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First Condition
The first condition of equilibrium is that the net force in all directions must be zero.
Learning Objective

Identify the first condition of equilibrium
Key Points
 There are two conditions that must be met for equilibrium.
 The same conditions apply both to static equilibrium and dynamic equilibrium.
 If net force is zero, then net force along any direction is zero.
Terms

translation
Motion of a body on a linear path, without deformation or rotation, i.e. such that every part of the body moves at the same speed and in the same direction; also (in physics), the linear motion of a body considered independently of its rotation.

torque
A rotational or twisting effect of a force; (SI unit newtonmeter or Nm; imperial unit footpound or ftlb)

force
A physical quantity that denotes ability to push, pull, twist or accelerate a body which is measured in a unit dimensioned in mass × distance/time² (ML/T²): SI: newton (N); CGS: dyne (dyn)
Full Text
First Condition of Equilibrium
An object that is motionless or undergoes no translational and rotational acceleration is said to be in equilibrium. That is, the net force and net torque on the object is zero in all directions. For an object to truly be in equilibrium, two conditions must be met.
The first condition states that the net force acting on the object must be zero. This means that for each axis of motion, the forces acting along that particular axis must sum to zero.
Expressed as an equation, this is simply:
Note that if net
and illustrate situations where
Dynamic Equilibrium
This car is in dynamic equilibrium because it is moving at constant velocity. There are horizontal and vertical forces, but the net external force in any direction is zero. The applied force Fapp between the tires and the road is balanced by air friction, and the weight of the car is supported by the normal forces, here shown to be equal for all four tires.
Static Equilibrium
This motionless person is in static equilibrium. The forces acting on him add up to zero. Both forces are vertical in this case.
But remember, for true equilibrium, condition 1 is only half the picture.
Static Equilibrium
This motionless person is in static equilibrium. The forces acting on him add up to zero. Both forces are vertical in this case.
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Key Term Reference
 acceleration
 Appears in these related concepts: Mass, FreeFalling Objects, and Applications and ProblemSolving
 axis
 Appears in these related concepts: Adding and Subtracting Vectors Graphically, Area Between Curves, and Components of a Vector
 constant velocity
 Appears in these related concepts: FourDimensional SpaceTime, TwoComponent Forces, and Constant Velocity
 dynamic
 Appears in these related concepts: Competitive Dynamics, Translational Equilibrium, and General ProblemSolving Tricks
 equation
 Appears in these related concepts: A General Approach, Equations and Inequalities, and Equations and Their Solutions
 equilibrium
 Appears in these related concepts: Ecological Succession, Ecosystem Dynamics, and Balance and Determining Equilibrium
 friction
 Appears in these related concepts: ProblemSolving With Friction and Inclines, Inelastic Collisions in Multiple Dimensions, and The First Law: Inertia
 motion
 Appears in these related concepts: Motion with Constant Acceleration, Newton and His Laws, and Motion Diagrams
 net force
 Appears in these related concepts: Stability, Balance, and Center of Mass, Conservation of Energy in Rotational Motion, and The Second Law: Force and Acceleration
 normal
 Appears in these related concepts: Vectors in the Plane, Arc Length and Curvature, and Normal Forces
 normal force
 Appears in these related concepts: Friction: Static, Center of Mass of the Human Body, and Banked and Unbacked Highway Curves
 static
 Appears in these related concepts: Time and Motion, Alternative Views, and Motion
 static equilibrium
 Appears in these related concepts: Equipotential Lines, Variation of Pressure With Depth, and Conductors and Fields in Static Equilibrium
 velocity
 Appears in these related concepts: Centripetial Acceleration, Force, and Average Velocity: A Graphical Interpretation
 weight
 Appears in these related concepts: Gauge Pressure and Atmospheric Pressure, Locating the Center of Mass, and Weight of the Earth
Sources
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Cite This Source
Source: Boundless. “First Condition.” Boundless Physics. Boundless, 21 Jul. 2015. Retrieved 02 Dec. 2015 from https://www.boundless.com/physics/textbooks/boundlessphysicstextbook/staticequilibriumelasticityandtorque8/conditionsforequilibrium74/firstcondition3086359/