4.4 Newton’s Third Law of Motion: Symmetry in Forces
3 min read•june 18, 2024
of Motion is all about action and reaction. It states that for every , there's an equal and opposite force pushing back. This concept explains how objects interact and move in the world around us.
From rockets blasting off to swimmers propelling through water, Newton's Third Law is at play everywhere. It helps us understand why we can walk, drive cars, and even sit in chairs without falling through. This law is key to grasping how forces shape our daily lives.
Newton's Third Law of Motion
Newton's third law of motion
States for every action force, there is an equal and opposite reaction force
If object A exerts a force on object B, object B simultaneously exerts an equal force on object A in the opposite direction (two billiard balls colliding)
Forces always occur in pairs, known as action-reaction force pairs
Action force and reaction force have the same magnitude but opposite directions (rocket exhaust pushing down, rocket moving up)
Action and reaction forces act on different objects
Forces are equal in magnitude and opposite in direction, but they do not cancel each other out because they act on different objects (person jumping off a boat, boat moves backward)
Law implies that forces are interactions between two objects
A single object cannot exert a force on itself; forces always involve the interaction of two objects (book pushing down on table, table pushing up on book)
Real-world applications of Newton's third
Sports:
Swimmer pushes against water (action force), water pushes back on swimmer (reaction force), propelling them forward
In tug-of-war, each team pulls on rope (action force), rope pulls back on each team (reaction force)
Transportation:
Car's wheels push against road (action force), road pushes back on wheels (reaction force), enabling car to move forward
In rocket launch, rocket pushes on exhaust gases (action force), exhaust gases push back on rocket (reaction force), propelling it upward
Everyday activities:
When walking, foot pushes against ground (action force), ground pushes back on foot (reaction force), allowing forward motion
Sitting on chair, body exerts downward force on chair (action force), chair exerts upward force on body (reaction force), supporting weight
Force symmetry in motion systems
in forces means action and reaction forces are always equal in magnitude but opposite in direction
Action and reaction forces act on different objects, so they do not cancel each other out
Instead, forces cause changes in the motion of the objects involved (two ice skaters pushing off each other)
In a collision between two objects:
Action force is the force exerted by object A on object B
Reaction force is the force exerted by object B on object A
Forces are equal in magnitude and opposite in direction, but they act on different objects, causing changes in their motion (car hitting tree)
In a system where one object pushes against another:
Action force is the force exerted by the first object on the second object
Reaction force is the force exerted by the second object on the first object
Forces are equal in magnitude and opposite in direction, but they act on different objects, enabling the first object to move or change its motion (pushing a shopping cart)
Forces are vectors, having both magnitude and direction
Key concepts in motion and force
is the resistance of an object to changes in its motion
is a measure of an object's and affects the force required to change its motion
Acceleration is the rate of change of velocity, resulting from the application of force
is the product of force and the time over which it is applied, causing a change in momentum
Key Terms to Review (22)
Action-Reaction Pairs: Action-reaction pairs refer to the equal and opposite forces that two interacting objects exert on each other, as described by Newton's Third Law of Motion. This concept is fundamental to understanding the development of the force concept and the symmetry of forces in physics.
Adhesive forces: Adhesive forces are the attractive forces between unlike molecules. They play a significant role in phenomena such as capillary action and the wetting of surfaces.
Atomic mass: Atomic mass is the weighted average mass of an atom's naturally occurring isotopes, measured in atomic mass units (amu). It reflects both the mass and relative abundance of each isotope.
Conservation of Momentum: Conservation of momentum is a fundamental principle in physics which states that the total momentum of a closed system is constant unless an external force acts on the system. This means that the total momentum before an event, such as a collision, is equal to the total momentum after the event.
Conservation of momentum principle: The principle of conservation of momentum states that the total linear momentum of an isolated system remains constant if no external forces are acting on it. This means that the momentum before and after a collision or interaction is the same.
Equilibrium: Equilibrium is a state of balance or stability, where the forces acting on a system are in a state of balance, and the system remains at rest or in a constant state of motion. This concept is fundamental in various areas of physics, including mechanics, thermodynamics, and electromagnetism.
Force: Force is a vector quantity that represents the interaction between two objects, causing a change in the motion or shape of one or both objects. It is a fundamental concept in physics that describes the push or pull experienced by an object due to the influence of another object or system.
Friction: Friction is the resistive force that occurs when two surfaces interact, opposing the relative motion between them. It acts parallel to the surfaces in contact and can be either static or kinetic.
Impulse: Impulse is the product of the average force applied to an object and the time duration over which it is applied. It is also equal to the change in momentum of the object.
Impulse: Impulse is a vector quantity that represents the change in momentum experienced by an object over a given time interval. It is the product of the force acting on an object and the time interval over which that force is applied.
Inertia: Inertia is the resistance of an object to any change in its state of motion. It is directly proportional to the mass of the object.
Inertia: Inertia is the tendency of an object to resist changes in its state of motion. It is a fundamental property of matter that describes an object's resistance to changes in its velocity or direction of motion.
Isaac Newton: Isaac Newton was a renowned physicist and mathematician, credited with formulating the laws of motion and universal gravitation, which laid the foundation for classical mechanics. His work revolutionized our understanding of motion and forces, influencing various fields in science and mathematics, and providing essential insights into the nature of physical interactions.
Mass: Mass is a fundamental property of matter that represents the amount of material in an object. It is a measure of the object's resistance to changes in its motion, and it is a scalar quantity, meaning it has magnitude but no direction. Mass is a crucial concept in physics, as it is a key factor in determining an object's behavior under the influence of forces.
Newton's Third Law: Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another object, the second object exerts an equal and opposite force on the first. This principle of action and reaction forces is fundamental to understanding the dynamics of various physical systems, from collisions to rocket propulsion.
Newton’s third law of motion: Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. This means that forces always come in pairs, acting on two interacting objects.
Normal force: The normal force is the perpendicular contact force exerted by a surface on an object resting on it. It counteracts the weight of the object.
Normal Force: The normal force is a contact force that acts perpendicular to the surface of an object in response to an external force pressing the object against the surface. It is a fundamental concept in classical mechanics, particularly in the study of Newton's laws of motion.
Spontaneous symmetry breaking: Spontaneous symmetry breaking occurs when a system that is symmetric under some symmetry group goes into a vacuum state that is not symmetric. This phenomenon plays a crucial role in explaining the behavior of fundamental particles and cosmological events.
Symmetry: Symmetry refers to the balanced and proportional arrangement of parts or elements in an object or system, where the parts or elements on one side mirror or correspond to those on the other side. This concept of balance and regularity is fundamental in various fields, including physics, mathematics, and design.
Tension: Tension is the force transmitted through a string, rope, cable, or similar object when it is pulled tight by forces acting from opposite ends. This concept is crucial in understanding how forces interact in various systems, as it provides insights into how objects transmit forces and maintain equilibrium.
Thrust: Thrust is the force applied on a surface in a direction perpendicular or nearly perpendicular to that surface. It often results from the expulsion of mass, as seen in rockets and jet engines.