In understanding Hooke's law, we can advance an illustration: A bridge, when used by traffic during the day, is subjected to loads of varying magnitude. Before a steel bridge is erected, therefore, samples of the steel are sent to a research laboratory, where they undergo tests to find out whether the steel can withstand the loads to which it is likely to be subjected.
Hooke's law states that the force required to extend or compress a substance, a spring or any elastic material by given distance is proportional to that distance so long as the elastic limit has not been exceeded.
In explaining the application of Hooke’s law and elasticity, we shall use hypothetical experimentation for better clarity. A simple laboratory method of measuring the elasticity of two long thin steel wires, P, Q, are suspended beside each other from a rigid support B, such as a girder at the top of the ceiling. The wire P is kept taut by a weight A attached to its end and carries a scale M graduated in centimeters. The wire Q carries a vernier scale V which is alongside the scale M.
When a load W such as 1kg block is attached to the end of Q, the wire increases in length by an amount which can be read from the change in the reading on the vernier V. if the load is taken off and the reading on V returns to its original value, the wire is said to be elastic for loads from zero to 1 kg block, a term adopted by analogy with an elastic thread. When the load W is increased to 2 kg block the extension (increase in length) is obtained from V again; and if the reading on V returns to original value when the load is removed the wire is said to be elastic at least for loads from zero to 2 kg.
The extension of a thin wire such as Q for increasing loads may be found by experiments to be as follows:
| W (kgf) | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Extension (mm.) | 0 | 0.14 | 0.28 | 0.42 | 0.56 | 0.70 | 0.85 | 1.01 | 1.19 |
Elastic Limit
When the extension, e, is plotted against the load, W, a graph is obtained which is a straight line OA, followed by a curve ABY rising slowly. At first and then very sharply, up to about 5 kg, then, the results in the table show that the extension increased by 0.14 mm for every kg which is added to the wire. Further, the wire returned to its original length when the load was removed. For loads greater than about 5 kg, however, the extension increases relatively more and more, and the wire is thus permanently strained, and A corresponds to its elastic limit.
Hooke’s Law
From the straight line graph OA, we deduced that the extension is proportional to the load or tension in the wire when the elastic limit is not exceeded. This is known as Hooke’s Law , after ROBERT HOOKE, founder of the Royal Society, who discovered the relation in 1676. The law shows that when a molecule of a solid is displaced farther from its mean position, the restoring force is proportional to its displacement. One may therefore conclude that the molecules of a solid are undergoing simple harmonic motion.
The measurements also show that it would be dangerous to load the wire with weights greater than 5 kilogrammes, the elastic limit, because the wire then suffers a permanent strain. Similar experiments in the research laboratory enable scientists to find the maximum load which a steel bridge, for example, should carry for safety. Rubber samples are also subjected to similar experiments, to find the maximum safe tension in rubber belts used in machinery.
Yield Point, Breaking Stress, Ductile and Brittle Substances
Carefully conducted experiments to prove Hooke's law show that, for mild steel and iron for example, the molecules of the wire begin to ‘slide’ across each other soon after the load exceeds the elastic limit, that is, the material becomes plastic. This is indicated by the slight ‘kink’ at B beyond A and it is called the yield point of the wire. The change from an elastic to a plastic stage is shown by a sudden increase in the extension, and as the load is increased further the extension increases rapidly along the curve YN and the wire then snaps. The breaking stress of the wire is the corresponding force per unit area of cross-section of the wire. Substances such as those just described, which elongate considerably and undergo plastic deformation until they break, are known as ductile substances. Lead, copper and wrought iron are ductile. Other substances, however, break just after the elastic limit is reached; they are known as brittle substances. Glass and high carbon steels are brittle.
Brass, bronze and many alloys appear to have no yield point. These materials increase in length beyond the elastic limit as the load is increased without the appearance of a plastic stage. These materials do not obey Hooke's law
The strength and ductility of a metal, its ability to flow, are dependent on defects in the metal crystal lattice. Such defects may consist of a missing atom at a site or a dislocation at a plane of atoms. Plastic deformation is the result of the ‘slip’ of planes. The latter is due to movement of dislocations, which spreads across the crystal.
Scientific Exploits Of Robert Hooke
The 17th century English physicist Robert Hooke developed this law that states that the force F applied to a spring is proportional to the distance of the extension or compression x. This linear relationship is known as the Hooke's law of elasticity.
Modern theory of elasticity generalizes this principle so that the proportionality factor is not a real number but a function of vectors called a tensor.
Definition of Hooke's Law
Hooke's law of elasticity states that the force (F) required to extend or compress a spring by some distance (x) scales linearly with that distance. It is named after the 17th-century British physicist Robert Hooke, who discovered this empirical law by experimenting with a spring suspended from a rod with weights attached to it. The law can also be written as Fs = kx, where k is a constant factor characteristic of the spring (i.e., its stiffness) and x is small compared to the total possible deformation of the spring.
The law applies to most elastic materials under certain loading conditions, such as when the elongation or compression vectors (Fs and x) have the same direction. This condition is known as linear elasticity, and Hooke's law describes the relationship between stress and strain for such materials, with stress being proportional to displacement.
However, the law does not hold for all materials, and it fails when forces are applied that exceed a material's elastic limits. This failure to comply with the law is called non-Hookean behavior. For example, a helical spring only obeys the law within its elastic range, and it becomes non-Hookean when stretched beyond this limit.
Generally, when a helical spring is not within its elastic range, the restoring force (Fs) is no longer proportional to the elongation/compression vector (x). This results in the loss of energy, which can be measured by graphing Fs vs x. This can be done using a spring scale or by making use of a manometer (i.e., a pressure gauge) that has a 'U'-shaped tube with a graduated scale.
The law of elasticity enables scientists and engineers to predict the elastic deformation of materials such as steel, before they construct or manufacture products from them. It is also used to model the behavior of bones, tendons, and ligaments in the human body, so that doctors can design treatments for injuries and diseases that affect these tissues. In addition, the law of elasticity is important in the field of biology, where it is used to explain why some animals are more susceptible to illness than others in similar environments.
Explanations of Hooke's Law
Hooke's law states that the strain of a body is directly proportional to the force that is applied to it. It also states that the restoring force is equal and opposite to the deformation of the material. This is a useful property to have when dealing with springs and other elastic materials. This law is used in many real world applications, from the spring balance on your desk to the suspension cable on your bicycle. It is also used in other everyday items, such as the string on a guitar or the amount of wind that sways a tall building.
Another important principle of Hooke's law is that a material will return to its original state when the force is removed. This is a fundamental property of most materials, and it is why we are able to bend and stretch objects so easily. However, the energy that is lost through this process is minimal and is mostly accounted for by natural friction between the object's components.
Although Hooke's law is not completely accurate, it is a good approximation for most solid bodies and small deformations. It is a particularly useful tool for engineers who need to understand the properties of elastic materials. It is also useful for acoustics and other disciplines that deal with the elasticity of materials.
The law is most often expressed in terms of stress and strain. Stress is the force per unit area that causes a material to change shape, while strain is the relative material deformation caused by the stress. Typically, Hooke's law is expressed as: F = -kx.
In general, the Young's modulus of a material is proportional to the stress and strain of a material. This means that a material with a larger Young's modulus will be stiffer than a material with a smaller Young's modulus.
However, there are some exceptions to this rule. Brittle materials can only be stretched so far before they break, and they do not exhibit linear elasticity. Therefore, they are not governed by the same principles as ductile materials.
Examples of Hooke's Law Application
Hooke's law states that the deformation of a material, measured as the change in its length, is proportional to the force exerted on it. This principle is used to create many everyday products. For example, when a spring is stretched outwards from its original length, the resulting force is proportional to the distance the spring is stretched. When the load is removed, the spring reverts back to its original length and shape. This ability to return to its normal state is also referred to as a restoring force.
For most elastic materials, Hooke's law is only true for a small range of deformations. This is because most materials have a natural tendency to deform in only one direction. For this reason, a more accurate description of elasticity is achieved by using stress and strain. For most elastic materials, the magnitude of a stress F and the displacement X are proportional to each other (assuming that both are small). However, the direction of the displacement varies depending on the material. For this reason, it is often necessary to introduce a constant of proportionality, called the modulus of elasticity, into the relationship.
The modulus of elasticity is usually determined experimentally for each type of material. For example, a spring is likely to have a much larger modulus than the cross section of a piece of steel. The ratio between the stress and the strain is then determined by dividing the elastic modulus by the cross-sectional area of the material.
The law of elasticity is fundamental in many branches of physics. It is the basis for many mechanical devices such as manometers, spring scales and the balance wheel of a clock. It is also a key principle in materials science and provides important insights into the behavior of structures made from ductile materials such as steel and concrete. In biomechanics, it is used to understand the behavior of bones, tendons and ligaments in the human body. It is also used to design buildings and other engineering structures that use tensile and compressive forces. A misunderstanding of the law can result in catastrophic failure of these structures, such as the collapse of the World Trade Center towers.
The basic premise of Hooke's law is that the deformation (or change in shape) of an elastic object or material is proportional to the force exerted on it. This force can be applied in the form of pressure or tension, and is normally represented by a vector in physics, called the stress-strain (or Young's modulus) tensor. This tensor is a vector matrix of real numbers, and if shown on a graph, will show a direct linear relationship between stress and strain, up to a certain limit, known as the elastic limits.
Beyond this point, the material deviates from its original shape and does not revert to its original state upon removal of the force. This is a result of the fact that the total amount of strain required to induce the desired change in shape is greater than the material's elastic limit.
Nevertheless, for small changes in shape and for most solid materials, Hooke's law is a very close approximation and can be used in engineering and science to predict the behavior of such materials. In the case of springs and ductile metals such as steel, this is vital for designing structures that will withstand normal loads without excessive deformation.
In more advanced applications, Hooke's law is used to describe a more complex relationship between stresses and strains that involve multiple components. These new models are more sophisticated and take into account all of the possible atoms and molecules in the material, making them much more accurate than earlier, simpler models. Generally speaking, however, these more complicated models are only valid for extremely small changes in shape and for very brittle materials that exhibit no elasticity at all.
Overall, Hooke's law is a useful general rule that can be applied to a wide range of situations and materials. It is particularly useful for describing the behavior of elastic objects such as springs and rubber bands, but it can also be used to explain how certain forces, such as the wind, can affect a tall building or even an entire city. Hooke's law is most commonly referred to as the law of elasticity, but it is more accurately described as the linear-elastic (or Hookean) law.