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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

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)

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

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.
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: Position, Displacement, Velocity, and Acceleration as Vectors, FreeFalling Objects, and The Second Law: Force and Acceleration
 axis
 Appears in these related concepts: Area Between Curves, Components of a Vector, and Conservation of Angular Momentum
 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 Time
 equation
 Appears in these related concepts: A General Approach, Equations and Inequalities, and Equations and Their Solutions
 equilibrium
 Appears in these related concepts: The Function and Nature of Markets, Demand Schedules and Demand Curves, and Supply Schedules and Supply Curves
 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: Newton and His Laws, Diffusion, and Electric Fields and Conductors
 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, LongTerm Approach, and General ProblemSolving Tricks
 static equilibrium
 Appears in these related concepts: Stability, Balance, and Center of Mass, Equipotential Lines, and Variation of Pressure With Depth
 velocity
 Appears in these related concepts: Arc Length and Speed, Centripetial Acceleration, and Graphical Interpretation
 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, 08 Jan. 2016. Retrieved 03 May. 2016 from https://www.boundless.com/physics/textbooks/boundlessphysicstextbook/staticequilibriumelasticityandtorque8/conditionsforequilibrium74/firstcondition3086359/