Physics Experiment - Friction
Introduction
Friction plays an important role in our daily activities such as walking, writing, skating, driving a vehicle on a surface, lighting a matchstick and much more. Without friction, nothing is possible. In this class 11 friction experiment, we'll talk about types of friction, laws of limiting friction, the origin of friction, and the importance and application of friction.
Table of Content
Aim
Theory
Laws of limiting friction
Conclusion
Aim
Friction
Theory
The resistance provided by surfaces in touch as they move past one another is referred to as friction.
Types of Friction
1. Static Friction
The frictional force that exists between surfaces while they are at rest in relation to one another is known as static friction. When a tiny amount of force is applied, the static force's magnitude is similar in the other direction.
The examples of static friction are as follows:
Skiers battling the snow
Rubbing two hands together to produce heat
Coefficient of Static Friction
The symbol for the static friction coefficient is µs. The product of the coefficient of static friction and normal force is used to calculate the maximum amount of static friction, and the amount of static friction is either less than or equal to this value. It is specified as
$F_s$ (max) = µs η and Fs ≤ µs η
Where
Fs is the force of static friction
µs is the coefficient of static friction
η is the normal force
$F_s$ max is the maximum static friction force.
2. Sliding Friction
The resistance that is formed between any two items when they are sliding against one another is known as sliding friction.
Some examples of sliding friction:
The brick is sliding across the floor.
Two cards in a deck are rubbing against one another.
Coefficient of Sliding Friction
The symbol for the sliding friction coefficient is µs. The sum of the normal force and the coefficient of sliding friction is known as the force of sliding friction. It is specified as
Fs = µs. Fn
3. Rolling Friction
The weakest type of friction, rolling friction is described as the force that opposes a ball's or wheel's motion.
The instances of rolling friction are as follows:
The wood rolling on the ground
The moving vehicles' wheels
Coefficient of Rolling Friction
The symbol for the rolling friction coefficient is r. The sum of the normal force and the rolling friction coefficient is known as the rolling friction force. It is specified as
Fs = µr. Fn
4. Fluid friction
The friction that develops between fluid layers as they move in relation to one another is known as fluid friction.
The instances of fluid friction are as follows:
The way ink flows out of pens
Swimming
Origin of Friction
The roughness of surfaces causes friction. When two surfaces are moving relative to one another, the roughness or imperfections of the surfaces allow for interlocking. Elevations and depressions on the bodies' lower surfaces and bottom surfaces interlock.
Molecules on the body's lower and bottom surfaces are attracted to one another by force. The term for it is sticky force.
Laws of Limiting Friction
There are four laws of limiting friction:
Law of Direction: Reducing friction acts in a direction tangential to the surfaces in contact (i.e., opposite to the direction of motion).
Law of Magnitude: Limiting friction has a magnitude that is directly proportionate to the normal response that is exerting pressure on the body.
Law of Dependence: The law of dependency states that friction can be reduced by altering the type (rough or polished) and material (hard or soft) of the surfaces in contact.
Law of Independence: Friction can be reduced regardless of the size and shape of the surfaces coming into contact.
Coefficient of friction
The normal reaction has a direct relationship with the limiting friction.
μ =F/R
F=μ R
Here F is the limiting friction, R is the normal reaction and μ is called the coefficient of friction.
Importance of Friction
In daily life, friction is vital. Many acts might not be able to be performed without them.
A motion cannot begin without friction. On a surface with no friction, we cannot walk.
If there is no friction, the brakes will not function.
A motion cannot be conveyed without friction. If there is no friction between the belt and the wheels, the belt will not be able to rotate the machine wheels one by one.
Knots won't remain if there is no friction. It will be challenging to pack, tie, and bandage.
The process of grinding cannot occur without friction. Spices and wheat won't be ground.
Nails won't stay in place if there is no friction. A piece of furniture will be challenging to put together.
Application of Friction
When matchsticks are lit, it is possible because of friction.
Friction is used when pistons move inside a cylinder.
Due to the friction between the board and the pen, writing on both books and boards is feasible.
Disadvantages of Friction
Although friction has various applications, it is bad in the following situations.
The wear and tear of parts and excessive heat are caused by friction between the various machine parts. The machine's life is shortened as a result.
Unnecessary energy expenditure to remove friction As a result, the output is never more than the input.
Ways to Reduce Friction
The following techniques can be used to minimise friction:
Polishing and rubbing the surface to make it smoother. Elevations lose height when rubbed. Deeper depressions are filled with polishing. Interlocking decreases in the absence of peaks and valleys. As a result, friction is reduced.
However, friction is increased when surfaces in contact are excessively smooth. The molecules on the two surfaces are in close touch, which is why the friction rises as a result of the molecules' increasing attraction to one another.
Using the right lubricant to lubricate the surfaces that are in contact. Two surfaces are kept apart and from locking together by a lubricant. It lessens friction.
Grease should be used for heavy bodies, heavy oil should be used for bodies of moderate weight, and light oil should be used for light bodies when choosing a lubricant.
Adding a soft material layer to the surfaces or surface. Strong molecular tensions between the surfaces of hard materials cause higher friction. They are lined (coated) with soft material to decrease friction between them without compromising their strength. Less friction is created when soft material surfaces come into contact.
Converting the friction of rolling into that of sliding friction.
By modifying the form of a body travelling through the air, friction due to air can be minimised in jets, planes, swiftly moving automobiles, etc.
Ways to Increase Friction
Several methods can increase friction:
Chiselling the surface in order to make it rough.
Sanding the outside.
Strong material lining.
Creating slide friction by converting rolling friction.
Lab Manual Questions
1. What do you understand by the self-adjusting nature of the force of friction?
Ans. As static friction wants the objects to stay at rest and not move, it is seen as a self-adjusting force. Because of this, if an external force is applied, the static friction force will match it until it exceeds the threshold of motion.
2. Why don't we study the limiting friction between the two surfaces using a spherical body?
Ans. The best shape for reducing friction is a sphere since only a small portion of a spherical object comes into contact with the opposing surface. This is the reason why a spherical body was not selected to explore limiting friction between the two surfaces since there is only one point of contact between the body and the plane at any given instant, and this point has no motion connected to the plane.
3. Why is there no possibility of zero friction?
Ans. Since there will always be some friction on every surface, it is impossible to completely eliminate it. A surface cannot have a zero-friction surface, hence a frictionless surface is not feasible.
4. Why are unpolished surfaces typically preferred in experiments related to friction?
Ans. Lessening the area of contact between imperfections on surfaces results in less friction, which reduces the likelihood that the force may have an adverse effect and improves the accuracy of the outcome.
Viva Questions
1. What is the angle of friction?
Ans. The angle of friction is the result of a normal reaction and limiting friction with the normal reaction. The static friction coefficient is equal to the tangent of the friction angle.
3. What is Limiting Friction?
Ans. Limiting friction is the highest amount of static friction that is present when a body is about to slide over the surface of another body.
4. Which is easier: rolling or sliding?
Ans. It is simpler to roll a body along a surface than it is to slide (drag) it because rolling friction is smaller than sliding friction.
5. Is Zero friction possible?
Ans. Although theoretically friction acting on a surface may be zero, this is never practical. It can be drastically reduced, but it can never be zero because of contamination and wear.
6. What kind of friction is not moving?
Ans. While sliding friction happens when objects glide against one another, static friction happens when objects are still. Rolling friction and fluid friction are other types of friction. A surface cannot be moved by an item due to static friction.
7. Does ice have friction?
Ans. Most experts agree that ice's low friction is caused by a thin layer of liquid water that covers its surface. Therefore, skaters on delicate metal blades can glide across the ice rink without incident but come to a grinding halt when they reach the wooden floor beyond.
8. How does friction affect speed?
Ans. Friction always resists motion and reduces the speed of an object moving across a surface. Transforming a portion of an object's kinetic energy into thermal energy will slow it down.
9. Is a higher or lower friction angle preferable?
Ans. Friction angle is the physical representation of particle interlocking. The friction angle should be higher the more tightly the particles lock together, and vice versa.
10. What is the angle of response?
Ans. The angle of repose is formed by an inclined plane and the horizontal when a body is placed on it, and it simply begins to slide.
11. Why is friction less on snow?
Ans. A thin lubricating water layer produced while skiing can be used to explain the low sliding friction between skis and snow. For sliding to take place, this skinny water film is necessary. The snow's free water content creates the water film.
Practical Based Question
1. The force of friction
Always act in the direction of motion
Always act against the direction of motion
Has no particular direction
has a direction based on the size of the body
Ans. The force of friction always acts against the direction of motion.
2. If no force is applied to the moving object, it will stop due to
Tension
Momentum
Impulse
Friction
Ans. If no force is applied to the moving object, it will stop due to friction.
3. If two rough surfaces are polished and put in contact, the friction force will?
Increase
Decrease
Remain constant
Either increase or decrease
Ans. If two rough surfaces are polished and in contact, the friction force between them will decrease.
4. Which of the following forces is contact force?
Force of gravity
Magnetic force
Force of friction
Electrostatic force
Ans. The Force of friction is the contact force.
5. Which of the following is the wrong example of friction?
Sledge sliding across the snow.
A person sliding down a slide.
A person lifting a heavy object.
Washing machine pushed along a floor.
Ans. A person lifting a heavy object is the wrong example of friction.
6. Which of the following produces the least friction?
Sliding friction
Rolling friction
Composite friction
Static friction
Ans. Rolling friction produces the least friction.
7. The unit of coefficient of friction in the SI unit of the system is?
N
Unit less
N/M
Ans. The unit of coefficient of friction in the SI unit of the system is unit less.
8. Which of the following cannot be charged by friction?
A copper rod
A glass rod
A plastic pen
An inflated balloon
Ans. Frictional charging is not possible with copper rods.
9. The static coefficient of friction is greater than the kinetic coefficient of friction.
True
False
Ans. The static coefficient of friction is greater when the static frictional force is divided by the kinetic frictional force because the static frictional force is greater than the kinetic frictional force.
10. Limiting friction of a body depends on _____
Area of contact of surfaces
The volume of the smaller body on a larger surface
Nature of surfaces
The periphery of the contact surfaces
Ans. Limiting friction only depends on the surfaces' coefficients of friction, or the smoothness or roughness of the surface, because it depends on more than anything else.
Conclusion
Through this experiment, we discovered the significance of friction in daily motion. Without friction, no one in motion will be able to stop, since friction is a force that resists motion. We came to know the types of friction, laws of limiting friction and much more. Also, the importance and application of friction.
FAQs on CBSE Physics Experiment Friction
1. What is limiting friction? State the important laws of limiting friction from an exam perspective.
Limiting friction is the maximum value of static friction that comes into play when a body is just about to slide over the surface of another body. It is the point where an object transitions from rest to motion.
For the CBSE 2025-26 exams, the four laws of limiting friction are an important topic. They are:
- Law of Direction: The force of limiting friction always acts in a direction opposite to the direction in which the body is about to move.
- Law of Magnitude: The magnitude of the limiting friction (F) is directly proportional to the magnitude of the normal reaction (R) between the two surfaces in contact. That is, F ∝ R.
- Law of Dependence: The value of limiting friction depends on the nature and material of the surfaces in contact (i.e., their roughness or smoothness).
- Law of Independence: The force of limiting friction is independent of the apparent area of contact between the two surfaces, as long as the normal reaction remains constant.
2. Differentiate between static, kinetic, and rolling friction.
The key differences between static, kinetic, and rolling friction, frequently asked in Class 11 Physics exams, are as follows:
- Static Friction (fₛ): This is the friction that exists between two surfaces when there is no relative motion. It is a self-adjusting force, meaning its magnitude matches the applied force up to a maximum value called limiting friction.
- Kinetic Friction (fₖ): Also known as sliding friction, this force acts between surfaces in relative motion. Its value is nearly constant and is typically less than the limiting static friction. It is calculated as fₖ = μₖN, where μₖ is the coefficient of kinetic friction.
- Rolling Friction (fᵣ): This is the resistance that occurs when a round object (like a wheel or a ball) rolls over a surface. It arises due to the slight deformation of the surfaces at the point of contact. Rolling friction is significantly smaller than both static and kinetic friction, which is why wheels are used to move heavy objects.
3. Why is the coefficient of static friction (μs) generally greater than the coefficient of kinetic friction (μk)?
This is a crucial conceptual question. The coefficient of static friction is greater than the coefficient of kinetic friction because of the nature of surface imperfections at a microscopic level.
- When an object is at rest, the microscopic irregularities (crests and troughs) of the two surfaces have more time to settle and interlock deeply, creating stronger bonds. A greater force is needed to break these initial interlocks. This maximum required force corresponds to limiting static friction.
- Once the object is in motion, the surfaces slide past each other rapidly. The irregularities do not get enough time to form deep interlocks. As a result, the average force needed to maintain the motion (kinetic friction) is less than the force required to start it.
4. Define the angle of friction and angle of repose. Derive the relationship between them.
Both are important 3-mark questions for the Friction chapter.
Angle of Friction (θ): It is the angle that the resultant of the limiting friction (Fₗ) and the normal reaction (R) makes with the normal reaction. From the vector diagram, we can see that tan θ = Fₗ / R. Since we know that Fₗ = μₛR, we get tan θ = μₛR / R, which simplifies to tan θ = μₛ.
Angle of Repose (α): It is the minimum angle of inclination of a plane with the horizontal such that a body placed on it just begins to slide down. By resolving the weight (mg) into components, the force causing it to slide is mg sin α, and the normal reaction is mg cos α. At the point of sliding, mg sin α = Fₗ = μₛR = μₛ(mg cos α). This gives us tan α = μₛ.
Since both tan θ = μₛ and tan α = μₛ, the relationship is that the angle of friction is equal to the angle of repose (θ = α).
5. List some key advantages and disadvantages of friction in our daily lives.
Friction is often called a necessary evil. Here are its important advantages and disadvantages:
Advantages of Friction:
- It allows us to walk without slipping.
- Brakes in vehicles work because of the friction between the brake shoes and the wheels.
- It is essential for writing on paper or a blackboard.
- Nails and screws hold objects together due to friction.
- Transfer of motion through belts in machinery is possible due to friction.
Disadvantages of Friction:
- It causes wear and tear of machine parts, like tyres and shoe soles.
- Friction produces heat, which can damage machinery and reduce efficiency.
- A significant amount of energy is wasted in overcoming friction, which reduces the efficiency of engines.
6. Explain why friction is considered a 'self-adjusting force'.
Static friction is called a 'self-adjusting force' because its magnitude automatically adjusts to be equal and opposite to the applied external force, as long as the external force is less than or equal to the limiting friction. For example, if you push a heavy cabinet with a force of 10 N and it doesn't move, the force of static friction is also 10 N. If you increase your push to 20 N and it still doesn't move, the static friction increases to 20 N. It keeps adjusting itself until your applied force overcomes the maximum possible static friction (limiting friction), at which point the cabinet starts to move.
7. A block of mass 'm' is on a rough horizontal surface with a coefficient of static friction μs. How do you find the minimum horizontal force needed to start its motion?
This is a typical numerical problem setup for Class 11 exams. To find the minimum horizontal force (F) required to start the motion, you must overcome the limiting friction (fₗ). Here are the steps:
- Draw a Free Body Diagram: Show the block with all forces acting on it: weight (mg) acting downwards, normal reaction (N) acting upwards, applied horizontal force (F), and static friction (fₛ) acting opposite to F.
- Balance Vertical Forces: Since there is no vertical motion, the upward forces equal the downward forces. Therefore, the normal reaction N = mg.
- Apply the Condition for Motion: The block will just begin to move when the applied force F is equal to the force of limiting friction (fₗ).
- Calculate Limiting Friction: The formula for limiting friction is fₗ = μₛN.
- Find the Force: Substitute the value of N from step 2 into the formula from step 4. So, F = fₗ = μₛN = μₛmg. This is the expression for the minimum horizontal force required.
8. Explain with a vector diagram why it is easier to pull a lawn roller than to push it.
This is a classic Higher Order Thinking Skills (HOTS) question. The reason lies in how the applied force affects the normal reaction, and consequently, the force of friction.
- When Pulling: The applied force (F) is directed at an angle upwards. This force has a horizontal component (Fcosθ) that moves the roller and a vertical component (Fsinθ) that acts upwards. This upward component reduces the effective downward force. The normal reaction becomes N = mg - Fsinθ. Since friction is proportional to N, the frictional force is reduced, making it easier to pull.
- When Pushing: The applied force (F) is directed at an angle downwards. The vertical component (Fsinθ) now acts downwards, adding to the weight. The normal reaction becomes N = mg + Fsinθ. This increased normal reaction leads to a greater frictional force, making it harder to push.
Therefore, as pulling decreases the opposing frictional force, it is easier than pushing.
9. What are the common methods to reduce friction? Explain the role of lubricants.
From a practical and exam point of view, knowing how to reduce friction is important. The main methods are:
- Polishing Surfaces: Making surfaces smoother reduces the interlocking of irregularities.
- Lubrication: Applying a substance like oil or grease between two surfaces creates a thin film that separates them, reducing direct contact and thus friction. For example, engine oil reduces friction between pistons and cylinders.
- Using Ball Bearings: This is a clever method to convert sliding friction into much weaker rolling friction. Hard steel balls are placed between the moving parts, allowing them to roll instead of slide over each other. This is used in bicycle wheels and motor axles.
- Streamlining: This involves designing the shape of objects (like cars, aeroplanes) to be pointed and curved to reduce fluid friction (air drag).
10. If surfaces are polished to be extremely smooth, the force of friction increases instead of decreasing. Why does this happen?
This is a fascinating exception to the general rule and a great conceptual question. While moderate polishing reduces friction by smoothing out large-scale irregularities, excessive polishing can increase friction. This is because when surfaces become extremely smooth at a molecular level, the distance between the molecules of the two surfaces becomes very small. At such close proximity, the intermolecular forces of attraction (adhesive forces) become very strong. These forces pull the surfaces together, and a significant force is then required to overcome this adhesion, leading to an increase in the frictional force. This phenomenon is also known as cold welding.
