Newtons Third Law of Motion - Definition, Examples, FAQs
When two objects interact, they always exert forces on each other. For example, when a ball hits a wall, the wall pushes the ball back, causing it to bounce. Similarly, although the Earth pulls us downward due to gravity, we also pull the Earth upward with an equal force. These simple but powerful observations are explained by Newton’s Third Law of Motion. According to this law, for every action, there is an equal and opposite reaction. It highlights a fundamental symmetry in nature: forces always occur in pairs and act on different bodies simultaneously. Newton’s Third Law is essential for understanding motion in everyday life as well as in physics problems related to walking, swimming, rockets, and gravitational interactions. For students, this law forms a strong foundation for analyzing motion and solving exam-oriented numerical and conceptual questions.
This Story also Contains
- What is Newton’s Third Law of Motion?
- Examples of Newton’s Third Law
- Action–Reaction Pair
- Applications of Newton’s Third Law of Motion
- Third Law Contained in Second Law
What is Newton’s Third Law of Motion?
If you are an Avengers fan, you might have wondered how Iron Man flies using his suit. The answer lies in Newton’s Third Law of Motion.
Newton’s Third Law of Motion states: For every action, there is an equal and opposite reaction.
This means that when one body exerts a force on another body, the second body simultaneously exerts a force of equal magnitude but in the opposite direction on the first body. These forces act along the same line but on different bodies.
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Examples of Newton’s Third Law
1. Rocket launch
A rocket moves upward by pushing hot gases downward. The downward force on gases is the action, and the upward force exerted by gases on the rocket is the reaction, causing the rocket to move up.
2. Swimming
A swimmer pushes water backward with hands and feet. The water pushes the swimmer forward with an equal and opposite force, helping the swimmer move ahead.
Action–Reaction Pair
An action–reaction pair refers to the two forces described in Newton’s Third Law of Motion.
For every action, there is an equal and opposite reaction.
When one object exerts a force on another object (action), the second object simultaneously exerts an equal force in the opposite direction (reaction) on the first object.
- Action and reaction forces are equal in magnitude
- They act in opposite directions
- They act on two different bodies, not on the same body
- They occur simultaneously
Examples
Walking: Foot pushes the ground backward (action); ground pushes the foot forward (reaction)
Rocket launch: Rocket pushes gases downward; gases push the rocket upward
Swimming: Swimmer pushes water backward; water pushes swimmer forward
Applications of 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 law explains how motion occurs when forces act in pairs on different bodies.
Walking: When we walk, our foot pushes the ground backward. In return, the ground pushes us forward, allowing us to move.
Swimming: A swimmer pushes water backward with hands and legs. The water pushes the swimmer forward with an equal force.
Rocket Propulsion: A rocket moves upward by expelling hot gases downward. The gases exert an equal and opposite force on the rocket, pushing it up.
Recoil of a Gun: When a bullet is fired forward, the gun moves backward due to reaction force.
Flight of Birds and Aircraft: Birds and airplanes push air downward, and air pushes them upward, helping them fly.
Jumping from a Boat: When a person jumps from a boat, the boat moves backward as a reaction.
Third Law Contained in Second Law
Newton's Third Law of Motion is said to be contained in Newton's Second Law because the principle of action-reaction can be derived using the second law and the law of conservation of momentum.
According to Newton's Second Law,
$
F=\frac{d p}{d t}
$
Consider two interacting bodies $\mathbf{A}$ and $\mathbf{B}$.
If body $A$ exerts a force on body $B$, the momentum of $B$ changes. Similarly, $B$ exerts a force on $A$, changing its momentum.
Since no external force acts on the system,
$
\frac{d\left(p_A+p_B\right)}{d t}=0
$
This gives,
$
\frac{d p_A}{d t}=-\frac{d p_B}{d t}
$
Using the second law,
$
F_{A B}=-F_{B A}
$
Thus, the force exerted by $A$ on $B$ is equal in magnitude and opposite in direction to the force exerted by B on A, which is exactly Newton's Third Law.
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Frequently Asked Questions (FAQs)
Newton's Third Law 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 force back on the first object, but in the opposite direction. These forces always occur in pairs and act on different objects.
Examples of Newton’s third law of motion are:-
An elastic band is being pulled.
Swimming or rowing a boat are two options.
When pushing an object, there is static friction.
Walking.
Standing on the ground or sitting in a chair are both acceptable options.
The rocket's upward thrust.
Resting against a tree or a wall.
Slingshot.
In collisions between two objects, Newton’s third law of motion is naturally applied. When two things collide, forces of equal size and opposite direction are experienced by both objects. As a result of such forces, one object often accelerates (gains momentum) while the other slows down (lose momentum).
When two bodies contact, Newton’s third law of motion states that they apply forces to each other that are equal in magnitude and opposing in direction. The law of action and reaction is another name for the third law.