What is Frictional Force?
Frictional Force refers to the force generated by two surfaces that contact and slide against each other. These forces are mainly affected by the surface texture and quantity of force requiring them together. The angle and position of the object affect the volume of frictional force.
The main reason behind friction between objects appears is due to the forces of attraction, known as adhesion, between the points of contact regions of the surfaces, which are always minutely irregular. Friction arises from shearing these “fused” junctions and from the action of the irregularities of the harder surface tilling across the softer surface.
If an object is placed against an object, then the frictional force will be the same as the weight of the object. If an object is pushed against the surface, then the frictional force will be increased and become extra than the weight of the object. The full amount of friction force that a surface can apply upon an object can be easily measured with the use of the given formula:
Ffrict = μ Fnorm
Two basic experimental facts describe the friction of sliding solids. First, the volume of friction is nearly independent of the area of contact. If a brick is pulled along a table, the frictional force is similar whether the brick is lying flat or standing on end. Second, friction is directly proportional to the weight that presses the surfaces together. If a load of three bricks is pulled along a table, the friction is three times more than if one brick is pulled. Thus, the ratio of friction F to load L is the same. This constant ratio is called the coefficient of friction and is typically symbolized by the Greek letter mu (μ).
Mathematically, μ = F/L. Because both friction and load are calculated in units of force (such as pounds or newtons), the coefficient of friction is dimensionless. The value of the coefficient of friction for a case of one or more bricks sliding on a clean wooden table is about 0.5, which indicates that a force is equal to half the weight of the bricks is required just to overcome friction in keeping the bricks moving forward with a constant speed. The frictional force is directed oppositely to the motion of the object. Because the friction thus far described rises between surfaces in relative motion, it is called kinetic friction.
If you have a block of brick in your hand and try to slide it on the ground by force then it does not slide that easily, it happens because of frictional force. In the subject of physics, Frictional force is one of the most important topics and hence students must have a good and clear understanding of the same. Therefore, to help the students of physics in understanding the topic of Frictional force, Vedantu provides a complete explanation of the same in an easy language.
Types of Frictional Forces
Friction is the force that competes with motion between any surfaces that are in touching base. Static, kinetic, sliding, and rolling friction takes place between solid surfaces. Fluid friction takes place in liquids and gases. All four types of friction are described below:
1. Dry Friction
a. Static Friction
b. Kinetic Friction
c. Rolling Friction
d. Sliding Friction
2. Fluid Friction
Dry Friction: Dry friction is the force that competes with one solid surface gliding across another solid surface. Dry friction always opposes the surfaces sliding kin to one another and can have the effect of any opposing motion or causing motion in bodies. The most commonly used for dry friction is coulomb friction. This kind of friction can further be divided into static friction and kinetic friction. These two types of friction are explained in the diagram below. First, imagine a box on a surface. A pushing force is applied parallel to the surface and is slowly being increased. A gravitational force, a normal force, and a frictional force are also acting upon the box.
(a) Static Friction
Static friction occurs earlier to the box slipping and moving. In this area, the friction force will be equal in scale and opposite in direction to the pushing force itself. As the degree of the pushing force increases so does the friction force. If the amount of the pushing force continues to rise, eventually the box will start to slip. As the box begins to slip the type of friction resisting the motion of the box changes from static friction to what is called kinetic friction. The point just before the box slips is called an impending motion. This can also be assumed as the maximum static friction force before slipping. The total of the maximum static friction force is the same as the static coefficient of friction times the normal force existing between the box and the surface. This coefficient of friction is a property that depends on both materials and can typically be looked up in tables.
Static Friction Examples
Static friction acts on two bodies in contact, i.e., one body over the other. Let us consider a few real-life examples:
While walking, our feet’s backward motion exerts pressure on the road as the other foot goes forward.
Static friction generates when two fabrics slide over each other.
(b) Kinetic Friction
Kinetic friction occurs beyond the point of coming motion when the box is sliding. With kinetic friction, the amount of the friction force opposing motion will be the same as the kinetic coefficient of friction times the normal force between the box and the surface. The kinetic coefficient of friction also rests upon the two things in contact but will almost always be less than the static coefficient of friction.
(c) Rolling Friction
Rolling friction happens when a wheel, ball, or cylinder roll freely above a surface, as in ball and roller bearings. The main cause of friction in rolling appears to be the distribution of energy involved in the twisting of the objects. If a hardball is rolling on a level surface, the ball is somewhat packed down and the level surface is somewhat indented in the regions of contact. The elastic bend or compression produced at the leading section of the part in contact is interference to motion that is not fully compensated as the substances spring back to typical shape at the trailing section. The internal losses in the two substances are parallel to those that keep a ball from bouncing back to the level from which it is released. Coefficients of sliding friction are usually 100 to 1,000 times greater than coefficients of rolling friction for corresponding materials.
Rolling Friction Examples
Rolling friction is one of the types of kinetic or dynamic friction, which comes into force when one body rolls over the other. For example, when a circular disc, a wheel, a ring, or a sphere rolls over the surface, a force that opposes its motion is the rolling friction.
Other examples are:
A basketball rolling on the court comes to rest after some time.
A tire rolling on the road or skateboard wheels rolling on the road are all examples of rolling friction.
The shape, size, and weight are the factors that affect the motion of vehicles; for example, a twenty-wheeler heavy loaded truck will have higher rolling friction than that of a TATA Nano Car.
Painting the walls using paint rollers.
(d) Sliding Friction
Sliding friction is friction that acts on objects when they are slipping over a surface. Sliding friction is weaker than static friction. That’s why it’s easier to slide a piece of equipment over the floor after you start moving than it is to get it moving in the first place. Sliding friction can be valuable. For instance, you use sliding friction when you write with a pen. The pen “point” slides easily over the paper, but there’s just sufficient friction between the pen and paper to leave a mark.
Sliding Friction Examples
When an object slides over the other, an opposing force that comes into play is the sliding friction. Illustrative sliding friction examples are as follows:
Pushing a lawn roller on the grass.
Rubbing one’s hands together causes heat because of the sliding friction.
Opening a sliding door/window.
Dragging mouse on the surface.
Sliding chalk to make art on the floor.
Speed car racing.
We must know that static friction is the strongest frictional force, followed by sliding friction and rolling friction is the weakest.
Fluid Friction
Fluid friction takes place between fluid layers that are moving opposite to each other. This internal conflict to flow is named viscosity. In everyday terms, the viscosity of a fluid is branded as its “thickness”.
All actual fluids give some resistance to shearing and therefore are viscous. It is very helpful to use the concept of an ideal fluid that offers no resistance to shearing and so is not viscous.
Fluid Friction Examples
While swimming, a swimmer experiences an opposing force; this force is the fluid friction force.
While sucking a drink through a straw, you experience some friction; that friction is fluid friction.
During skydiving, the parachute slows down because of the air resistance.
Flying a paper made an aeroplane.
Examples of Fluid Friction
If there is a wet surface between two thin glass plates, you will see that plates get stuck and the bottom plate doesn’t fall when you hold only the top one.
How to calculate the Force of Friction
Calculate the force of friction using the formula:
F = μn
Where N is the normal force and μ is the friction coefficient for your tools and whether they are stationary or moving. The normal force is equivalent to the weight of the object, so this can also be written as:
F = μmg
Given m is the mass of the object and g is the acceleration due to gravity. The friction behaves to oppose the motion of the object.
Calculating the Force of Friction
Find the Normal Force.
The “normal” force defines the force that the surface an object is resting on exerts on the object. For a motionless object on a flat surface, the force must exactly resist the force due to gravity, otherwise, the object would move, according to Newton’s laws of motion. The “normal” force (N) is the term for the force that does this.
It constantly acts perpendicular to the surface. This means that on a sloppy surface, the normal force would still point straight away from the surface, while the force of gravity would point directly downwards.
The normal force can be simply defined in most cases by:
N = mg
Here, m denotes the mass of the object, and g represents the acceleration due to gravity, which is 9.8 m/s2. This just matches the “weight” of the object.
For sloppy surfaces, the strength of the normal force is minimizing the more the surface is inclined, so the formula becomes:
N = mg cos (θ)
With θ represent the angle the surface is inclined to.
For a simple case calculation, consider a flat surface with a 2-kg block of wood resting on it. The normal force would point directly upwards (to support the weight of the block), and you would measure:
N = 2 kg × 9.8 N/kg = 19.6 N
Find the Right Coefficient.
The coefficient relies on the object and the specific situation you’re working with. If the object is not already moving across the surface, you use the coefficient of static friction μstatic, but if it is moving you will use the coefficient of sliding friction μslide.
Usually, the coefficient of sliding friction is less than the coefficient of static friction. In other words, it is easier to slide something that is already sliding than to slide something unmoving.
The nature of the material also disturbs the coefficient. For instance, if the block of wood from earlier was on a brick surface, the coefficient would be 0.6, but for clean wood, it can be somewhere from 0.25 to 0.5. The static coefficient of friction for ice on ice is 0.1. Again, the sliding coefficient decreases even more, to 0.03 for ice on ice and 0.2 for wood on wood.
The formula for the force of friction states:
F = μN
For instance, consider a woodblock of 2-kg mass on a wooden table, being pushed from still. In this case, you can use the static coefficient, with μstatic = 0.25 to 0.5 for wood. Taking μstatic = 0.5 to take full advantage of the potential effect of friction, and remembering the N = 19.6 N from earlier, the force is:
F = 0.5 × 19.6 N = 9.8 N
Remember that friction only offers force to resist motion, so if you start pushing it slightly and get firmer, the force of friction will escalate to a maximum value, which is what you have just calculated. Physicists sometimes mark Fmax to make this point clear.
Once the block is in motion, you use μslide = 0.2, in this case:
Fslide = μslide N
= 0.2 × 19.6 N = 3.92 N
Problems on Frictional Force
Problem 1 – A 50 N of force is applied to the 6 kg of the box. If the coefficient of friction is 0, 3, calculate the acceleration of the box?
Solution 1– Fnormal = 60 N – 40 N = 20 N
Friction force is – Ffriction = µ.Fnormal = 0, 3.20N = 6N
Net force in –Y to Y = zero,
But, in –X + X direction net force is not zero
Fnet = m.a
Fnet = m.a
Fx – Ffriction = m.a
Fx – Ffriction = m.a
30 N – 6 N = 6 a
a = 4m/s2
Problem 2 - A block of mass M = 10 kg is placed on a surface that is inclined at angle θ = 45°.
Mentioning that μs = 0.5 is the coefficient of static friction is between the block and the surface.
What will be the minimum force F required to prevent slipping?
What will be the maximum force F that can be applied without causing the block to slip?
Solution 2 - The minimum force essential to stop slipping is the minimum force that will inhibit the block from sliding down the incline.
Fmin = 10 g sin(45°) - 10 g cos(45°) x 0.5.
The maximum force that can be applied without causing the block to slip is the maximum force that can be applied without causing the block to slide up the incline.
Fmax = 10 g sin(45°) + 10 g cos(45°) x 0.5.
Fmin = 34.65 N, Fmax = 103.94 N
FAQs on Frictional Force
1. How many types of Frictional forces are there?
There are mainly four types of Frictional forces which are given below: -
Dry Friction: Dry friction explains the reaction between kinetic friction and static friction. That is to say, it resists the sliding movement of moving solid on non-moving solid.
Fluid Friction: As the name suggests it is the friction of viscous fluid.
Rolling Friction: It is the force that opposes the motion of moving rolling solid, such as rolling of the wheels, or ball, or any such solid body.
Sliding Friction: This is kinetic friction, and is created when the two rolling bodies slide along with each other.
2. What causes Frictional force?
In physics, there is a concept of adhesion, which refers to the holding of the particles together with each other, and these particles are always dissimilar. And this adhesion is one of the major causes behind the frictional force. Because in metal or solid bodies the adhesion occurs when the surface of two solid bodies touches each other, which are always at the microscopic level irregular. When such an event occurs, the frictional force arises, due to the dissimilarities of the harder surface and the softer surface.
3. Is understanding the topic of frictional force difficult?
No, understanding the topic of frictional force is not difficult at all. First of all, since the topic of frictional force is included in the syllabus, it in itself means that it has the level of difficulty which the students can easily manage because the syllabus for the physics is designed by taking many factors into consideration and the intellectual capacity of the students is one of the important aspects. And also, the topic is designed in such a manner that students can easily understand it without much problem.
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