Friction Class 11 Physics: Complete Formula Sheet & Free PDF Download for JEE & NEET
Friction Formula Sheet — Competishun
Friction Class 11 Physics: Complete Formula Sheet & Free PDF Download for JEE & NEET
Friction is one of the most frequently tested topics in JEE and NEET physics. It appears in direct formula-based questions, in problems involving connected bodies, and in almost every mechanics problem where surfaces are in contact. A clear understanding of static and kinetic friction, the angle of friction, and inclined plane problems can easily get you 2-3 correct questions in the exam.
This page gives you a complete Friction formula sheet for Class 11 physics, covering all essential formulas, laws, and problem-solving strategies. Download the free PDF below and keep it handy for quick revision before your JEE Main, JEE Advanced, or NEET exam.
Download the Friction Formula Sheet PDF
Get all friction formulas, including static and kinetic friction, angle of friction, inclined plane motion, and more in one clean PDF, free. Perfect for JEE & NEET revision.
Download Free PDFWhat is Friction in Physics?
Friction is the force that opposes the relative motion or the tendency of relative motion between two surfaces in contact. It is a contact force that acts parallel to the surfaces. Friction is not a fundamental force but arises from electromagnetic interactions between atoms on the surfaces in contact.
This chapter covers the following key topics in the formula sheet:
Types of Friction: Static & Kinetic
There are two main types of friction that you will encounter in problems:
| Type | Definition | Formula |
|---|---|---|
| Static Friction | Opposes the tendency of relative motion. It acts when the surfaces are at rest relative to each other. | fs ≤ μs N |
| Kinetic Friction | Opposes actual relative motion. It acts when the surfaces are sliding past each other. | fk = μk N |
| Key fact: μk < μs. It is harder to start motion than to keep it going. | ||
Laws of Friction
These laws define how friction behaves in different situations. They are essential for solving problems correctly.
- First Law: The force of friction always acts in a direction opposite to the relative motion or the tendency of relative motion.
- Second Law: For two surfaces in contact, the force of friction is proportional to the normal reaction (N).
- Third Law: The force of friction is independent of the apparent area of contact (for moderate pressures).
- Fourth Law: Kinetic friction is independent of the relative velocity of the two surfaces (for moderate speeds).
Angle of Friction & Angle of Repose
These two concepts are closely linked and frequently appear in JEE and NEET questions. They provide a geometric way to understand friction.
| Concept | Definition | Formula |
|---|---|---|
| Angle of Friction (λ) | The angle between the resultant contact force and the normal reaction. | tan λ = μs |
| Angle of Repose (θr) | The maximum angle of an inclined plane at which a block just begins to slide. | tan θr = μs |
| The angle of friction is equal to the angle of repose. This is a classic JEE one-liner. | ||
Friction on an Inclined Plane
Inclined plane problems are extremely common. Here are the essential formulas you need.
| Situation | Formula |
|---|---|
| Normal force on incline | N = mg cos θ |
| Component of gravity down the slope | mg sin θ |
| Friction (block sliding down) | f = μk mg cos θ |
| Acceleration down a rough incline | a = g (sin θ − μk cos θ) |
| Acceleration up a rough incline | a = g (sin θ + μk cos θ) |
| Condition to not slide (static) | tan θ ≤ μs |
| Condition to just slide (static) | tan θ = μs |
| The condition tan θ ≤ μs is the most important one to remember for static equilibrium on an incline. | |
The Friction vs. Applied Force Graph
Understanding the graph of friction versus applied force is crucial for visualizing how friction behaves.
- Region OA: Applied force is small. Static friction adjusts itself to exactly balance it. No motion.
- Point A: The applied force reaches the maximum static friction (μsN). This is the limit of static friction.
- Region AB: The object starts to slide. Kinetic friction (μkN) takes over, which is slightly lower than the maximum static friction.
- Region BC: As the applied force increases further, kinetic friction remains constant (μkN).
All Friction Formulas at a Glance
Here is a quick reference table of all the key formulas from the Friction chapter.
| Formula | What It Means |
|---|---|
| fs ≤ μs N | Static friction (up to its maximum value) |
| fk = μk N | Kinetic friction (constant while sliding) |
| tan λ = μs | Angle of friction |
| tan θr = μs | Angle of repose |
| N = mg cos θ | Normal force on an inclined surface |
| a = g (sin θ − μk cos θ) | Acceleration down a rough inclined plane |
| tan θ ≤ μs | Condition for a block to not slide on an incline |
| Memorize these formulas and practice applying them to connected-body problems and inclined-plane scenarios. | |
Worked Examples
| Problem | Solution |
|---|---|
| A 10 kg block is on a horizontal surface. μs = 0.4. What is the maximum static friction? | N = mg = 100 N. fs,max = μsN = 0.4 × 100 = 40 N. |
| What is the angle of repose for μs = 0.5? | tan θr = μs = 0.5. θr = tan⁻¹(0.5) = 26.6°. |
| A block slides down an incline with μk = 0.3 and θ = 30°. Find acceleration (g = 10). | a = g(sinθ − μkcosθ) = 10(0.5 − 0.3×0.866) = 10(0.5 − 0.26) = 2.4 m/s². |
| A block is on an incline with μs = 0.6. Will it slide for θ = 25°? | tan 25° = 0.466. Since tan θ (0.466) < μs (0.6), the block will not slide. |
| Practise these types of problems to become comfortable with applying friction formulas in different scenarios. | |
Common Mistakes to Avoid
- Using f = μN for static friction always: This formula gives the maximum static friction. The actual static friction can be less than this value and adjusts itself to balance the applied force.
- Confusing static and kinetic coefficients: Remember μs > μk. Do not use μs when the object is already sliding.
- Forgetting the normal force on an incline: N = mg cos θ, not mg. This is a very common error.
- Ignoring the direction of friction: Friction always opposes the relative motion or the tendency of relative motion. Make sure you correctly identify this direction in your free body diagrams.
- Not checking the condition for sliding: Always check tan θ vs. μs first before assuming the block will move on an incline.
Why Friction Matters for JEE and NEET
- High weightage: Friction appears in nearly every JEE Main, JEE Advanced, and NEET physics paper, usually in 2-3 questions.
- Foundation for mechanics: It is a core concept in the Laws of Motion chapter and is used in many other mechanics problems.
- Direct scoring: Many friction questions are formula-based and can be solved quickly if you know the formulas and concepts well.
- Connects to other topics: Friction is linked to work (friction does negative work), motion on an incline, and circular motion (as a centripetal force).
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Frequently Asked Questions — Friction
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