Courses
Courses for Kids
Free study material
Offline Centres
More
Store Icon
Store

Block on Block Friction Problems Explained for JEE and NEET

ffImage
hightlight icon
highlight icon
highlight icon
share icon
copy icon
SearchIcon

How to Draw Free Body Diagrams for Block on Block Friction

Understanding block on block problems friction is vital for JEE Main aspirants mastering Newtonian mechanics. These scenarios ask you to analyze how friction operates between two or more stacked blocks, usually when a force is applied to one or both. Calculating the resulting motion, accelerations, and frictional forces is a common exam problem. Clarity on this topic builds a strong base for related concepts like contact forces and relative motion.


Typical block on block friction setups present two blocks in contact on a horizontal surface. Questions often specify mass, applied force, and friction coefficients between surfaces. Analyzing the situation with the right diagrams and laws prevents errors and saves exam time.


It’s important to relate these problems to core mechanics ideas such as Newton’s laws of motion, friction, and contact force. For JEE, being able to draw neat free-body diagrams and identify forces helps solve even advanced multi-block friction numericals.


Concept of Block on Block Problems Friction

Block on block problems friction refers to any situation involving two (or more) blocks in contact where friction acts at their interface. The bottom block may also interact with a rough surface. JEE Main questions typically ask for acceleration, relative motion, the direction and magnitude of frictional force, or contact force between the blocks.


These problems test your ability to identify all forces and apply Newton’s Second Law to each block separately. Similar ideas appear in static friction, relative motion, and laws of motion numericals.


Key Laws and Formulas Used in Block on Block Problems Friction

  • Your starting point is Newton’s second law: F = ma, where F is net force, m is mass, a is acceleration.
  • Friction force magnitude: f = μN, where μ is the coefficient of friction, N is normal reaction.
  • Static friction: the force that must be overcome to start slipping; maximum value is fmax = μsN.
  • Kinetic friction: applicable once slipping occurs, always fk = μkN.
  • Contact force between blocks: often found by considering forces on one block and calculating reaction at the interface.

A correct free-body diagram is essential before applying formulas. Refer to free body diagram basics and always mark directions of forces, especially friction.


Stepwise Approach: Solving Block on Block Problems Friction

  1. Read the question carefully for given values (mass, force, μ).
  2. Draw accurate free-body diagrams for each block, showing friction, weight, normal, applied force.
  3. Decide if blocks move together (no slipping) or slide relative to each other based on maximum static friction.
  4. Apply Newton’s second law to each block, writing separate equations.
  5. Solve the system for required quantities: acceleration, friction, or contact force.

In JEE, always state assumptions such as “block moves with, or relative to, the other.” Errors often come from not checking the static friction limit first.


Worked JEE Example: Block on Block Problems Friction

Two blocks, A (2 kg) on top of B (4 kg), rest on a horizontal table. Coefficient of friction between A and B is 0.4, between B and ground is 0.5. A horizontal force F = 18 N is applied to B. Find acceleration of both blocks and frictional force acting between them.

  • Total weight: (2 + 4) × 9.8 = 58.8 N.
  • Max static friction at A–B: fmax = 0.4 × 19.6 = 7.84 N (19.6 N is A’s weight).
  • Max friction B–ground: 0.5 × 58.8 = 29.4 N.
  • If F = 18 N: possible for both blocks to move together if required friction is ≤7.84 N.
  • Combined acceleration: a = F/(mA + mB) = 18/6 = 3 m/s2.
  • Friction needed at A–B: f = mA × a = 2 × 3 = 6 N (less than max).

So, both blocks move together; acceleration = 3 m/s2, friction force at A–B = 6 N (direction: right for A, left for B).


Common Mistakes and Best Practices in Block on Block Problems Friction

  • Forgetting to check maximum static friction before assuming blocks move together.
  • Incorrectly choosing the direction of friction force — remember it always opposes relative motion.
  • Omitting or mislabeling forces in the free-body diagram.
  • Confusing normal reaction on different blocks; it’s not always equal to the block’s weight if vertical forces act.
  • Not including the friction between block and ground if only focusing on interface friction.

Term Symbol SI Unit
Friction force f Newton (N)
Coefficient of friction μ Dimensionless
Normal force N Newton (N)
Mass m kg
Acceleration a m/s2

Encountering terms like block on block friction questions or “relative motion in blocks” in JEE tests is usual. These are often combined with relative velocity or laws of motion practice paper questions.


You can deepen your understanding by solving numericals involving connected blocks, and by reading Vedantu’s structured topic notes. Practice boosts both speed and accuracy in block on block friction.


More Practice and Exam Tips for Block on Block Problems Friction


Expert Vedantu faculty recommend you always start from FBDs, double-check friction types, and only use approximations (like ignoring air resistance) when stated. Consistent practice with prior JEE questions unlocks confidence before the exam.


Preparation tips for JEE Main physics suggest covering blocks-and-friction early in your revision, since this theme recurs each year.


Mastering block on block problems friction ensures readiness for a wide class of Newton’s law application numericals, enhancing your scoring potential.

Competitive Exams after 12th Science

FAQs on Block on Block Friction Problems Explained for JEE and NEET

1. How do you solve block on block friction problems for JEE?

Block on block friction problems are solved using free-body diagrams, Newton’s laws, and the concepts of static and kinetic friction. To solve these questions efficiently in JEE:

  • Draw a free-body diagram (FBD) for each block.
  • Identify all forces such as normal force, friction force, and applied force.
  • Apply Newton's Second Law (F = ma) to each block separately.
  • Use the correct friction formula: f = μN, where μ is the coefficient of friction and N is the normal force.
  • Check whether the friction is static or kinetic based on relative motion.
  • Solve the resulting equations to find acceleration, frictional force, and contact force.
This systematic approach helps avoid common errors in friction direction and application.

2. What formula is used to find contact force between two blocks?

Contact force between two blocks is determined using Newton’s third law and the equation of motion for each block.

  • When two blocks (mass m1 over m2) are in contact and placed on a surface, the contact force (Fcontact) on one block by the other is typically:
    Fcontact = mblock × acceleration (a)
  • First, find the system's acceleration (a) using ΣF = (m1 + m2) × a
  • Then, for the desired block, apply F = m × a considering only forces acting on it.
Don’t forget to include frictional forces acting between blocks where relevant in your calculations.

3. What is block on block friction?

Block on block friction refers to the resistance force that acts between two blocks in contact when one is placed over the other and a force is applied.

  • Friction opposes the relative motion between the two surfaces of the blocks.
  • If neither block moves relative to each other, static friction applies.
  • If they slide over each other, kinetic friction acts.
  • This situation is common in JEE and NEET physics questions to test concepts like Newton's laws, free-body diagrams, and relative motion.

4. How do you draw the free body diagram for two-block friction problems?

To draw an FBD for two-block friction problems, represent all forces acting on each block separately.

  1. Draw each block as a rectangle.
  2. Show weight (mg) downwards and normal force (N) upwards.
  3. Add applied forces (like a push or pull).
  4. Include friction forces at the point of contact—direction opposes potential or actual motion.
  5. Indicate contact forces between blocks (internal forces).
Label all forces and check that Newton’s third law is satisfied for pairs of contact forces. Accurate FBDs are crucial for solving block on block questions in exams.

5. Are block-on-block friction questions important for NEET and Boards?

Yes, block on block friction problems are important for NEET, JEE Main, and Board exams.

  • They test understanding of Newton’s laws, friction, and forces.
  • Mastery helps in solving complex numericals quickly and accurately.
  • Regularly appear in NEET and CBSE Class 11 & 12 Physics syllabus.
  • Practice improves exam confidence and speed.

6. Can you download PDF notes of block on block friction problems?

Yes, block on block friction problem PDFs are available for download from many educational websites.

  • PDFs usually contain important concepts, formulae, solved examples, and practice numericals.
  • These resources help students practice for JEE, NEET, and Board exams.
  • Look for PDFs labelled for Class 11 Physics, Laws of Motion, or Friction numericals.

7. What are the most common mistakes in block on block friction problems?

Common mistakes in block-on-block friction problems include misunderstanding friction direction and formula application. Watch out for:

  • Incorrect direction of friction force - always opposes potential or actual relative motion.
  • Confusing static and kinetic friction.
  • Drawing incomplete or inaccurate FBDs.
  • Forgetting to include all forces, especially contact force.
  • Incorrect use of relative acceleration or velocity concepts.
Reviewing each step helps reduce exam errors and boosts accuracy.

8. How do you use relative velocity in block-on-block friction problems?

Relative velocity helps determine whether friction is static or kinetic and the movement between two blocks.

  • If both blocks move together, the relative velocity is zero, so static friction acts.
  • If one block slides over the other, the relative velocity is non-zero, so kinetic friction acts.
  • Use the formula: vrelative = vblock1 – vblock2
Always check the direction and value of friction based on this concept.

9. When does kinetic friction apply instead of static friction in these problems?

Kinetic friction is used when two blocks slide over each other. It applies when:

  • The force acting on a block exceeds the maximum static friction.
  • There is Relative motion between block surfaces.
Otherwise, if blocks move together without relative sliding, use static friction in your calculations.

10. How to find acceleration of blocks in block on block friction problems?

Finding the acceleration of blocks in friction problems involves applying Newton’s laws and accounting for all forces.

  1. Draw FBDs for each block.
  2. Write equations of motion for each (ΣF = ma).
  3. Include friction and contact forces appropriately.
  4. Solve the simultaneous equations for acceleration (a) of block(s).
Understanding these steps is essential for JEE/NEET style numericals.