is all about action and reaction forces. It states that for every force, there's an equal and opposite force acting back. This principle applies to all types of forces, from everyday pushes to gravitational pulls.
Understanding action-reaction pairs is key to solving physics problems. By identifying these pairs and applying Newton's second law, we can analyze complex systems and predict object motion. This law also connects to momentum conservation in collisions and explosions.
Newton's Third Law
Newton's third law of motion
States that for every , there is an equal and opposite
If object A exerts a force on object B, then object B exerts an equal and opposite force on object A (, colliding billiard balls)
Action and reaction forces always act on different objects
Equal in magnitude but opposite in direction
Occur simultaneously
Applicable to all types of forces, including (, , ) and (, , )
Action-reaction force pairs
Identify the two objects involved in the interaction
Determine the action force exerted by one object on the other
Identify the reaction force exerted by the second object on the first
Examples of ():
Person pushing a wall: person exerts a force on the wall (action), wall exerts an equal and opposite force on the person (reaction)
Earth's gravity pulling on a book: Earth exerts a gravitational force on the book (action), book exerts an equal and opposite gravitational force on Earth (reaction)
Car accelerating on a road: road exerts a friction force on the car's tires (action), car's tires exert an equal and opposite friction force on the road (reaction)
: rocket exerts a force on the exhaust gases (action), exhaust gases exert an equal and opposite force on the rocket (reaction, causing )
Problem-solving with Newton's third law
Define and identify objects within the system
Identify acting on the system
Apply Newton's third law to determine action-reaction force pairs within the system
Use action-reaction force pairs and external forces to set up for each object in the system
Apply Newton's second law (Fnet=ma) to each object, considering the acting on it
Solve equations of motion to determine desired quantities (acceleration, velocity, displacement)
Consider the following when solving problems:
Action and reaction forces do not cancel each other out because they act on different objects (book on a table, person standing on the ground)
Net force on an object is the vector sum of all forces acting on that object, including external forces and forces from action-reaction pairs
When the system is in equilibrium, the net force on each object within the system is zero (, static objects)
Conservation of Momentum and Impulse
Newton's third law is closely related to the in
is the change in momentum of an object due to a force acting over time
Conservation of momentum states that the total momentum of a closed system remains constant
In collisions and explosions, the total momentum before and after the event is conserved
Key Terms to Review (28)
Action Force: An action force is a force exerted by one object on another object, as described by Newton's Third Law of Motion. It is the force that one object applies to a second object, causing the second object to experience an equal and opposite reaction force.
Action-Reaction Force Pairs: Action-reaction force pairs are the equal and opposite forces that two interacting objects exert on each other, as described by Newton's Third Law of Motion. These paired forces arise when one object applies a force on another object, and the second object simultaneously applies an equal and opposite force on the first object.
Balanced Forces: Balanced forces are a state of equilibrium where the net force acting on an object is zero, meaning the object is not accelerating. This concept is central to Newton's Third Law, which states that for every action, there is an equal and opposite reaction.
Conservation of Momentum: Conservation of momentum is a fundamental principle in physics that states the total momentum of a closed system remains constant unless an external force acts upon it. This principle applies to various topics in mechanics, including Newton's Third Law, linear momentum, impulse and collisions, types of collisions, center of mass, and rocket propulsion.
Contact Forces: Contact forces are the forces that arise when two objects are in physical contact with each other. These forces act at the interface between the objects, and their magnitude and direction depend on the nature of the contact and the properties of the materials involved.
Electrostatic Force: Electrostatic force is the attractive or repulsive force that exists between stationary electric charges. It is a fundamental force in nature that governs the interactions between charged particles and plays a crucial role in various physical phenomena.
Equations of Motion: Equations of motion are mathematical expressions that describe the relationship between the position, velocity, acceleration, and time of an object in motion. These equations are fundamental in understanding and analyzing the motion of objects, including projectile motion and the application of Newton's laws of motion.
External Forces: External forces are the forces that act on an object from outside the object's system. These forces are not generated within the object itself, but rather originate from the object's interaction with its surrounding environment.
Force Pairs: Force pairs refer to the concept of action and reaction forces that arise between two interacting objects, as described by Newton's Third Law of Motion. These paired forces are equal in magnitude but opposite in direction, acting on the respective objects involved in the interaction.
Friction: Friction is a force that opposes the relative motion between two surfaces in contact. It arises due to the microscopic irregularities on the surfaces, which create resistance to sliding or rolling. Friction is a fundamental concept in physics that plays a crucial role in various topics, including solving problems, understanding forces, and analyzing energy transformations.
Gravitational Force: Gravitational force is the attractive force that exists between any two objects with mass. It is the force that causes objects to be pulled towards each other, and is the fundamental force responsible for the motion of celestial bodies and the behavior of objects on Earth.
Impulse: Impulse is the product of the average force and the time interval over which it acts on an object. It is equal to the change in momentum of the object.
Impulse: Impulse is a quantity that describes the change in momentum of an object over a given time interval. It is the product of the net force acting on an object and the time interval during which that force is applied. Impulse is a fundamental concept in physics that connects the ideas of force, time, and momentum, and is essential for understanding topics such as solving problems in physics, forces, Newton's laws, and collisions.
Law of Conservation of Momentum: The Law of Conservation of Momentum states that the total linear momentum of a closed system remains constant if no external forces are acting on it. This principle is fundamental in analyzing collisions and interactions in mechanics.
Magnetic Force: Magnetic force is a type of force that acts between magnetic objects, such as magnets or objects with magnetic fields. It is a fundamental force in nature that arises from the motion of electric charges and the intrinsic magnetic moments of elementary particles.
Net force: Net force is the total force acting on an object, taking into account both the magnitude and direction of all individual forces. It determines the object's acceleration according to Newton's second law of motion, which states that an object will accelerate in the direction of the net force. Understanding net force is crucial for analyzing how forces interact and influence motion, as it helps explain concepts like inertia, action-reaction pairs, and equilibrium conditions.
Newton's Cradle: Newton's cradle is a device that demonstrates the conservation of momentum and energy. It consists of a series of identical pendulums, usually metal balls, suspended from a frame in a line.
Newton’s third law: Newton's third law states that for every action, there is an equal and opposite reaction. This means that forces always come in pairs, acting on two different objects.
Newton's Third Law: Newton's Third Law, also known as the law of action and reaction, states that for every action, there is an equal and opposite reaction. This fundamental principle of physics describes the relationship between forces acting on interacting objects.
Non-Contact Forces: Non-contact forces are forces that can act on an object without any physical contact between the object and the source of the force. These forces can influence the motion and behavior of objects from a distance, without the need for direct touch or interaction.
Normal Force: Normal force is the support force exerted by a surface perpendicular to the object resting on it, preventing the object from falling through the surface. It plays a crucial role in balancing other forces acting on an object, particularly in scenarios involving gravity and acceleration.
Reaction Force: A reaction force is the force exerted by a surface on an object in contact with it. It is the counterpart to the force the object exerts on the surface, in accordance with Newton's Third Law of Motion.
Reciprocal Interactions: Reciprocal interactions refer to the mutual and interdependent relationship between two or more entities, where the actions and effects of one directly influence the other, and vice versa. This concept is particularly relevant in the context of Newton's Third Law, which describes the balanced forces that arise between interacting objects.
Recoil: Recoil is the backward movement or force that occurs when an object is propelled forward, such as the backward motion of a gun when it is fired. This concept is closely related to Newton's Third Law of Motion, Linear Momentum, and the Conservation of Linear Momentum.
Rocket Propulsion: Rocket propulsion is the use of a rocket engine to generate thrust and propel a vehicle, such as a spacecraft or missile, through the vacuum of space or the Earth's atmosphere. It is a fundamental principle of physics that enables the movement of objects by the rearward acceleration of a propellant.
System Boundaries: System boundaries define the limits or scope of a physical system being analyzed, separating it from its surrounding environment. They determine which components and processes are included within the system and which are considered external factors.
Tension: Tension is a force that acts to pull or stretch an object, often along the length of a string, rope, or cable. It is a vector quantity, meaning it has both magnitude and direction, and it plays a crucial role in various physics concepts related to forces, motion, and equilibrium.
Thrust: Thrust is the force applied to an object to move it in a specific direction, typically produced by engines or propulsion systems. It is an application of Newton's Third Law, where every action has an equal and opposite reaction.