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First Condition
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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 an object to be in equilibrium.
 The first condition is that the net force on the object must be zero for the object to be in 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
For an object to be in equilibrium, it must be experiencing no acceleration. This means that both the net force and the net torque on the object must be zero. Here we will discuss the first condition, that of zero net force.
In the form of an equation, this first condition is:
In order to achieve this conditon, the forces acting along each axis of motion must sum to zero. For example, the net external forces along the typical x and yaxes are zero. This is written as
The condition
Below, the motionless person is in static equilibrium. The forces acting on him add up to zero. Both forces are vertical in this case.
Person in Static Equilibrium
This motionless person is in static equilibrium.
Below, the 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 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.
A Car in Dynamic Equilibrium
This car is in dynamic equilibrium because it is moving at constant velocity. The forces in all directions are balanced.
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Key Term Reference
 acceleration
 Appears in these related concepts: Position, Displacement, Velocity, and Acceleration as Vectors, Scientific Applications of Quadratic Functions, and Centripetial Acceleration
 axis
 Appears in these related concepts: Area Between Curves, Regional Terms and Axes, and Components of a Vector
 constant velocity
 Appears in these related concepts: FourDimensional SpaceTime, Constant Velocity, and B.3 Chapter 3
 dynamic
 Appears in these related concepts: Competitive Dynamics, Translational Equilibrium, and General ProblemSolving Tricks
 equation
 Appears in these related concepts: Equations and Inequalities, Graphs of Equations as Graphs of Solutions, and What is an Equation?
 equilibrium
 Appears in these related concepts: Homogeneous versus Heterogeneous Solution Equilibria, Calculating Equilibrium Concentrations , and Humidity, Evaporation, and Boiling
 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 Diagrams, TwoComponent Forces, and Moving Source
 net force
 Appears in these related concepts: Diffusion, Electric Fields and Conductors, 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: Center of Mass of the Human Body, Internal vs. External Forces, and Banked and Unbacked Highway Curves
 static
 Appears in these related concepts: Friction: Static, Time and Motion, and Alternative Views
 static equilibrium
 Appears in these related concepts: Stability, Balance, and Center of Mass, Variation of Pressure With Depth, and Conductors and Fields in Static Equilibrium
 velocity
 Appears in these related concepts: Velocity of Blood Flow, RootMeanSquare Speed, and Rolling Without Slipping
 weight
 Appears in these related concepts: Gauge Pressure and Atmospheric Pressure, Weight of the Earth, and B.9 Chapter 9
Sources
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Cite This Source
Source: Boundless. “First Condition.” Boundless Physics Boundless, 17 Jun. 2016. Retrieved 24 Feb. 2017 from https://www.boundless.com/physics/textbooks/boundlessphysicstextbook/staticequilibriumelasticityandtorque8/conditionsforequilibrium74/firstcondition3086359/