Skip to main content

Newton's Third Law Example 4.12 Explained for NEET Students

 

Dr.Sanjaykumar Pawar


 
Physics diagram showing a wooden block and iron cylinder on a soft floor with labeled forces, normal reaction, weight, acceleration, and action-reaction pairs explained for NEET students.
Example 4.12 illustrating Newton's Third Law, Free Body Diagrams, and Action-Reaction Pairs for NEET Physics preparation.

NEET Notes - Example 4.12

Example 4.12 – Newton's Laws & Action-Reaction Pairs

Example 4.12 See Fig. 4.15. A wooden block of mass 2 kg rests on a soft horizontal floor. When an iron cylinder of mass 25 kg is placed on top of the block, the floor yields steadily and the block and the cylinder together go down with an acceleration of 0.1 m s –2. What is the action of the block on the floor (a) before and (b) after the floor yields ? Take g = 10 m s–2. Identify the action-reaction pairs in the problem.

Given Data

  • Mass of wooden block = 2 kg
  • Mass of iron cylinder = 25 kg
  • Acceleration downward = 0.1 m s-2
  • Acceleration due to gravity, g = 10 m s-2

Part (a): Before the Floor Yields

Step 1: Forces Acting on the Block

  • Weight of block (downward)
  • Normal reaction of floor (upward)
Weight = mg
= 2 × 10
= 20 N

Step 2: Apply Newton's First Law

The block is at rest. Therefore, net force on it is zero.

R = 20 N

Step 3: Force Exerted by Block on Floor

According to Newton's Third Law:

Force by floor on block = Force by block on floor
Answer (a): 20 N downward

Part (b): After the Floor Starts Yielding

Step 1: Consider Block + Cylinder as One System

Total Mass = 2 + 25 = 27 kg

Step 2: Forces on the System

  • Weight of system = 270 N (downward)
  • Normal reaction R' (upward)
Weight = Mg
= 27 × 10
= 270 N

Step 3: Apply Newton's Second Law

W - R' = Ma
270 - R' = 27 × 0.1
270 - R' = 2.7
R' = 270 - 2.7
R' = 267.3 N

Step 4: Force Exerted on Floor

By Newton's Third Law:

Force on floor = 267.3 N downward
Answer (b): 267.3 N downward

Action-Reaction Pairs

Case (a): Block at Rest

Action Reaction
Earth pulls block downward (20 N) Block pulls Earth upward (20 N)
Block pushes floor downward Floor pushes block upward

Case (b): Block + Cylinder Moving Downward

Action Reaction
Earth pulls system downward (270 N) System pulls Earth upward (270 N)
System pushes floor downward (267.3 N) Floor pushes system upward (267.3 N)
Cylinder pushes block downward Block pushes cylinder upward

Important NEET Concept

Action-Reaction Forces:

  • Always equal in magnitude.
  • Always opposite in direction.
  • Act on different bodies.
  • Occur simultaneously.

Common NEET Mistake

Many students think:

  • Weight ↓
  • Normal Reaction ↑

form an Action-Reaction Pair.

Wrong!

Both forces act on the same object. Therefore, they cannot be an action-reaction pair.

NEET Shortcut

Ask this question:

Do these two forces act on different objects?

  • Yes → Action-Reaction Pair ✔
  • No → Not an Action-Reaction Pair ✘

Final Answers

Before floor yields = 20 N downward

After floor yields = 267.3 N downward

Exam Tip

  1. Always draw a Free Body Diagram (FBD).
  2. Mark all external forces.
  3. Apply Newton's First Law if the body is at rest.
  4. Apply Newton's Second Law if the body accelerates.
  5. Use Newton's Third Law only for identifying action-reaction pairs.

This is one of the most important NEET concepts from Newton's Laws of Motion.

Internal Links
Related Newton's Laws Articles
Newton's First Law of Motion Explained
Newton's Second Law Numerical Problems
Newton's Third Law Complete Notes
Free Body Diagram (FBD) Master Guide
Contact Forces and Normal Reaction Force
Weight vs Mass Explained
Common Mistakes in Newton's Laws
Laws of Motion NEET Revision Notes
Most Important NEET Physics Numericals
Mechanics Formula Sheet for NEET
Labels:

Comments

Post a Comment