Frictional Force

Frictional Force - Types of Frictional Force

Frictional Force refers to the force generated by two surfaces that contacts 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 becomes 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
F_{frict}=\mu. F_{norm}

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.

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

1. 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 as impeding 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.

(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 twisting of the objects. If a hard ball 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 to 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.

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.

1. 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 which offers no resistance to shearing and so is not viscous.

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

    F=\mu N

    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
    F=\mu mg

    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

    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)

    N=mg cos(\theta)

    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 that 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 that is 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. 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
    F=\mu N

    For instance, consider a wood block 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

    Fslide=\mu slide 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 box. If the coefficient of friction is 0, 3, calculate 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 which is inclined at angle θ = 45°.
    Mentioning that μs = 0.5 is the coefficient of static friction is between 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