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Frictional force can be defined as the force that opposes motion when one object’s surface comes into contact with another object’s surface. Friction reduces the machine’s mechanical advantage or in other words the ratio of output to input. Friction is one of the most important physical phenomena, affecting everything from matches to machines to molecular structures.
Static, sliding and rolling friction take place between solid surfaces. Static friction is the strongest force, which is followed by sliding friction and then rolling friction is the weakest. Fluid friction can be seen in fluids, which are liquids or gases.
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Key Terms: Frictional Force, Forces, Friction, Inertia, Coefficient of Friction, Physical phenomena, liquids, gases, solid, Static friction, sliding friction, rolling friction, Dry friction, Fluid friction
What is Frictional Force?
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“Frictional Force is the force created when two surfaces come into contact with each other and slide against each other.” These types of forces get affected by the texture of the surface and the amount of force required by both surfaces together. The volume of the frictional force gets affected by the position and angle of the object.
When an object is placed against another object, the frictional force is equal to the weight of the object. And, when an object is pushed against the surface, the frictional force increases and becomes greater than the object’s weight.
The total amount of frictional force that a surface applies to an object can be easily measured with the following formula:
| Ffriction = μ.Fnorm Ffriction = Frictional Force, μ = Coefficient of Friction, Fnorm = Normal Force |
Types of Frictional Forces
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Friction is the force that opposes motion between any surfaces that are in contact with one another. Dry friction which includes static, kinetic, sliding, and rolling friction generally occurs between solid surfaces whereas the fluid friction takes place in liquids and gases. The friction is mainly classified as:
- Dry Friction
- Fluid Friction
Let’s discuss the types of frictional forces in detail.
Dry Friction
The force that competes with one solid surface sliding across another is known as Dry Friction. The force of dry friction is generally calculated using Coulomb friction.
Static Friction
Static Friction is defined as the friction that occurs when individuals attempt to move a stationary object on a surface without actually causing any relative motion between the body and the surface which it is on. The static frictional force is a self-regulating force which means that it will always be equivalent to and opposite of the applied force.
Example: The backward motion of the feet exerts pressure on the road while walking and the other foot goes forward.
| F = -fr (When no motion takes place) r = reaction force because of the weight W, F = external force, fr = friction |
Kinetic Friction
The force that acts between two moving surfaces is known as Kinetic Friction. The strength of the force is dependent on the kinetic friction coefficient of the two materials.
Kinetic friction is defined as a force that acts between moving surfaces. The equation of kinetic friction is given as:
| Fk= μk η Fk = Kinetic Friction Force, η = Normal Force, μk = Coefficient of Kinetic Friction |
Rolling Friction
Rolling Friction is defined as the force that resists the motion of a rolling body on a surface. Example: When a wheel or circular disc rolls over any surface, the force opposing its motion is known as rolling friction. Given below are the laws of rolling friction:
- Law 1: The force of rolling friction decreases as the smoothness increases.
- Law 2: Rolling friction is expressed as a product of a fractional power of the load and constant. F = kLn
- Law 3: Rolling friction force (F) is directly proportional to load as well as inversely proportional to the radius of curvature. F=μ×W/r
The equation of rolling friction is:
| Fr=μrW Fr = Resistive Force of Rolling Resistance, μr = Coefficient of Rolling Resistance, W = Weight of the Rolling Body |
Sliding Friction
Sliding Friction is defined as the resistance created by any two objects that slide against each other. This is also known as kinetic friction and is defined as the force required to keep one surface sliding along another. It depends upon two variables i.e. material and weight of the object.
Example: While writing with a pen you are using sliding friction. The pen point easily slides over the paper but there is a sufficient amount of friction between the pen and paper to create a mark on the paper.
The equation for sliding force is:
| FS = μSFn FS = Force of Sliding Friction, μS = Coefficient of Sliding Friction, Fn = Normal Force |
Fluid Friction
The force that prevents the flow of fluid is known as Fluid Friction. It is a scenario in which the fluid offers resistance between the two surfaces. If both surfaces provide high resistance then it is known as high viscous.
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| Related Chapters to this Topic | ||
|---|---|---|
| NCERT Solutions for Class 11 Laws of Motion | Inclined plane acceleration | Force of friction |
| Momentum and Inertia | Force and motion | Effects of forces |
Calculation of Frictional Force
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First, find out the normal force.
N = mg
Where,
- m = mass of the object
- g = 9.8 m/s2 which is acceleration due to gravity
For sloppy surfaces, the strength of the normal force decreases as the surface become more inclined. Hence, the formula becomes:
N = mg cos (θ)
Here θ represents the angle the surface is inclined to.
For calculation, consider a flat surface having a 2kg block of wood on it. So, the normal force will be:
N = mg
= 2 kg x 9.8 N/kg
= 19.6 N
The formula for the force of friction will be:
F = μN
Where,
- N = 19.6 N
- μstatic = 0.25 to 0.5 for wood
Taking μstatic = 0.5 the force of friction will be: F = 0.5 x 19.6 N = 9.8 N
Solved Examples on Frictional ForceExample 1: A 5 kg box on a horizontal table is pushed by a horizontal force of 15 N as shown on the right. If the coefficient of friction is 0.4, will the box move? Solution: The sketch shows the forces acting on the box. Note that the weight of a box of mass 5 kg is 5g where g = 9.8 ms–2 Since the vertical forces are in equilibrium, R = 5g Therefore the maximum possible friction is F = μR = 0.4 x 5g = 19.6 N The pushing force, 15 N, is less than this and so cannot overcome the friction. The box will not move. Example 2: A block of Mass M is moving with a velocity v on straight surface.What is the shortest distance and shortest time in which the block can be stopped if μ is coefficient of friction? Solution: Force of friction opposes the motion Force of friction=μN=μmg Therefore retardation =μmg/m=μg From v2=u2+2as or s=v2/2μg from v=u+at or t=v/μg |
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Things to Remember
- Frictional forces is a part of CBSE class 11 physics syllabus unit 3 chapter 5, laws of motion.
- It carries a total of 14 periods and 4 to 7 marks.
- Frictional forces help in slowing down or stopping things in motion.
- Friction refers to the rate of resistance an object in motion shows relative to another object.
- Frictional force can be denoted by f = μN
Sample Questions
Ques 1. A 5 kg box on a horizontal table is pushed by a horizontal force of 15 N as shown on the right. If the coefficient of friction is 0.4, will the box move? (4 marks)
Ans: The weight of a 5 Kg mass box is 5g and g = 9.8 ms–2.
The vertical forces are in equilibrium, R = 5g.
Therefore, the maximum possible friction will be:
F = μR
= 0.4 × 5g
= 19.6 N
The pushing force, 15 N, is less than the maximum possible friction therefore the box will not move.
Ques 2. A block of mass (M) = 10 kg is placed on a surface that is inclined at an angle θ = 45°. (Note that μs = 0.5 is the coefficient of static friction between the block and the surface.) (4 marks)
a) What will be the minimum force F required to prevent slipping?
b) What will be the maximum force F that can be applied without allowing the block to slip?
Ans: The minimum force to prevent slipping is:
Fmin = 10 g sin(45°) - 10 g cos(45°) x 0.5.
The maximum force that can be applied without making the block to slip is:
Fmax = 10 g sin(45°) + 10 g cos(45°) x 0.5.
Fmin = 34.65 N
Fmax = 103.94 N
Ques 3. A large block of ice is being pulled across a frozen lake. The block of ice has a mass of 300 kg. The coefficient of friction between two ice surfaces is small: μk = 0.05. What is the force of friction that is acting on the block of ice? (4 marks)
Ans: The normal force on an object on a flat surface can be referred to as:
N = mg.
So, the force of friction will be:
Ff = μN
After placing the value of N we get:
Ff = μmg
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.
The force of friction acting on the block of ice is 147 N.
Ques 4. When a man walks on a rough surface, what is the angle between the force of friction and the instantaneous velocity of the person? (1 mark)
Ans: Here, the instantaneous velocity will be in the forward direction. When the man pushes the ground to move, the friction opposes the man from slipping. So, it is because of friction we move on the ground which means the friction is in the direction of motion i.e., in the forward direction. So, the angle will be zero.
Ques 5. The block shown below moves with a constant velocity on a horizontal surface. Two of the forces on it are shown. A frictional force exerted by the surface is the only other horizontal force on the block. The frictional force is. (3 marks)
Ans: When the block moves with constant velocity on a horizontal surface, the acceleration of the block is zero. As per Newton's Second Law, the net external force will be zero with zero acceleration.
Fnet = 0
Taking right direction as positive force direction we will get:
5 N + f N + (-3) N = 0 => f = -2 N
Where, f is the frictional force.
The value of frictional force is 2N in the leftward direction.
Ques 6. What are the types of dry friction? (2 marks)
Ans: The types of dry frictional force are:
- Static Friction
- Kinetic Friction
- Rolling Friction
- Sliding Friction
Ques 7. What happens to the coefficient of friction when the weight of the body is doubled? (1 mark)
Ans: The coefficient of friction remains the same.
Ques 8. Automobile tyres are generally uneven in shape, why? (1 mark)
Ans: Automobile tyres are generally uneven in shape in order to increase the frictional force.
Ques 9. How does lubricant help in minimizing friction? (1 mark)
Ans: When lubricant is applied it acts as a thin layer separating the two surfaces in contact. Thus giving rise to liquid friction which is generally much smaller.
Ques 10. What is cold welding? (2 marks)
Ans: Generally there is a decrease in the frictional force with an increase in the smoothness of the surface. However with an increase in smoothness beyond a certain point the binding force of adhesion rises which in turn increases friction. This occurrence is known as cold welding.
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