Key concepts
Springs
Elasticity
Weight
Distance

Introduction
Have you ever played with a Slinky, used a pinball machine, written with a click pen or ridden in a car? If so, then you have made use of a spring! Springs are in machines all around us and have many useful purposes. In this activity you will learn another cool use for a spring: making a scale to weigh objects.

Background
Springs are usually spirals made out of metal. They have the useful property that they are very stretchy—the scientific term for this is elastic. If you squish a spring or pull on it, then let go, it will bounce back to its original shape. There are limits to this behavior, however. If you pull (or push) too much, you might take the spring past its elastic limit. After that point, there will be some permanent change—or deformation—in the spring, and it will not fully recover its original shape.

Luckily, the elastic behavior of a spring is defined by a well-known equation called Hooke's law, which states that the restoring force of a spring (how hard the spring pushes or pulls to get back to its original length) is proportional to the distance the spring has been stretched (or compressed) from its original length. This law is expressed mathematically as F = kx, where F is force, x is the spring’s change in length and k is a number called the spring constant, which is different for springs of different sizes or materials. For a given spring, however, k remains the same as long as you stay within the spring's elastic limit. This makes Hooke's law useful because if you can measure either the force or change in length, you can use the spring constant to calculate the other value.

You might think of a scale as something that you stand—or place objects—on, measuring mass pushing down on it. But you can also weigh objects from a hanging scale, in which mass pulls on it. In this project you will make a simple spring scale by hanging weights from a spring. First you have to calibrate the scale using known weights, but after that you can measure the weight of an unknown object by hanging it from the spring, and measuring how far the spring stretches.

Materials

• A spring (You can find a selection of springs at a hardware store or get one by disassembling a click pen or some toys—with permission, of course. Ask an adult to help you take apart a pen or toy if necessary.)
• Paper clips
• Small plastic bucket or a paper or plastic cup with string tied through holes near the top to form a handle
• Objects to use as weights, such as coins (The weight of the objects you use will depend on how strong your spring is.)
• Kitchen scale
• Ruler
• Paper and pencil or pen
• Various small household objects to measure