Make Your Own Spring Scale
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 used a spring. Springs are in machines all around us and have many useful purposes. In this activity you will learn how a spring can be used to make a scale to weigh objects.
This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.
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 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. Hooke's law 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 the force, x is the change in length of the spring, and k is a number called the spring constant, which is different for springs of different sizes or materials. However, for a given spring, k remains constant 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.
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.
Extra: using Hooke's Law and your data, can you calculate the spring constant, k, for your spring? Hint: k is the slope of a graph of force vs. distance.
Extra: what happens if you stretch the spring past its elastic limit, but continue the experiment?
Extra: can you figure out how to do the experiment by pushing on the spring (compression) instead of pulling on it (tension)? Hint: this may be difficult because the spring will buckle, or suddenly bend in half. You can prevent this by constraining the spring's motion somehow (like putting it inside the body of a click pen or other cylinder).
Observations and Results
While the exact weights and distances you measure will depend on the individual spring, in general you should see that any spring follow's Hooke's Law within its elastic limit. That means that the relationship between weight and distance is linear. If you double the weight, the amount the spring stretches will double. For example, say your spring has an unstretched length of 10cm. When you add a certain amount of weight, it stretches to 11cm (a change of 1cm from its unstretched length). When you add double that weight, it should stretch to 12cm (a change of 2cm from its unstretched length).
Once you have collected a few data points, this allows you to easily "look up" the weight of an unknown object simply by using a ruler to measure the change in length of the spring. This is how real spring scales work – the springs are calibrated so that when they are stretched by a certain amount, it corresponds to a known weight. This fails, however, if you stretch a spring beyond its elastic limit. That will cause some permanent change in the spring's shape, so it will not return to its original length, and your measurements are no longer valid.
More to Explore
Ben Finio, PhD, Science Buddies
Science Buddies |
Springs, elasticity, weight, distance
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