

Physics Experiment- To Study the Relationship Between the Normal Reaction and the Limiting Friction
Physics Experiment – To Study the Relationship Between the Normal Reaction and the Limiting Friction and to Find the Coefficient of Friction Between a Block and a Horizontal Surface
A force known as the force of friction develops as a reaction to the applied force when two surfaces come in contact. When bodies are at rest, static friction takes place and the experimental observations manifest that the coefficient of static friction is the limiting frictional force, which is a dimensionless constant, but depends on the nature of surfaces in contact.
In daily life static friction helps us to stand on a surface and rest the objects on a table.
Table of Content
Aim
Theory
Observations
Results
Aim
To find the relationship between the normal reaction and the limiting friction force.
To find the frictional coefficient between a block and a horizontal surface.
Theory
When two objects touch each other while at rest, static friction takes place. The least amount of force needed to move the body from rest is the limiting value of limiting friction force. There is a stage where the body is almost ready to move when the external force F is increased. Limiting friction force is the force of friction at this point, when it is at its greatest.
The force resisting motion is referred to as the kinetic or sliding friction when the applied force F, is raised further (beyond the limiting frictional force). Limiting friction is greater than kinetic friction. The coefficient of kinetic friction is the measurement of the force of kinetic friction.
Experimentally it is found that limiting frictional force is
Where the coefficient of friction is being taken as .Although it has no dimensions, it depends on the kind of surfaces coming into touch. Here, the force which is being applied on the kinetic friction, where the coefficient of kinetic friction is being taken as
Therefore,
Three laws of static frictions states that:
The nature and state of the polished two surfaces in contact determine the amount of limiting friction.
As long as the usual reaction doesn't change, the amount of limiting friction is independent of the area and shape of the surfaces in contact.
The amount of normal reaction (R) between the two surfaces in contact is directly proportional to the amount of limiting friction (F).
Therefore, we can say that
So, coefficient of friction will be,
Apparatus Required
A wooden block
30 g or 60 g of slotted weight
Tabletop or other horizontal surface with a frictionless pulley attached to one end of it
Thread
Spring balance
Weight box
Procedure
Least count and range of the spring balance should be determined first.
Place the wooden block on the table with the frictionless pulley at one end and use a spring balance to determine its mass (see the below figure for reference).
Pass a thread over the pulley and tie one end to the hook on the wooden block.
Allow the thread's other free end to hang vertically by tying it to the weight hanger.
Increase the mass until the block barely begins to slide when the table is tapped.
Record the total weights added in the observation table after adding up the total mass added to the weight hanger and multiplying it by the acceleration caused by gravity, g. These results should be in the frictional limiting force, F.
The mass of the wooden block, M, along with the acceleration brought on by gravity, g, are multiplied to produce the normal reaction, R.
The experiment should be repeated with various masses on the block, and in each instance, the estimated value is confirmed to be a constant for the specified pair of surfaces.
Draw a graph with the limiting friction, F, along the Y axis and the normal reaction, R, along the X axis. The graph will be an upward-sloping straight line.
By measuring the slope of the graph, the coefficient of friction can also be determined.

Relationship Between the Force of Limiting Friction and Normal Reaction
Observation
Weight of the wooden block, W=_________N
Number of Observations | Weight Kept on the Wooden Block, W (N) | Normal Reaction(N) | Total Weights on Weight Hanger= Limiting Friction, F (N) | Coefficient of Friction |
1 | ||||
2 | ||||
3 | ||||
4 |
Results
The value of limiting friction is precisely related to normal reaction since is a constant.
The coefficient of friction between two specified surfaces, =_________
Precautions
The surface on which the wooden block will be placed must be dust free and plane.
Between the block and the pulley, the thread portion should be horizontal.
The pulley should have less friction.
Lab Manual Questions
1. Why is the thread between the block and the pulley kept horizontal?
Ans: By pulling the block horizontally, the combined weight of the pan and weights becomes effective and therefore the thread remains horizontal.
2. What would happen if the pulley had friction?
Ans: In a frictionless pulley the pan's weight and the effectiveness of the weights is maintained without any loss, however, for Pulley's effective value will be decreased by friction.
3. In what way does frictional force follow?
Ans: In the way opposite to the direction of motion of the object.
4. What kind of friction occurs when a body rolls over another body?
Ans: Rolling friction
Viva Questions
1. What is limiting friction's alternate name?
Ans: The amount of friction is at its highest. The term "limiting friction" also refers to the friction's maximum value.
2. What differentiates limiting friction from static friction?
Ans: Due to the fact that static friction acts when a body is lying over the surface of another body without moving and it is a self-adjusting force. The greatest amount of static friction known as limiting friction, is reached when that body exceeds the force of static friction.
3. Find the normal force acting on a body whose limiting friction is 60 N and whose coefficient of friction is 12 N.
Ans: We know that,
Where F and are the limiting friction and coefficient of limiting friction respectively,
Taken, N as the normal force,
4. Why do roads get slippery after it rains?
Ans: Between our feet and the road, a thin layer of rainwater accumulates. It functions as a lubricant to stop imperfections on the surface of the wheel and the road from interlocking. Slipping occurs as the amount of friction decreases.
5. Why are brake surfaces kept flat when the area of the surface doesn't increase friction?
Ans: More surface area under the same force results in less pressure. It doesn't leave scratches on the surface being braked.
6. What happens to the coefficient of friction when the weight of the body is being doubled?
Ans: The coefficient of friction remains the same when the weight of the body is being doubled.
7. What role does lubricant serve in reducing the friction?
Ans: A thin layer of lubricant is created between the two surfaces that are in contact. Consequently, liquid friction, which is significantly smaller, plays the role of sliding friction.
8. What makes ball bearings so effective in reducing friction?
Ans: The reason for this is that ball bearings converts sliding friction into rolling friction.
9. Why does there come a limit after which further polishing of a surface increases the frictional resistance rather than decreasing it?
Ans: Normally, friction reduces as surface smoothness improves; however, when a surface is made excessively smooth, binding force of adhesion increases and friction increases. Therefore, the term given for this phenomena is cold welding.
10. Why in hilly areas, sand is thrown on a track covered in snow?
Ans: It is done so to prevent the tires of a vehicle from slipping on the snow as sand increases the force of friction when it is being thrown on the snow.
Practical Based Questions
1. What surface will have the least friction out of the following?
Plywood
Plastic
Ice
Vinyl floor
Ans: C) Ice
2. Out of which factor from the following causes bicycle tyres to wear out?
Electrostatic force
Muscular force
Frictional force
None of the above
Ans: C) Frictional force
3. There is a very thin layer of lubricant between the two surfaces in contact when this occurs, and this is referred to as,
Rolling friction
Greasy friction
Film friction
Solid friction
Ans: C) Film friction
4. How can friction be reduced?
Oiling
Polishing
Use of ball bearings
All of the above
Ans: D) All of the above
5. When a body is just about to slide over the surface of another body, the most significant amount of static friction that can be measured is known as
Sliding friction
Limiting friction
Rolling friction
None of the above
Ans: B) limiting friction
6. The reason why meteors burn up in the atmosphere is
Gravitational force
Electrostatic force
Magnetic force
Frictional force
Ans: D) Frictional force
7. What is the most common way to frequently to lessen friction?
Lubrication
Elimination of the motion
Warning signs
None of the above
Ans: A) Lubrication
8. When does the force of friction come into play, if the surfaces in contact tend to move or move with respect to each other?
If all the objects are solid
One of the two object is in gaseous form
One of the two object is in solid form
regardless of the objects' state as solids, liquids, or gases.
Ans: D) regardless of the objects' state as solids, liquids, or gases.
9. On what conditions, force of friction depends on,
Materials of objects in contact
Nature of the surface in contact
Both A and B
None of the above
Ans: C) Both A and B
10. From which of the following, increasing the friction is very necessary?
Handle bar of the bicycle
Ball bearings in the front wheel
Chain of a bicycle
None of the above
Ans: A) Handle bar of the bicycle
Conclusion
This experiment tells the coefficient of friction between a block and a horizontal surface in order to analyse the relationship between the force of limiting friction and normal reaction. A force opposing the object originates in response to the applied force and acts in the opposite direction when a body tries to slide over another body. Between two bodies in touch with one another, there occurs static friction. The limiting amount of static friction is referred to as "limiting friction."
FAQs on CBSE Physics Experiment To Study the Relationship Between the Normal Reaction and the Limiting Friction
1. How can you experimentally verify the relationship between limiting friction and normal reaction? What are the key precautions for this Class 11 experiment?
The relationship that limiting friction is directly proportional to the normal reaction (fL ∝ N) can be verified using a wooden block on a horizontal tabletop. The procedure involves:
- Placing the block on the table and adding known weights on top of it. The total weight (block + added weights) equals the Normal Reaction (N).
- Using a pan attached via a frictionless pulley to apply a horizontal force. The weight in the pan is increased until the block just begins to slide. This weight gives the limiting friction (fL).
- Repeating this for different values of N.
A graph of fL (y-axis) versus N (x-axis) will be a straight line passing through the origin, which proves the direct proportionality. Key precautions include ensuring the pulley is frictionless, the string is horizontal, and the tabletop surface is clean and uniform.
2. A block of mass 5 kg rests on a horizontal surface. If the coefficient of static friction (μs) is 0.4, what is the maximum horizontal force that can be applied to the block without moving it? (Take g = 10 m/s²)
This is a typical numerical question for the CBSE 2025-26 exam. Here is the step-by-step solution:
- First, calculate the Normal Reaction (N), which is equal to the weight of the block on a horizontal surface: N = mg = 5 kg × 10 m/s² = 50 N.
- The maximum horizontal force that can be applied without moving the block is equal to the limiting friction (fL).
- Use the formula: fL = μs × N.
- Substitute the values: fL = 0.4 × 50 N = 20 N.
Therefore, the maximum horizontal force that can be applied before the block starts to move is 20 N.
3. What are the key differences between static friction, limiting friction, and kinetic friction?
Understanding these three types of friction is crucial for Class 11 Physics exams.
- Static Friction (fs): This is a self-adjusting force that opposes the tendency of motion when a body is at rest. Its value can be anything from zero up to its maximum limit.
- Limiting Friction (fL): This is the maximum value of static friction. It is the force that must be overcome to just start the motion of a body over a surface. It is not self-adjusting.
- Kinetic Friction (fk): This is the frictional force that acts when a body is in actual motion over another surface. Its value is constant for a given pair of surfaces and is generally less than the limiting friction.
4. State the important laws of limiting friction that are frequently asked in exams.
The laws of limiting friction are fundamental for solving problems in mechanics. For the CBSE 2025-26 syllabus, the key laws are:
- The force of limiting friction is directly proportional to the normal reaction (N) between the surfaces in contact.
- The direction of the force of friction is always opposite to the direction in which the body tends to move.
- The force of friction is independent of the area of contact between the two surfaces, provided the normal reaction is constant.
- The force of friction depends on the nature and smoothness of the surfaces in contact (e.g., wood on concrete has a different friction from steel on ice).
5. How does friction help a car negotiate a turn on a level circular road? What is the formula for its maximum safe speed?
This is a key application-based question. When a car takes a turn, it requires a centripetal force to move in a circular path. On a level (unbanked) road, this necessary force is provided entirely by the force of static friction between the car's tyres and the road. If the required centripetal force exceeds the limiting friction, the car will skid. The maximum safe speed (vmax) is determined by the maximum available static friction and is given by the formula: vmax = √(μsrg), where μs is the coefficient of static friction, r is the radius of the turn, and g is acceleration due to gravity.
6. Why is the coefficient of kinetic friction (μk) generally less than the coefficient of static friction (μs)?
This is a key conceptual point that explains the behaviour of friction. The difference arises from the interactions at a microscopic level.
- When an object is at rest, the microscopic irregularities (crests and troughs) on the two surfaces get time to interlock firmly, forming 'cold welds'. A larger force is needed to break these bonds, which corresponds to the higher static friction.
- Once the object is in motion, the surfaces do not get enough time to settle into these deep interlocks. They essentially glide over each other, resulting in a weaker opposing force, which is the lower kinetic friction.
7. Is friction always a disadvantage? Justify your answer with two examples where it is essential.
No, this is a common misconception. Friction is often called a 'necessary evil' but is absolutely indispensable. While it does cause energy loss and wear, life would be impossible without it.
- Walking and Running: We can push against the ground to move forward only because of the static friction between our footwear and the surface. On a frictionless surface like perfect ice, walking would be impossible.
- Braking Systems: The brakes in vehicles rely entirely on friction. Brake pads press against the rotors, using kinetic friction to convert the vehicle's kinetic energy into heat, thus slowing it down safely.
8. From an exam perspective, why is it considered easier to pull a lawn roller than to push it?
This is a classic application-based question that tests the understanding of vector components. The answer lies in how the applied force affects the normal reaction, and consequently, the friction.
- When Pushing: The applied force is directed downwards at an angle. Its vertical component adds to the weight of the roller, thus increasing the total normal reaction. Since friction is proportional to the normal reaction, the opposing frictional force also increases.
- When Pulling: The applied force is directed upwards at an angle. Its vertical component acts against the weight, thus decreasing the total normal reaction. This reduces the frictional force that needs to be overcome.
Since the opposing friction is less when pulling, it is easier than pushing.
9. How does rolling friction compare to sliding friction, and why is this difference important in modern engineering?
Rolling friction is the resistance that occurs when a round object (like a wheel or ball) rolls on a surface. It is significantly smaller than sliding friction for the same load and surfaces. This happens because rolling motion involves minimal interlocking of surface irregularities; the resistance mainly comes from the slight deformation of the object and the surface at the point of contact. This principle is vital in engineering. For instance, ball bearings are used in almost all rotating machinery to replace sliding motion with rolling motion, which dramatically reduces energy loss due to friction and increases the efficiency and lifespan of the machine.











