Action and Reaction Forces: How Newton's Third Law Applies in Daily Life
In physics, understanding how forces act and react is essential for analyzing the motion of objects in nature and engineered systems. When two objects interact, they influence each other through forces. This happens whether the objects are in contact, like a chair and a person, or separated by distance, such as planets and gravity. The concept of equal and opposite forces forms the foundation for laws of motion, helping students predict and interpret movement in everyday and exam-oriented problems.
Newton's Third Law of Motion: Concept and Statement
Newton's third law of motion is one of the basic principles for understanding forces in physics. The law can be stated as:
- For every action, there is an equal and opposite reaction.
This means that whenever one object applies a force to a second object, the second object applies an equally strong force back to the first, but in the opposite direction. These forces are called action and reaction forces. They always occur in pairs and act on different objects.
Understanding Action and Reaction Forces
When you sit in a chair, your body pushes down on the chair due to gravity. At the same time, the chair pushes up on you with an equal force. These two forces are mutually interacting, forming an action-reaction force pair.
Some forces are due to direct physical contact, like frictional or normal forces. Others act at a distance, like gravitational or magnetic forces. In every case, Newton's third law applies: the force exerted by object A on object B is matched by an equal and opposite force from B on A.
Mathematical Expression
Newton's third law can be represented mathematically as:
Here, FAB is the force exerted by object A on object B, and FBA is the force exerted by object B on object A. Both are equal in magnitude and opposite in direction.
Examples of Newton's Third Law in Daily Life
- Fish swimming: A fish uses its fins to push water backward. The water, in turn, pushes the fish forward with an equal force, allowing it to move.
- Bird flying: Birds push air downward with their wings. The air pushes the bird upward with an equal force, helping it fly.
- Car movement: When a car’s tires push the road backwards, the road pushes the tires forward with equal force, moving the car ahead.
- Sitting on a chair: Your body pushes down on the chair; the chair pushes up on you.
Table: Action-Reaction Force Pairs
| Scenario | Action Force | Reaction Force |
|---|---|---|
| Fish swimming | Fish pushes water backward | Water pushes fish forward |
| Bird flying | Wings push air downward | Air pushes bird upward |
| Car moving | Wheels push road backward | Road pushes wheels forward |
| Sitting on chair | Body pushes chair downward | Chair pushes body upward |
Solving Problems with Newton's Third Law
When approaching physics problems about forces and motion, identify all interaction pairs. For each interaction, apply the principle that the forces are equal in size but opposite in direction, and remember they act on different bodies.
- Identify interacting objects: Find object A and object B involved in the interaction.
- Assign forces: Label the force from A on B and the force from B on A.
- Apply the law: Set up FAB = –FBA.
- Analyze the results: Consider motion, acceleration, or effect on each object, knowing the forces are equal and opposite.
Practice Concept Questions
| Situation | Common Question | Key Principle |
|---|---|---|
| Bus hits firefly | Which force is greater: bus on firefly or firefly on bus? | Both forces are equal in strength and opposite in direction. |
| Rifle firing bullet | Why does the gun recoil? | Bullet pushed forward, gun backward, forces equal and opposite. |
| Person pulling rope tied to wall or elephant | Does the force change? | The force exerted is the same in both cases. |
Key Takeaways and Applications
- Action and reaction forces always occur in pairs, are equal in magnitude, and opposite in direction.
- The two forces act on different objects and cannot cancel each other out.
- This law helps explain motion in swimming, flying, running, and many engineering designs.
- Always identify the pair and direction before solving physics problems.
Next Steps for Deeper Learning
- Explore related topics like Laws of Motion, Newton's First Law, and Second Law of Motion for complete understanding.
- Practice identifying force pairs in problems on Force and Motion and Interaction of Forces.
- Apply the concept in situations like friction, dynamics, and momentum.
Summary Table: Core Points of Newton’s Third Law
| Aspect | Description |
|---|---|
| Law Statement | Every action has an equal and opposite reaction. |
| Mathematical Form | FAB = –FBA |
| Force Pair | Acts on two different bodies |
| Direction | Opposite directions, equal magnitude |
| Typical Examples | Swimming, flying, car movement, sitting |
For more examples, conceptual notes, and practice questions, visit Newton's Third Law of Motion resource at Vedantu. Strengthen your understanding by applying these principles in different real-world scenarios and exam questions.
FAQs on Newton's Third Law of Motion: Definition, Formula, Examples
1. What is 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:
- Whenever one object exerts a force on another, the second object exerts a force of equal magnitude but in the opposite direction on the first object.
- Action and reaction forces always act on different bodies.
2. Give two daily life examples of Newton's Third Law of Motion.
Examples of Newton's Third Law in daily life:
- Walking: When you walk, your foot pushes the ground backward (action). The ground pushes your foot forward (reaction), making you move ahead.
- Swimming: A swimmer pushes water backward (action), and the water pushes the swimmer forward (reaction).
3. Do action and reaction forces act on the same body?
No, action and reaction forces always act on different bodies. They form a force pair between two objects in interaction, not on the same object.
4. State the formula for Newton's Third Law of Motion.
The formula for Newton's Third Law is:
FAB = –FBA, where:
- FAB: Force exerted by object A on B
- FBA: Force exerted by object B on A
- The forces are equal in magnitude and opposite in direction.
5. How does Newton's Third Law help a rocket move in space?
Rockets move forward in space by expelling exhaust gases backward (action). The expelled gases push the rocket forward with an equal and opposite force (reaction), according to Newton's Third Law.
- No need for air or material to push against—action and reaction happen between the rocket and the gases it ejects.
6. What is the difference between Newton's First, Second, and Third Laws of Motion?
Newton's Three Laws differ as follows:
- First Law (Inertia): A body remains at rest or uniform motion unless acted upon by a net force.
- Second Law: The rate of change of momentum is proportional to the force applied (F = ma).
- Third Law: Every action has an equal and opposite reaction, always on different objects.
7. Why does a gun recoil when a bullet is fired?
When a bullet is fired forward (action), the gun experiences a backward force (reaction). The momentum gained by the bullet is balanced by an equal and opposite momentum of the recoiling gun, demonstrating Newton's Third Law.
8. Is Newton's Third Law applicable in equilibrium situations?
Yes, Newton's Third Law applies in all situations, including equilibrium. In equilibrium, action and reaction forces are still equal and opposite but the net change in motion may be zero if balanced by other forces.
9. What is an action-reaction force pair? Give an example.
An action-reaction force pair consists of two equal and opposite forces exerted by two interacting bodies on each other.
Example: A book lying on a table exerts a downward force (its weight) on the table; the table exerts an equal upward normal force on the book.
10. Can action and reaction forces cancel each other out?
No, action and reaction forces do not cancel each other out because they act on different bodies. Each object experiences its own force separately, influencing its motion according to Newton's Second Law.
11. How is the conservation of momentum related to Newton's Third Law?
The law of conservation of momentum is a direct consequence of Newton's Third Law. In closed systems, the total momentum before an interaction equals the total momentum after, because action and reaction forces are equal, opposite, and act for the same duration.
12. Name some common misconceptions about Newton's Third Law.
Common misconceptions include:
- Believing action and reaction act on the same object (they don't).
- Assuming the objects must move for the law to apply (not true).
- Thinking the two forces act one after another, rather than simultaneously.
