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# Friction

Engineering Mechanics: Friction

Contents

Define the term Friction 1

State different kinds of Friction 2

State the Laws of Friction 3

Recall the following terms related to the Friction i. Angle of Friction ii. Angle of Repose iii. Cone of Friction 4

Describe the following applications of Friction i. Wedges ii. Ladders iii. Rope 5

Define the following terms i. Load and Effort ii. Mechanical Advantage iii. Velocity Ratio iv. Input and Output v. Ideal Machine and Ideal Effort 6

Describe the concept of Practical Machine 7

Recall the Law of Machine 7

Classify Pulley system 8

Recall the concept of Wheel and Axle 9

Define the term Screw Jack 9

Recall the principle of Screw Jack 10

Explain the concept of lead of Screw Jack 10

Define and classify Winch Crab 11

# Define the term Friction

Friction is a force that opposes the relative motion between two surfaces that are in contact. It is the force that resists motion between two bodies when they are trying to slide against each other. Friction is a fundamental concept in engineering mechanics and plays a critical role in the design and analysis of many types of structures and machines.

Friction is an important factor in determining the behavior of a structure under load. It can cause a reduction in the efficiency of mechanical systems, increase wear and tear, and increase energy consumption. In order to design structures and machines that are safe and efficient, engineers must understand the nature of friction and its impact on the performance of systems.

There are two types of friction: static friction and kinetic friction. Static friction is the force that resists the initiation of motion between two surfaces. It is greater than the force of kinetic friction, which is the force that resists the motion of two surfaces that are already in motion.

Friction depends on several factors, including the roughness of the surfaces in contact, the normal force acting on the surfaces, and the coefficient of friction between the surfaces. The coefficient of friction is a dimensionless number that represents the ratio of the force of friction to the normal force acting on the surfaces. The coefficient of friction depends on the materials in contact and can be found experimentally or calculated using models.

In summary, friction is a critical factor in the design and analysis of structures and machines, and a basic understanding of this force is essential for engineers working in this field.

# State different kinds of Friction

There are two main types of friction: static friction and kinetic friction.

1. Static Friction: Static friction is the friction that opposes the initiation of motion between two surfaces in contact. It is the force that prevents two surfaces from sliding against each other when a force is applied. The force of static friction is proportional to the normal force acting on the surfaces and is determined by the coefficient of static friction, which is a material property. The coefficient of static friction is usually greater than the coefficient of kinetic friction, which represents the force of friction between two surfaces in motion.
2. Kinetic Friction: Kinetic friction is the friction that opposes the motion between two surfaces in contact. It is the force that resists the sliding of two surfaces against each other when they are already in motion. Kinetic friction is proportional to the normal force acting on the surfaces and is determined by the coefficient of kinetic friction, which is a material property. The coefficient of kinetic friction is usually less than the coefficient of static friction.

There are also other types of friction that can be observed in different types of systems and environments, such as:

1. Rolling Friction: Rolling friction is the force that resists the rolling motion of a wheel or a ball. It is smaller than the force of static or kinetic friction and is determined by the coefficient of rolling friction, which is a material property.
2. Fluid Friction: Fluid friction is the force that opposes the motion of a fluid or the relative motion of two fluids. It is determined by the viscosity of the fluid and the velocity of the fluid flow.
3. Air Friction: Air friction is the force that opposes the motion of an object through the air. It is determined by the density of the air, the velocity of the object, and the shape of the object.

In summary, friction is a complex phenomenon that can take different forms and can depend on a wide range of factors, including the materials in contact, the normal force acting on the surfaces, and the velocity of the relative motion. Understanding the different types of friction is important for engineers who design and analyze mechanical systems.

# State the Laws of Friction

The laws of friction are principles that describe the behavior of friction in mechanical systems. There are two main laws of friction:

1. Coulomb’s law of friction: Coulomb’s law of friction states that the maximum friction force between two surfaces in contact is proportional to the normal force acting on the surfaces, and is independent of the area of contact between the surfaces. Mathematically, this law can be expressed as:

Ffmax = μN

where Ffmax is the maximum friction force, μ is the coefficient of friction, and N is the normal force acting on the surfaces. The coefficient of friction is a material property that depends on the type of materials in contact and the roughness of the surfaces.

1. Amonton’s law of friction: Amonton’s law of friction states that the force of friction between two surfaces in contact is proportional to the normal force acting on the surfaces, and is independent of the velocity of the relative motion between the surfaces. Mathematically, this law can be expressed as:

Ff = μ * N

where Ff is the friction force, μ is the coefficient of friction, and N is the normal force acting on the surfaces.

In summary, Coulomb’s law of friction and Amonton’s law of friction are fundamental principles that describe the behavior of friction in mechanical systems. These laws are used to predict the force of friction in a wide range of applications, from the design of brakes and clutches to the analysis of sliding and rolling motions.

# Recall the following terms related to the Friction i. Angle of Friction ii. Angle of Repose iii. Cone of Friction

1. Angle of Friction: The angle of friction is the minimum angle of inclination at which an object begins to slide when it is placed on a rough surface. It is defined as the angle between the horizontal plane and the inclined plane that is just enough to make the object start to slide. The angle of friction depends on the coefficient of friction between the two surfaces in contact and the normal force acting on the surfaces.
2. Angle of Repose: The angle of repose is the maximum angle of inclination at which an object can be placed on a rough surface without slipping. It is defined as the angle between the horizontal plane and the inclined plane that is just enough to make the object start to slip. The angle of repose depends on the coefficient of friction between the two surfaces in contact and the normal force acting on the surfaces.
3. Cone of Friction: The cone of friction is a graphical representation of the relationship between the frictional force acting on an object and the normal force acting on the surfaces in contact. The cone of friction is a three-dimensional cone with its base at the origin and its vertex at the point where the frictional force is equal to the maximum frictional force. The angle between the vertical axis and the cone axis represents the angle of friction, while the angle between the cone axis and the cone surface represents the angle of repose.

In summary, the angle of friction, angle of repose, and cone of friction are important concepts related to the study of friction. These terms are used to describe the behavior of objects in contact with rough surfaces and to predict the frictional forces acting on the objects. Understanding these concepts is essential for analyzing and designing mechanical systems that involve friction.

# Describe the following applications of Friction i. Wedges ii. Ladders iii. Rope

1. Wedges: Wedges are tools that are used to split objects or materials apart. They use friction to generate a large force with a small applied force. A wedge is usually a triangular shape with a sharp edge that is driven into an object. The applied force generates a compressive force in the object, which is transmitted to the base of the wedge. This compressive force generates a frictional force at the interface between the wedge and the object, which is responsible for splitting the object apart. Wedges are commonly used in construction and manufacturing to split timber, stone, and other materials.
2. Ladders: Ladders are used to access high places and are supported by friction between the ladder rungs and the ground. Ladders rely on friction to provide stability and prevent slipping. The frictional force between the ladder rungs and the ground is proportional to the normal force acting on the rungs, and is determined by the coefficient of friction between the two surfaces. Ladders are designed with a sufficient angle of inclination so that the frictional force between the rungs and the ground is greater than the weight of the person climbing the ladder.
3. Rope: Ropes are used to transmit forces and are supported by friction between the rope and the pulley or other objects. Ropes rely on friction to provide stability and prevent slipping. The frictional force between the rope and the pulley is proportional to the normal force acting on the rope, and is determined by the coefficient of friction between the two surfaces. Ropes are commonly used in construction and manufacturing to lift heavy objects and to transmit forces between objects.

In summary, friction is an important concept that is used in many applications. Wedges, ladders, and ropes are three examples of applications where friction is used to generate large forces, provide stability, and prevent slipping. Understanding the principles of friction is essential for designing and analyzing these and other applications.

# Define the following terms i. Load and Effort ii. Mechanical Advantage iii. Velocity Ratio iv. Input and Output v. Ideal Machine and Ideal Effort

1. Load and Effort: Load and effort are terms used to describe the forces involved in a machine. The load is the force that a machine is designed to lift, move, or support. The effort is the force applied by the operator or other source to lift, move, or support the load. The load and effort forces are related by the mechanical advantage of the machine.
2. Mechanical Advantage: Mechanical advantage is a measure of the ability of a machine to amplify the force applied by the operator, or to reduce the force required by the operator to lift, move, or support a load. The mechanical advantage of a machine is defined as the ratio of the load force to the effort force, and it determines how easily the machine can perform a specific task. The higher the mechanical advantage, the less effort force required to lift, move, or support a load.
3. Velocity Ratio: Velocity ratio is a measure of the relationship between the velocity of the effort and the velocity of the load in a machine. The velocity ratio is defined as the ratio of the velocity of the effort to the velocity of the load, and it determines how quickly the machine can perform a specific task. The higher the velocity ratio, the more quickly the load can be lifted, moved, or supported by the machine.
4. Input and Output: Input and output are terms used to describe the forces involved in a machine. The input is the force applied by the operator or other source to lift, move, or support the load. The output is the force exerted by the machine to lift, move, or support the load. The input and output forces are related by the mechanical advantage and velocity ratio of the machine.
5. Ideal Machine and Ideal Effort: An ideal machine is a theoretical machine that has no friction, no wear, no heat loss, and no other sources of inefficiency. An ideal machine has an infinite mechanical advantage and an infinite velocity ratio, and it is used as a benchmark to compare the efficiency of real machines. The ideal effort is the minimum force required to lift, move, or support a load in an ideal machine. It is used as a benchmark to compare the effort required by real machines.

In summary, load, effort, mechanical advantage, velocity ratio, input, output, ideal machine, and ideal effort are all important concepts in the study of machines and mechanisms. Understanding these concepts is essential for analyzing and designing machines, and for determining how efficiently a machine can perform a specific task.

# Describe the concept of Practical Machine

A practical machine is a type of machine that is used in real-world applications and is designed to perform work efficiently by converting applied effort into useful work output. In contrast to an ideal machine, which is a theoretical machine that operates with 100% efficiency, a practical machine has some level of friction, wear and tear, or other losses that reduce its overall efficiency. As a result, the actual output of a practical machine is lower than the ideal output, and the amount of effort required to perform the same amount of work is higher. To determine the efficiency of a practical machine, the input effort and the output work must be measured and compared, taking into account the losses that occur due to friction and other factors.

# Recall the Law of Machine

The Law of Machines, also known as the Law of Mechanics, is a fundamental principle in the study of mechanics and machines that states that the mechanical advantage (MA) of a machine is equal to the ratio of the output force to the input force, or the ratio of the distance the output force moves to the distance the input force moves. Mathematically, it can be expressed as:

MA = Output force / Input force = Output distance / Input distance

This law is important in the design and analysis of machines, as it allows engineers to determine the amount of effort required to perform a given amount of work, and to optimize the design of a machine to minimize effort while maximising output. It also provides a way to compare the efficiency of different types of machines, as the mechanical advantage provides a measure of the machine’s ability to amplify or reduce the effort applied to it. In practice, the actual mechanical advantage of a machine is always lower than the ideal mechanical advantage due to losses due to friction and other factors, which must be taken into account when designing or analyzing a machine.

# Classify Pulley system

Pulley systems are classified based on several factors, including the number of pulleys, the arrangement of the pulleys, and the direction of the applied force. Some common classifications of pulley systems are:

1. Fixed Pulley Systems: A fixed pulley system has a pulley that is attached to a stationary support and does not move. This type of pulley system changes the direction of the applied force, but does not change its magnitude.
2. Movable Pulley Systems: A movable pulley system has a pulley that is attached to an object that moves, such as a crane or a block and tackle. This type of pulley system both changes the direction of the applied force and amplifies its magnitude.
3. Block and Tackle Systems: A block and tackle system is a combination of fixed and movable pulleys, where multiple pulleys are arranged in series to provide a large mechanical advantage. This type of pulley system can amplify the applied force to a significant extent.
4. Compound Pulley Systems: A compound pulley system is a combination of fixed and movable pulleys where the mechanical advantage of the system is greater than one. This type of pulley system amplifies the applied force even more than a block and tackle system.
5. Friction Pulley Systems: A friction pulley system has a pulley that is mounted on a shaft and rotates. The force is transmitted through the pulley by friction between the pulley and the belt. This type of pulley system is used in power transmission systems, such as in automobiles and industrial machinery.

# Recall the concept of Wheel and Axle

The wheel and axle is a simple machine that consists of a wheel and an axle that are mechanically connected and rotate together. The wheel is a circular component that rotates around the axle, which is a cylindrical component that runs through the center of the wheel. The wheel and axle work together to transfer rotational force, or torque, from one component to another.

The wheel and axle is used in many everyday devices, such as doorknobs, gears, and cranks. When a force is applied to the wheel, it rotates about the axle, causing the axle to rotate as well. The size of the wheel and axle determines the mechanical advantage of the machine. If the wheel is larger than the axle, it will take less force to turn the wheel than to turn the axle, giving the wheel and axle a mechanical advantage.

The mechanical advantage of the wheel and axle is proportional to the ratio of the radii of the wheel and axle. This means that the larger the wheel, the greater the mechanical advantage will be. The wheel and axle is a type of second-class lever, which means that the effort force is applied at a distance from the fulcrum. The wheel and axle is often used to increase torque, or rotational force, and to reduce the force required to perform a task.

# Define the term Screw Jack

A screw jack is a type of simple machine that uses a screw thread to convert rotational motion into linear motion, or vice versa. It is a device that is used to raise or lower a load by applying a force to a threaded rod that is rotated by a handle. The threaded rod is known as the screw, and the load is supported by a platform or plate that is attached to the end of the screw.

The screw jack operates on the principle of the screw thread. As the screw is rotated, it moves linearly along the axis of the screw, either lifting or lowering the load that is attached to it. The amount of lift or lowering is proportional to the distance that the screw rotates, as determined by the pitch of the screw thread.

Screw jacks are widely used in construction, machinery, and other industrial applications, where they are used to support heavy loads or to make precise adjustments to the position of a load. They are also used in car jacks, in theatre rigging, and in various other applications where linear motion is required.

Screw jacks are simple, reliable, and durable machines that provide a mechanical advantage over manual labour. They are often used to lift heavy loads with a relatively small amount of effort, making them an essential tool for many industrial and construction applications.