If you have ever toyed in isolation with the sort of spring encountered in everyday objects and tools – say, the small kind inside the bottom of a "clickable" ballpoint pen – you may have noticed that it has certain general properties that set it apart from most other objects.
One of these is that it tends to return to the same size after you either stretch or compress it. Another, perhaps less obvious property is that the more you stretch or compress it, the harder it is to stretch or compress it even more.
These properties apply wholly to an ideal spring, and to some extent to springs used for all manner of purposes in the real world. Most other objects do not behave in this way at all; those that resist deformation completely usually break when an applied force becomes strong enough, while others may stretch or be compressed but not return fully or at all to their original shape and size.
The unusual properties of springs, combined with a then-new conceptual framework about force and motion advanced chiefly by Galileo Galilei and Issac Newton, led to the discovery of Hooke's law, a simple but elegant relationship that has applies to countless engineering and industrial processes in the modern world.
A Vital Discovery: Hooke's Law
A spring is an elastic object, which means it has the various characteristics described in the previous section. That means that it resists being deformed (stretching and compression being two types of deformation) and also that it returns to its original dimensions provided the force remains within the spring's elastic limits.
Before the publication of Newton's laws, Robert Hooke (1635-1703) discovered through some simple experimentation that the amount of deformation of objects was proportional to the forces applied to deform that object, as long as they had the property he termed "elasticity." Hooke, in fact, was a prolific scientist across almost all imaginable disciplines, even if he is not a household name today, in large part because of the sheer number of accomplished scientists operating in throughout Europe in his time.
Hooke's Law Defined
Hooke's law is very easy to write, remember and work with, a luxury not often bestowed on physics students. In words, it simply says that the force required to keep a spring (or other elastic object) from being deformed further is directly proportional to the distance the object has already been deformed.
F = −kx
Here k is called the spring constant, and it is different for different springs, as you would anticipate. Hooke's law, which you can think of as a "spring force formula," is in play in a variety of different tools and aspects of life, such as archery bows and the shock absorbers and bumpers on automobiles.
For simple examples, you can use your own head as spring force calculator. For example, if you are told that a spring exerts a force of 1,000 N when stretched by 2 m, you can divide to get the spring constant: 1,000/2 = 500 N/m.
Hooke’s Law in a Spring-Mass System
Bear in mind that although people may think of springs more as "stretchable" than "compressible," if a spring is properly constructed (that is, has enough room between successive coils), it can be significantly compressed as well as stretched, and Hooke's law applies in both directions of deformation.
Imagine a system with a block sitting on a frictionless surface and connected to a wall by a spring that is at equilibrium, meaning that it is being neither compressed nor stretched. If you pull the block away from the wall and let it go, what do you think will happen?
At the moment you release the block, a force F, in accordance with Newton's second law (F = ma), acts to accelerate the block toward its starting point. Thus for Hooke's law in this situation:
F = -kx = ma
From here it is possible, using k and m, to predict the mathematical behavior of the oscillation, which is wavelike in nature. The block is at is fastest at the times it passes through its starting point in either direction and, more evidently, at its slowest (0) when it reverses direction.
- Theory vs. reality: What happens in this imaginary situation is that the block passes its starting point and oscillates back and forth across its starting point, being compressed by the same distance it was first stretched in each trip toward the wall and then zooming back out to where you pulled it, in a never-ending cycle. In the real world, the spring would not be ideal and its material would eventually lose its elasticity, but more importantly, friction in reality is unavoidable; its force soon reduces the magnitude of the oscillations, and the block returns to rest.
Energy in Hooke's Law
You have seen that a spring has inherent, or built-in, properties that can be leveraged to do work in a way that, say, bubble gum or a ball bearing cannot. As a result, springs can be described in terms of not just force but energy. (Work has the same fundamental unit as energy: the newton-meter or N⋅m),
To deform the spring, you or something else must do work on it. The energy you impart using your arm is "transferred" into elastic potential energy when the spring is held stretched. This is analogous to an object above the ground having gravitational potential energy, and its value is:
EP = (1/2)kx2
Say you use a compressed spring to launch an object along a frictionless surface. The energy in this ideal situation has been "converted" entirely into kinetic energy at the instant the object leaves the spring, where:
EK = (1/2)mv2
Thus if you know the mass of the object, you can use algebra to solve for the velocity v by setting EP (initial) to EK at "launch."