Hooke’s Law is a fundamental principle in physics that describes the behavior of spring and elastic materials, particularly how they deform in response to an applied force. The law is named after the English physicist Robert Hooke, who first proposed it in the 17th century.
This article provides a complete guide to Hooke’s Law, including its definition, formula, applications, and limitations.
Hooke’s Law Explained
Also known as the Law of Elasticity, Hooke’s Law states that the resulting deformation of a spring is proportional to the force applied to it from equilibrium position. Mathematically, this can be written using the formula:
- F = restoring force of the spring [N or lb]
- k = spring constant [N/m or lb/ft]
- x = displacement from equilibrium position [m or ft]
The negative sign indicates that the restoring force of the spring is opposite in direction to the load applied. This is because elastic materials resist deformation due to the intermolecular forces that hold the atoms and molecules together.
It is also important to note that the value of the spring constant does not only depend on the material composition of the spring but also on its dimensions and shape.
The spring constant is a fundamental parameter that characterizes the stiffness of a spring. A spring with a higher spring constant requires more force to produce the same deformation compared to a spring with a lower k value, and vice versa. The spring constant is a crucial factor in the design and selection of springs for various applications in mechanical systems, such as suspension systems.
Graphically, Hooke’s Law is illustrated in the diagram below:
Suppose force F is able to stretch a spring from equilibrium position at a distance x, stretching the same spring at a distance 2x would require a force equal to 2F. Under these conditions, when the load is removed, the object reverts back to its original shape and size.
Hooke’s Law on Elastic Materials
In elastic materials, Hooke’s law is generally expressed in terms of stress and strain. Stress is the force per unit area applied to a material, while strain is the relative material deformation produced as a result.
According to Hooke’s Law, strain is proportional to the applied stress within a material’s elastic limit. Mathematically, this can be expressed as:
- σ = applied stress [N/m2 or psi]
- E = Young’s modulus [N/m2 or psi]
- ε = strain or relative deformation [unitless]
Also known as the modulus of elasticity, the Young’s modulus is a measure of the stiffness of a solid material. As the Young’s modulus increases, more stress is required to generate the same level of strain. Therefore, a perfectly rigid body would possess an infinite Young’s modulus.
Limitations of Hooke’s Law
Technically, all solid materials follow Hooke’s Law within the elastic region. However, since brittle materials have such insignificant elongations before fracture, it is almost impossible to accurately measure and observe Hooke’s Law.
Materials that follow Hooke’s Law are roughly referred to as “Hookean” materials. These include springs, ductile materials like steel, and other homogeneous and isotropic materials. For anisotropic materials, Hooke’s Law only applies to those exhibiting linear elasticity.
However, it is important to note that Hooke’s Law is only applicable for small elastic deformations within the proportional limit, as shown in the diagram below. Under these conditions, the object reverts back to its original shape and size when the load is removed.
The proportional limit bounds the region where the relationship of the stress and strain is linear. This is normally synonymous to the elastic limit or yield point— the point at which the material starts to experience plastic deformation. However, in some instances, they are not equal.
In addition, Hooke’s Law is applicable only to uniaxial stress conditions, where the stress is applied in one direction and there is no stress in any other directions. For multiaxial conditions, the calculation normally involves other elastic properties such as the shear modulus, bulk modulus, and Poisson’s ratio.
Applications of Hooke’s Law
Hooke’s Law has many real-life applications in engineering and science. In mechanical watches, car suspensions, and machinery, Hooke’s Law is used to design springs that can react a required load under a predictable deformation.
In construction and product design using ductile materials like steel, Hooke’s Law is used to predict the elastic deformation under tensile or compressive loads. This is essential to help engineers determine the safe working load of a structure and to ensure that it will not deform excessively under normal loads.
Lastly, in biomechanics, Hooke’s Law has also been used to model the behavior of bones, tendons, and ligaments in the human body. By understanding the elastic properties of these tissues, doctors and researchers can design treatments and therapies for injuries and diseases.