: Here’s what makes it dynamic:

  1. Steady Speed: The total outside force is still zero, so: $\Sigma F = 0$

  2. Steady Rotation: If the object is spinning, the total twist must also be zero: $\Sigma M = 0$

Important Point

• Motion vs. Stillness: The main difference is about movement. Static equilibrium is for things that are not moving, while dynamic equilibrium is for things that are moving at a steady speed. Knowing these differences can help you understand how forces work in different situations, whether you're figuring out tension in a bridge or forces on a moving car!