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Friction Force is a force that opposes motion when the surface of an object comes into contact with that of another object. It is a force generated by two surfaces that contact and slide against each other.
- It is affected by the surface texture and magnitude of the force acting on the objects.
- It decreases the mechanical advantage of a machine.
- Frictional force can be seen in almost every sphere of the physical world ranging from matchsticks to machines.
Friction Force Formula is given as
| Ff = μFn |
Here, Ff is the magnitude of friction, μ is the coefficient of friction, and Fn is the magnitude of the normal force is Fn.
Read More: NCERT Solutions For Class 11 Physics Laws of Motion
Key Terms: Friction Force, Friction Force Formula, Static Friction, Fluid Friction, Force, Normal Force, Coefficient of Friction
What is Friction Force?
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Friction Force is a force that resists the motion of the object when it comes in contact with the surface of another object. It is not a fundamental force but a derived force.
- It is generated when two surfaces come in contact and slide against each other.
- Friction force is affected by the surface texture and the force impelling them together.
- It is also affected by the angle and position of the object.
- The friction force will be equal to the object’s weight if the object is placed flat against an object.
- It will be more than the weight of the object if the object is pushed against the surface.
- The SI unit of frictional force is Newton (N).
- The dimensional formula of frictional force is [M L T-2].
Frictional Force
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Friction Force Formula
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Friction Force is defined as the hindering force that generates when the surface of two objects slides against each other. Friction Force Formula is given as
| Ff = μFn |
Where
- Ff: Magnitude of Friction
- µ: Coefficient of Friction
- Fn: Normal Force acting on the Object.
Normal Force Formula
Normal force is the force exerted upon an object that is in contact with another object. It is equivalent to the weight of the given object. Normal Force Formula is expressed as:
| Fn = mg |
Where
- m: Mass of the Object
- g: Acceleration due to Gravity.
Solved ExampleExample: A coolie is pulling a box of mass 10 Kg. Determine the standard force and the frictional force if the Coefficient of Friction μ is 0.3. Solution: Given that,
Normal Force is given using the formula, Fn = mg Fn = 10 Kg × 9.8 m/s2 = 98 N. Using the Friction Force Formula, we get, Ff = μFn Ff = 0.3 × 98 N = 29.4 N. Thus, the frictional force is 29.4 N. |
Read More: Laws of Motion Class 11 Important Questions
Calculation of Friction Force
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Friction Force applied on an object can be easily calculated with the help of the Friction Force Formula given as:
| Ffriction = µ x Fnormal |
Thus, in order to calculate the frictional force, we need to follow three major steps:
Step 1: Finding the Normal Force
Normal Force is defined as the support force exerted on an object that is in contact with another object. It is calculated using the formula:
N = mg
Here,
- m is the mass of the object.
- g is the acceleration due to gravity.
The strength of the normal surface is reduced when the surface is inclined, thus, in this case, the normal force formula is given as
N = mg cos(θ)
Here, θ is the angle to which the surface is inclined. For instance, to calculate the normal force of a 2 kg object, N = 2 kg × 9.8 N/kg = 19.6 N.
Step 2: Finding the Right Coefficient
The coefficient of the force depends on the object and the situation in which force is being applied to.
- When the object does not move across the surface, the coefficient of static friction μstatic is used.
- If the object is moving, the coefficient of sliding friction μslide is used.
- The coefficient is affected by the type of materials used.
For instance, the coefficient of a block on a brick surface would be 0.6, but if it was a block of clean wood, it would be around 0.25 to 0.5.
Step 3: Calculating Frictional Force
Friction Force Formula is given as
F = μN
Consider that a block of wood that weighs 2 kg needs to be pushed from the rest. The static coefficient of wood is given as 0.5. First, we will calculate the normal force as N = 2 kg × 9.8 N/kg = 19.6 N. Now, substitute the values of normal force and coefficient into the friction force formula, such that
F = 0.5 × 19.6 = 9.8 N
Types of Frictional Forces
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Frictional force can be classified into two major types based on the type of material in contact.
- Dry Friction
- Fluid Friction
Dry Friction
When two solid bodies are in contact with each other, whether they are moving (kinetic friction) or not moving (static friction), there is a force known as Dry friction.
- Static and kinetic friction are both proportional to the normal force exerted between the solid bodies.
- Different coefficients of friction are used to describe the interaction of different substances.
- It means certain substances have a higher resistance to movement than others for the same normal force between them.
Fluid Friction
Fluid is defined as a substance that can flow and can take the shape of the container in which it is kept.
- Fluid friction is the resistance that a liquid or gas offers to a moving object.
- In simpler terms, it is the force exerted by fluids on different objects.
- Air resistance force or drag force is the term given to the frictional force produced by the air.
- It is the fluid friction that makes it difficult to walk through a pool of water in the middle of a storm.
Read More: Laws of Motion MCQs with Answers
Examples of Frictional Force
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Some real-life examples of Frictional Force are as follows:
- Walking: There is a frictional force between the ground and the feet, thus, creating a solid connection between the foot and the ground.
- Sliding a Sled: Sliding a sled over an icy surface is another example of friction.
- Climbing: The ability of a climber to move on the rock is resisted by frictional force and can be felt easily.
- Rubbing Hands: When we rub our hands together, a force that opposes this motion of the hands is activated when they brush against one another.
Running (Example of Friction)
Solved Problems on Friction Force Formula
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Here are a few solved problems on the Friction Force Formula to understand the concept better:
Example 1: A man is pushing his boat on the shore across the mud to get into the water. The coefficient of friction is 0.400 and the boat has a mass of 40 kg. What will be the magnitude of the friction force acting on the boat?
Solution: Given that,
- Mass (m) = 40 Kg
- Coefficient of Friction (μ) = 0.4
- Friction Force (Ff) =?
We know that,
Fn = mg
Now, using the Friction Force Formula,
Ff = μFn
Ff = (0.400)(40.0 kg)(9.80 m/s2) = 156.8 N
Thus, the frictional force acting on the boat is calculated as 156.8 N.
Example 2: A boy of mass 40 Kg is slipping on the frost. Find the frictional force acting between the boy and the frost layer if the coefficient of friction acting is 0.45.
Solution: It is given that,
- Mass (m) = 40 Kg
- Coefficient of friction (μ) = 0.3
First, we will calculate the Normal Force using the formula,
Fn = mg
Fn = 40 Kg × 9.8 m/s2 = 392 N
Using the Friction Force Formula,
Ff = μ Fn
Ff = 0.45 × 392 N = 176.4 N.
Thus, the frictional force between the boy and the frost layer is 176.4 N.
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Things to Remember
- Friction Force is a force that resists the motion of an object when it comes in contact with the surface of another object.
- Surface texture and magnitude of the force affect the frictional force.
- Dry Friction and Fluid Friction are the two types of friction force.
- Friction Formula Formula is Ff = µFn, where, Fn is the normal force, µ is the coefficient of frictional force, and Ff is the friction.
- Frictional force is measured in Newtons and has the dimensional formula [M L T-2].
Previous Years Questions
- If the coefficient of static friction is μs, and coefficient of kinetic friction… (Haryana PMT 2007)
- If the coefficient of static friction between the tyres and road… (BCECE 2007)
- If the coefficient of static friction between the shoes of a runner… (UPSEE 2018)
- Assertion: Frictional forces are conservative forces. Reason… (AIIMS 2019)
- The minimum force required to start pushing a body up a… (AIEEE 2011)
- The limiting friction is… (AFMC 2007)
- Starting from rest, the time taken by a body sliding down on a rough… (BITSAT 2008)
- Which is true for rolling friction, static friction… (JIPMER 2006)
- A small block of mass m is kept on a rough inclined surface… (BITSAT 2014)
- A body can be at rest even when it is under action… (AIIMS 2013)
Sample Questions
Ques. A 5 kg body is moving through a surface with a 0.3 friction coefficient. Calculate the frictional force. (3 Marks)
Ans. Given that
- Mass m = 5kg
- Coefficient of Friction µ = 0.3
We know that,
Normal Force Fn = mg
Fn = 5 x 9.81 = 49.05 N
Using the Friction Force Formula,
Ff = µFn
Ff = 0.3 x 49.05 = 14.715 N
Thus, the frictional force is 14.715 N.
Ques. What are the factors affecting Frictional Force? (2 Marks)
Ans. Frictional Force is affected by two important factors:
- Nature of the two surfaces in contact.
- Nature of the force acting on both surfaces.
Ques. A block of ice of a mass of 300 kg is being pulled across a frozen lake. What is the frictional force acting on the block of ice if the coefficient of friction between two ice surfaces is 0.05? (3 Marks)
Ans. Given that,
- Mass m = 300 kg
- Coefficient of Friction µ = 0.05
We know that the normal force on an object is given by Fn = mg.
Using the Friction Force Formula,
Ff = µFn
Substituting the values in the above equation we get,
Ff =0.05 × 300 kg × 9.8 m/s2 = 147 kg-m/s2 or 147 N.
Thus, the frictional force acting on the block of ice is 147 N.
Ques. A circular racetrack with a radius of 300m is made at an angle of 15 degrees. 0.2 is the coefficient friction between the road and wheels of the race car.
(a) Calculate the maximum speed of the race car to avoid depletion of the tyres.
(b) Maximum permissible speed to limit slipping. (3 Marks)
Ans. The centripetal force which keeps the car in motion In a circular path without slipping is provided by the horizontal component of the normal force and the frictional force. At the maximum speed, the centripetal force is obtained from the normal reaction’s component and the frictional force is unnecessary. Thus,
- R = 300m
- θ= 15o
- g = 9.8ms-2
V0 = (Rg tan θ)1/2
V0 = 28.1 ms-1
Vmax = [Rg (µs + tanθ) /(1- µs tanθ)]½ = 38.1 ms-1
Ques. What is the net force acting on an object if some external force is applied to the object to keep it moving on a horizontal surface with constant velocity? (1 Mark)
Ans. There will be no acceleration if the object moves with constant velocity. According to Newton's second law of motion, there will be no net force on the object. The kinetic friction and the external force will be balanced.
Ques. What is Static Friction? Give examples. (2 Marks)
Ans. Static Friction is defined as the amount of resistance that exists between an object and the surface it is resting on. In order to move an object that is resting on a surface, one needs to apply a force that is greater than the frictional force generated by the surface. Some examples of static friction include walking, rock climbing, and other activities.
Ques. State a few examples of Frictional Force. (3 Marks)
Ans. Some examples of Frictional Force are as follows:
- A swimmer experiences an opposing force called the fluid friction force while swimming.
- When we suck a drink through a straw, we experience some friction called fluid friction.
- The parachute slows down because of the air resistance during skydiving.
Ques. What is Rolling Friction? (1 Mark)
Ans. Rolling friction is the resistance generated when one body is forced to roll across the surface of another. It is considerably less in magnitude in comparison to kinetic friction. Rolling friction can easily be seen in roller skates, ball bearings, etc.
Ques. Explain Fluid Friction. (3 Marks)
Ans. Fluid friction is a force that resists the flow of fluid. Here, the resistance is provided by the fluid between two surfaces. Some examples of fluid friction are as follows:
- We need to lubricate the door hinges for the smooth functioning of the door and avoid creaking sounds while opening and closing them.
- The water splashes out of the bucket when a ball is dropped in a bucket full of water due to the buoyancy of the fluid.
Ques. How to find the right coefficient of friction? (2 Marks)
Ans. The coefficient of friction depends on the object and the specific situation in which the friction is caused. The coefficient of static friction is used if the object is not moving across the surface, whereas if the object under consideration is moving. the coefficient of sliding friction is used. It is also affected by the type of materials used.
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