Newton's Third Law: CBSE Class 11 Physics Complete Guide
Newton’s Third Law of Motion – Easy NEET Notes (Line-by-Line Explanation)
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| Newton's Third Law of Motion: Every action force has an equal and opposite reaction force acting simultaneously on another body. |
Introduction
- Newton’s Second Law tells us how an external force causes acceleration.
- But a question arises: Where does this external force come from?
- In Newtonian mechanics, an external force always comes from another body or object.
Understanding Force Between Two Bodies
- Consider two bodies: A and B.
- If body B exerts a force on A, we may ask:
- Does A also exert a force on B?
- Newton's answer is Yes.
Example 1: Spring and Hand
- When you press a spring, your hand exerts force on the spring.
- The spring gets compressed.
- At the same time, the spring pushes back on your hand.
- You can feel this force.
Example 2: Earth and Stone
- Earth pulls a stone downward due to gravity.
- We may not notice it, but the stone also pulls the Earth upward.
- The force exerted by the stone on Earth is equal in magnitude.
- Since Earth is extremely massive, its acceleration is very small and difficult to observe.
Newton's Third Law of Motion
Statement
"To every action, there is always an equal and opposite reaction."
OR
"For every force, there is an equal and opposite force acting on another body."
Mathematical Form
Where:
- = Force on A by B
- = Force on B by A
- Negative sign (−) shows opposite direction.
Main Idea of Third Law
- Force never exists alone.
- Forces always occur in pairs.
- Whenever one body exerts a force on another body, the second body exerts an equal and opposite force on the first body.
- These two forces are called an action-reaction pair.
Important Points for NEET
Point 1: Action and Reaction Are Both Forces
- The words action and reaction simply mean forces.
- There is no special difference between them.
- A better statement is:
"Force on A by B is equal and opposite to force on B by A."
Point 2: Action Does Not Come Before Reaction
Common Misconception
- Many students think action happens first and reaction happens later.
Correct Concept
- Action and reaction occur simultaneously.
- They act at the same instant.
- Neither force is the cause nor the effect of the other.
Example
- When you hit a wall:
- Your hand exerts force on the wall.
- The wall exerts force on your hand at exactly the same moment.
Point 3: Action and Reaction Act on Different Bodies
Very Important NEET Point
- Action and reaction never act on the same body.
- They always act on two different bodies.
Example
Person pushes wall:
| Force | Acts On |
|---|---|
| Person pushes wall | Wall |
| Wall pushes person | Person |
- Since they act on different bodies, they cannot cancel each other.
Why Action and Reaction Do Not Cancel?
Example
Suppose:
- You push a wall with 100 N force.
- Wall pushes you back with 100 N force.
Students often say:
Net force = 100 N − 100 N = 0
This is wrong.
Reason
- 100 N force acts on the wall.
- 100 N reaction force acts on you.
- Since they act on different bodies, they cannot be added together.
Internal Forces in a System
Consider System (A + B)
- Here, force on A by B and force on B by A are internal forces.
- Internal forces always occur in equal and opposite pairs.
- Therefore, they cancel each other when considering the whole system.
Result
- Internal forces do not affect the motion of the entire system.
- Only external forces can change the motion of a system.
Real-Life Examples of Newton’s Third Law
1. Walking
- Foot pushes the ground backward.
- Ground pushes the foot forward.
- This forward reaction helps us walk.
2. Swimming
- Swimmer pushes water backward.
- Water pushes swimmer forward.
3. Rocket Launch
- Rocket throws gases downward.
- Gases push rocket upward.
4. Gun Recoil
- Bullet moves forward.
- Gun experiences backward recoil.
5. Jumping
- Person pushes ground downward.
- Ground pushes person upward.
NEET Quick Revision Points
✅ Force always occurs in pairs.
✅ Action and reaction are equal in magnitude.
✅ Action and reaction are opposite in direction.
✅ They act simultaneously.
✅ They act on different bodies.
✅ They never cancel each other on a single body.
✅ Internal forces cancel within a system.
✅ Only external forces can change the motion of a system.
One-Line NEET Definition
Newton’s Third Law states that whenever one body exerts a force on another body, the second body simultaneously exerts an equal and opposite force on the first body.
NEWTON'S THIRD LAW OF MOTION – CBSE CLASS 11 QUESTION BANK
A. Multiple Choice Questions (MCQs)
1. Newton's Third Law states that:
(a) Force is proportional to acceleration (b) Every object remains at rest (c) To every action there is an equal and opposite reaction (d) Momentum is conserved
Answer: (c)
2. Action and reaction forces:
(a) Act on the same body (b) Act on different bodies (c) Are unequal (d) Act at different times
Answer: (b)
3. Which of the following is an example of Newton's Third Law?
(a) Falling of an apple (b) Walking on the ground (c) Inertia of rest (d) Motion of planets
Answer: (b)
4. The force exerted by Earth on a stone and by the stone on Earth are:
(a) Unequal (b) Equal and opposite (c) Same direction (d) Zero
Answer: (b)
5. Action and reaction forces:
(a) Cancel each other (b) Act simultaneously (c) Are internal forces only (d) Produce no motion
Answer: (b)
B. Very Short Answer Questions (1 Mark)
1. State Newton's Third Law of Motion.
Answer: To every action, there is always an equal and opposite reaction.
2. Do action and reaction act on the same body?
Answer: No, they act on different bodies.
3. What is the direction of reaction force?
Answer: Opposite to the action force.
4. Can action and reaction occur separately?
Answer: No, they always occur together.
5. Give one example of Newton's Third Law.
Answer: Walking on the ground.
C. Short Answer Questions (2–3 Marks)
1. Why can a person walk on the ground?
Answer: The foot pushes the ground backward. The ground exerts an equal and opposite force on the foot in the forward direction. This reaction force enables the person to walk.
2. Why does a gun recoil when fired?
Answer: When the bullet moves forward, it exerts an equal and opposite force on the gun. As a result, the gun moves backward, called recoil.
3. Explain why action and reaction forces do not cancel each other.
Answer: Action and reaction forces act on different bodies. Since they act on different objects, they cannot be added together and therefore do not cancel each other.
4. What are internal forces?
Answer: Internal forces are forces acting between particles of the same system. They occur in equal and opposite pairs and cancel each other.
D. Long Answer Questions (5 Marks)
1. State and explain Newton's Third Law of Motion with examples.
Answer: Newton's Third Law states that to every action there is always an equal and opposite reaction.
Characteristics:
- Forces always occur in pairs.
- Action and reaction are equal in magnitude.
- They are opposite in direction.
- They act simultaneously.
- They act on different bodies.
Examples: • Walking • Swimming • Rocket propulsion • Gun recoil • Jumping from a boat
Thus, every force in nature has an equal and opposite counterpart.
2. Explain rocket propulsion using Newton's Third Law.
Answer: A rocket expels hot gases downward at high speed. The gases exert a downward force. According to Newton's Third Law, the gases exert an equal and opposite force on the rocket. This upward reaction force propels the rocket upward.
E. Assertion and Reason Questions
1.
Assertion (A): Action and reaction forces are equal and opposite.
Reason (R): They act on the same body.
Answer: Assertion is true but Reason is false.
2.
Assertion (A): A swimmer moves forward in water.
Reason (R): Water pushes the swimmer forward when the swimmer pushes water backward.
Answer: Both A and R are true and R is the correct explanation.
3.
Assertion (A): Action and reaction occur simultaneously.
Reason (R): There is no cause-effect relationship between them.
Answer: Both A and R are true and R is the correct explanation.
F. Fill in the Blanks
- Newton's Third Law states that every action has an equal and ______ reaction.
Answer: opposite
- Action and reaction forces act on ______ bodies.
Answer: different
- Walking is possible because of the ______ force of the ground.
Answer: reaction
- Forces always occur in ______.
Answer: pairs
- Internal forces of a system ______ each other.
Answer: cancel
G. Statement-Based Questions
1. Identify True or False:
(a) Action and reaction act on different bodies. Answer: True
(b) Action occurs before reaction. Answer: False
(c) Forces always occur in pairs. Answer: True
(d) Action and reaction can cancel each other. Answer: False
(e) Rocket propulsion is based on Newton's Third Law. Answer: True
H. Match the Columns
Column A
A. Walking
B. Gun
C. Rocket
D. Swimming
Column B
-
Water pushes swimmer
-
Ground pushes person
-
Gases push rocket upward
-
Recoil
Answers
A → 2
B → 4
C → 3
D → 1
I. Case Study Questions
Case Study
A student pushes a wall with a force of 50 N. The wall exerts a force of 50 N on the student. The student notices that the wall does not move.
Questions
1. Which law explains this situation?
Answer: Newton's Third Law of Motion.
2. What is the reaction force?
Answer: Force exerted by the wall on the student.
3. Are action and reaction equal?
Answer: Yes.
4. Why does the wall not move?
Answer: Because the wall is firmly supported and its acceleration is negligible.
5. Do action and reaction act on the same body?
Answer: No.
J. Important One-Mark CBSE Questions
- Define action-reaction pair.
- Give one example of Newton's Third Law.
- Why does a boat move backward when a person jumps forward?
- Why does a rocket move upward?
- Can action exist without reaction?
Answers
- Pair of equal and opposite forces acting on different bodies.
- Walking.
- Due to equal and opposite reaction force.
- Due to reaction force of escaping gases.
- No.
NEWTON'S THIRD LAW OF MOTION
│
├── Definition
│ │
│ └── To every action,
│ there is an equal
│ and opposite reaction.
│
├── Main Idea
│ │
│ ├── Force never exists alone
│ ├── Forces occur in pairs
│ └── Mutual interaction between bodies
│
├── Mathematical Form
│ │
│ └── FAB = -FBA
│
├── Action & Reaction
│ │
│ ├── Equal magnitude
│ ├── Opposite direction
│ ├── Act simultaneously
│ └── Act on different bodies
│
├── Important Points
│ │
│ ├── Action = Force
│ ├── Reaction = Force
│ ├── No cause-effect relation
│ ├── No time gap between them
│ └── Cannot cancel each other
│ on a single body
│
├── Internal Forces
│ │
│ ├── Present within a system
│ ├── Equal and opposite
│ └── Cancel each other
│
├── Examples
│ │
│ ├── Walking
│ │ ├── Foot pushes ground
│ │ └── Ground pushes foot
│ │
│ ├── Swimming
│ │ ├── Swimmer pushes water
│ │ └── Water pushes swimmer
│ │
│ ├── Rocket
│ │ ├── Gases move downward
│ │ └── Rocket moves upward
│ │
│ ├── Gun Recoil
│ │ ├── Bullet forward
│ │ └── Gun backward
│ │
│ └── Jumping
│ ├── Person pushes ground
│ └── Ground pushes person
│
└── NEET Revision
│
├── Forces occur in pairs
├── Equal magnitude
├── Opposite direction
├── Simultaneous action
├── Different bodies
├── Do not cancel on one body
└── Internal forces cancel
INTERNAL LINKS
Newton's First Law of Motion Notes
Newton's Second Law of Motion Explained
Laws of Motion Complete Chapter Notes
Force and Inertia Class 11 Physics
Momentum and Impulse Notes
Conservation of Momentum
Friction Class 11 Physics
Work, Energy and Power Notes
Circular Motion Class 11 Physics
NEET Physics Important Questions
CBSE Class 11 Physics MCQ Bank
Physics Formula Sheet for Class 11
Motion in a Straight Line Notes
Motion in a Plane Notes
Gravitation Complete Notes
