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The Third Law: Symmetry in Forces
The third law of motion states that for every action, there is an equal and opposite reaction.
Learning Objective

Define the Third Law of Motion
Key Points
 If an object A exerts a force on object B, object B exerts an equal and opposite force on object A.
 Newton's third law can be seen in many everyday circumstances. When you walk, the force you use to push off the ground backwards makes you move forward.
 Thrust is an application of the third law of motion. A helicopter uses thrust to push the air under the propeller down, and therefore lift off the ground.
Terms

symmetry
Exact correspondence on either side of a dividing line, plane, center or axis.

thrust
The force generated by propulsion, as in a jet engine.
Full Text
Sir Isaac Newton was a scientist from England who was interested in the motion of objects under various conditions. In 1687, he published a work called Philosophiae Naturalis Principla Mathematica, which contained his three laws of motion. Newton used these laws to explain and explore the motion of physical objects and systems. These laws form the bases for mechanics. The laws describe the relationship between forces acting on a body, and the motion is an experience due to these forces. Newton's three laws are:
 If an object experiences no net force, its velocity will remain constant. The object is either at rest and the velocity is zero or it moves in a straight line with a constant speed.
 The acceleration of an object is parallel and directly proportional to the net force acting on the object, is in the direction of the net force and is inversely proportional to the mass of the object.
 When a first object exerts a force on a second object, the second object simultaneously exerts a force on the first object, meaning that the force of the first object and the force of the second object are equal in magnitude and opposite in direction.
Newton's Third Law of Motion
Newton's third law basically states that for every action, there is an equal and opposite reaction. If object A exerts a force on object B, because of the law of symmetry, object B will exert a force on object A that is equal to the force acted on it:
In this example, F_{A} is the action and F_{B} is the reaction. You have undoubtedly witnessed this law of motion. For example, take a swimmer who uses her feet to push off the wall in order to gain speed . The more force she exerts on the wall, the harder she pushes off. This is because the wall exerts the same force on her that she forces on it. She pushes the wall in the direction behind her, therefore the wall will exert a force on her that is in the direction in front of her and propel her forward.
Newton's Third Law of Motion
When a swimmer pushes off the wall, the swimmer is using the third law of motion.
Take as another example, the concept of thrust. When a rocket launches into outer space, it expels gas backward at a high velocity. The rocket exerts a large backward force on the gas, and the gas exerts and equal and opposite reaction force forward on the rocket, causing it to launch. This force is called thrust. Thrust is used in cars and planes as well.
Key Term Reference
 Law
 Appears in these related concepts: Mechanical Work and Electrical Energy, Gauss's Law, and Models, Theories, and Laws
 acceleration
 Appears in these related concepts: Centripetial Acceleration, Position, Displacement, Velocity, and Acceleration as Vectors, and Graphical Interpretation
 application
 Appears in these related concepts: What is Potential Energy?, Work Done by a Variable Force, and Physics and Other Fields
 base
 Appears in these related concepts: Strong Bases, The Role of the Kidneys in AcidBase Balance, and Biology: DNA Structure and Replication
 force
 Appears in these related concepts: Work, Force, and Force of Muscle Contraction
 magnitude
 Appears in these related concepts: Adding and Subtracting Vectors Using Components, Multiplying Vectors by a Scalar, and Components of a Vector
 mass
 Appears in these related concepts: Mass Spectrometer, Mass, and Pop Art
 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
 parallel
 Appears in these related concepts: Charging a Battery: EMFs in Series and Parallel, Resistors in Parallel, and Combination Circuits
 plane
 Appears in these related concepts: Scalars vs. Vectors, Shape and Volume, and Shape
 velocity
 Appears in these related concepts: Rolling Without Slipping, RootMeanSquare Speed, and Applications and ProblemSolving
 work
 Appears in these related concepts: Heat and Work, Free Energy and Work, and Potentials and Charged Conductors
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Cite This Source
Source: Boundless. “The Third Law: Symmetry in Forces.” Boundless Physics. Boundless, 21 Jul. 2015. Retrieved 29 Nov. 2015 from https://www.boundless.com/physics/textbooks/boundlessphysicstextbook/thelawsofmotion4/newtonslaws46/thethirdlawsymmetryinforces2388442/