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Line of Action: Derivation and Importance
By BYJU'S Exam Prep
Updated on: September 25th, 2023
The Line of Action (LoA) refers to the path followed by the point of application of a force. In other words, it is the imaginary line along which the force is acting. It is an important concept in engineering and physics, as it determines the direction and magnitude of forces acting on a system. The line of action is usually represented as a straight line, but it can also be represented as a curved line sometimes.
Line of Action PDF (Download GATE Notes)
The direction of the force is given by the direction of the line of action, while the distance between the point of application and the line of action gives the magnitude of the force. In mechanics, the line of action is used to analyze and understand the behaviour of systems under different loads and conditions. It is particularly useful in analysing structures such as bridges and buildings, where it is important to understand the distribution of forces and stresses within the structure.
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What is the Line of Action of Force?
The Line of Action (LoA) of a force is the path followed by the point of application of the force. It is an imaginary line representing the direction in which the force is acting. The direction of the force is given by the direction of the line of action, and the distance between the point of application and the line of action gives the magnitude of the force. The line of action is usually represented as a straight line, but it can also be represented as a curved line sometimes.
In mechanics, the line of action is used to analyze and understand the behaviour of systems under different loads and conditions. It is particularly useful in analysing structures such as bridges and buildings, where it is important to understand the distribution of forces and stresses within the structure. The line of action can also be used to calculate the net force acting on an object or system by analyzing the individual forces acting on it and their lines of action. It’s worth noting that the line of action is a vector quantity, meaning it has both magnitude and direction. This contrasts a scalar quantity with only magnitude and no direction.
Derivation of Line of Action
The Line of Action (LoA) of a force is derived by considering the direction and magnitude of the force. The direction of the force is given by the direction of the line of action, which is typically represented as a vector. The direction of the vector can be determined by analyzing the direction of the force relative to a reference axis, such as the x, y, or z axis.
Line of Action Formula
The distance between the point of application and the line of action gives the magnitude of the force. This distance can be determined using the magnitude of the force vector and the angle between the force vector and the reference axis. For example, in twodimensional mechanics, if the force vector is represented by the coordinates (Fx, Fy) and the point of application is represented by the coordinates (Px, Py), then the line of action can be calculated by:
LoA = (Px + Fx, P_{y} + F_{y})
It’s worth noting that this is a general form of the equation, and the specific form of the equation can vary depending on the system type and the assumptions we make while deriving it.
In general, the line of action can be derived by analyzing the force’s direction and magnitude and considering the force’s point of application. The result is a vector representing the force’s direction and magnitude and the force’s point of application.
How to Calculate the Direction of Force by using a Line of Action?
The direction of a force can be determined using the Line of Action principle. The Line of Action is an imaginary line that passes through the point of application of the force and is parallel to the direction of the force. To calculate the direction of a force using the Line of Action, you need to follow these steps:

Identify the point of application of the force: This is the point where the force is applied to the object or the body.

Draw a vector representing the force: The vector should be drawn with its tail at the point of application and its head in the direction of the force.

Draw a line parallel to the direction of the force: This is the Line of Action. The line should pass through the point of application of the force.

Determine the direction of the force: The direction of the force is parallel to the Line of Action. You can use a protractor or a ruler to measure the angle between the Line of Action and a known reference direction, such as the xaxis or the yaxis.
For example, suppose you are calculating the direction of a force applied to a block on a table. The force is applied at a point on the block, and its vector is pointing to the right. To determine the direction of the force using the Line of Action, you would draw a line parallel to the vector passing through the point of application. The Line of Action would be parallel to the vector, and the direction of the force would be to the right. It can also be used for the system of forces to find the direction of the resultant force.
Importance of Line of Action
The Line of Action (LoA) of a force is an important concept in engineering and physics, as it determines the direction and magnitude of forces acting on a system. Understanding the line of action of a force allows for a better understanding of the behaviour of a system under different loads and conditions.
 Analysis of Structures: The line of action is particularly useful in analysing structures such as bridges and buildings, where it is important to understand the distribution of forces and stresses within the structure. By analyzing the line of action of the forces acting on a structure, engineers can determine the most efficient and safe design for the structure.
 Calculation of Net Force: The line of action can calculate the net force acting on an object or system by analyzing the individual forces acting on it and their lines of action. This is important in understanding the overall behaviour of a system and the forces acting on it.
 Stress Analysis: The line of action of a force is also used in stress analysis, which is the study of how forces are distributed within a material. By analyzing the line of action of the forces acting on a material, engineers can determine the most efficient and safe design for the material.
 Dynamics: The line of action is also important in studying how objects move and interact. By analyzing the line of action of the forces acting on an object, engineers can determine the most efficient and safe design for the object.
 Control of Vibration: The line of action of a force is also used in the control of vibration, which is the process of reducing or eliminating unwanted vibrations in a system. By analyzing the line of action of the forces acting on a system, engineers can determine the most efficient and safe way to control the vibrations.
Limitations of Line of Action

Assumption of a single point of application: The Line of Action assumes that the force is applied at a single point, which is not always the case. In realworld situations, forces can be distributed over an area or a surface, making it difficult to determine the exact point of application.

Inaccurate for nonparallel forces: The Line of Action is only accurate for forces that are parallel to each other. For nonparallel forces, such as a force that is applied at an angle, the Line of Action will not accurately represent the direction of the force.

Neglects the effect of friction and other factors: The Line of Action only considers the direction of the force and neglects other factors, such as friction, air resistance, and gravity. These factors can affect the behaviour of an object or a system and must be taken into account when analyzing forces.

Limited to statics: The Line of Action is only applicable in statics, where the object or system is in a state of equilibrium. In dynamic situations, where the object is moving or accelerating, the Line of Action may not accurately represent the direction and magnitude of the force.

Assumes rigid bodies: The Line of Action assumes that the object or system is rigid and does not deform under force. In reality, many objects and systems can deform under stress, which can affect the direction and magnitude of the force.
Overall, while the Line of Action is a useful concept for analyzing forces, it is important to be aware of its limitations and to use it in conjunction with other principles and methods for a more accurate analysis.
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