Friction Formula - Types, Calculation, Examples

The mechanical advantage of a machine is reduced by friction, or, to put it another way, the output-to-input ratio is reduced. Friction, a phenomenon that has an impact on everything from machines to molecular structures to matches, is one of the most significant occurrences in the physical universe.

What Is Frictional Force?

The physical phenomenon known as frictional force resists any relative motion or attempted motion between two surfaces that are in touch. Frictional force is what prevents motion when two things are in contact and one tries to move or slide over the other. The interactions between the tiny imperfections on the object's surfaces give rise to this force.

There are two main types of frictional forces:

• Static Friction : This type of friction prevents the initiation of motion between two surfaces that are in contact but not moving relative to each other. It's the force that you need to overcome to start an object moving. Once the object starts moving, static friction is typically overcome by other forces, like kinetic friction.
• Kinetic Friction : Also known as sliding or dynamic friction, this force opposes the relative motion between two surfaces that are already in motion with respect to each other. Kinetic friction is generally slightly weaker than static friction for most surfaces.

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Calculating The Frictional Of Force

With the aid of the following formula, the maximum amount of friction force that a surface can exert on an item can be quickly determined:

F _frict = µ•Fnorm

Here are some meticulous methods to follow when estimating the frictional force that will assist you in doing so.

• Find the Normal Force

When an object contacts another stable object, a known support force is applied to it. Most of the time, the normal force may be easily defined using the following formula:

N = mg

In this equation, m denotes the mass of the item, and g is the acceleration brought on by gravity. The strength of the normal surface decreases as an inclined surface grows more inclined, hence the formula becomes:

N = mg cos(θ)

θ represents the angle to the surface is inclined to. In a simple calculation, you would calculate the normal force of a 2-kg block of wood sitting on a surface as N = 2 kg × 9.8 N/kg = 19.6 N

• Finding the Right Co-efficient

Your choice of coefficient will rely on the nature of the object and the circumstances. Use the coefficient of static friction when an object isn't moving over the surface; nevertheless, while it is moving, use the coefficient of sliding friction.

The co-efficient is also impacted by the materials utilised. For instance, the coefficient for a block on a brick surface would be 0.6, whereas for a block of clean wood, it would be in the range of 0.25 to 0.5.

• Calculating Frictional Force

As was mentioned, F = N is the formula for frictional force.

Let's use the 2-kg block of wood that is resting on a table to be pushed from rest as an example. The static friction coefficient is taken into account in this situation. 0.5 is the static coefficient of wood.

With the information provided, we can get the normal force using the formula N = 2 kg 9.8 N/kg = 19.6 N. Knowing the static friction coefficient and normal force,we can use the formulas below to determine the frictional force:

F = 0.5 × 19.6 N = 9.8 N

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Different Types of Frictional Force

The main types of frictional forces are:

• Static Friction: When two surfaces come into touch but are stationary in relation to one another, friction of this kind develops. It prevents an object from starting to move when a force is applied. Once the force applied exceeds the maximum static friction, the object will start to move.
• Kinetic (or Dynamic) Friction: Kinetic friction occurs when two surfaces are in relative motion. The force of kinetic friction is typically less than static friction.
• Rolling Friction: This type of friction occurs when an object rolls over a surface. It is usually lower than sliding friction (kinetic friction) and is observed in situations where an object, like a wheel or a ball, is in rolling motion.
• Fluid Friction (Drag): Fluid friction, also known as drag, is the resistance an object experiences when moving through a fluid (liquid or gas). It is influenced by factors such as the shape of the object, the viscosity of the fluid, and the speed of motion.
• Lubricated Friction: This type of friction occurs when a lubricating substance, such as oil or grease, is used to reduce friction between two surfaces in contact. Lubricants help to minimize wear and heat generated by friction.
• Internal Friction: Also known as deformational or molecular friction, this type of friction arises from the interactions between molecules within a solid material when it is deformed. It contributes to the material's overall resistance to deformation.
• Traction: Traction is a specialized form of friction that comes into play when one surface provides a gripping force on another surface, allowing for the transfer of motion. This is commonly seen in applications like tire-road interactions.
• Air Resistance: Air resistance, also known as air friction or aerodynamic drag, is the frictional force experienced by an object moving through the air. It becomes significant at higher speeds and can affect the motion of objects like vehicles and projectiles.

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Inclined Surface

Component of Weight Parallel to the Incline

mg ⋅sin(θ)

1. Sliding Down an Incline (Constant Acceleration): If an object is sliding down an inclined plane without any external force applied (other than friction), you can use Newton's second law along the incline:

F _net = m ⋅ a = mg ⋅ sin(θ) − F _friction

Where,

F _net = Net force along the incline

α = Acceleration along the incline

2. Impending Motion (Maximum Friction): When you're calculating the maximum angle at which an object is on the verge of sliding down an incline, you set the friction force to its maximum value:

F _friction , _{max = μ max ⋅ N}