# Happy Birthday, Isaac Newton!

This week we celebrated Sir Isaac Newton’s birthday on January 4. Regarded as one of the greatest scientists of all time, the English physicist and mathematician attended Cambridge University, where he immersed himself in the work of Aristotle and then Descartes.

After years of research, Newton published “Principia” in 1687, a landmark work that established the universal laws of motion and gravity. Newton showed that this theory described all celestial bodies’ movements, including the Earth, substantiating Kepler’s laws of planetary motion.

In honor of his achievements, the international unit of force is called the newton. One newton is the force required to accelerate an object with the mass of one kilogram one meter per second per second. In other words, one newton is equal to one kilogram meter per second squared. When we step inside the elementary science classroom, instructors often thoughtfully connect the scientific method to Newton’s theory of gravity by discussing how he went about conceptualizing his theories after observing an apple fall from a tree in an orchard.

## The Spring Scale

The first spring scale was developed by Richard Salter in 1770; along with his nephews, Richard Salter founded George Salter & Co., patenting the spring balance in 1838. In fact, Salter Housewares is still a prominent British housewares brand today!

The spring scale is a type of weighing scale or force gauge and measures weight or force when an object is hung from its hook. The spring scale can only provide correct measurements within a frame of reference of constant acceleration in the spring axis. It is similar to how the Earth’s gravitational pull works, where acceleration is influenced by gravity.

Although this method of weighing produce is beginning to disappear, you may have seen spring scales used in the grocery store, where a small bin hanging from a numbered scale is used to weigh your apples. Spring scales have also been commonly used to weigh luggage.

## Spring Scales and the Lab

Most spring scales found in classrooms are calibrated to measure both the mass and weight of an object, but spring scales truly only measure weight. This is because spring scales rely on Hooke's Law, which states that the force needed to extend a spring is proportional to the distance extended. Therefore, spring scales are measuring a force, or weight, and not a mass. Because these relationships may appear complicated at first, it’s important to make sure students are using precise terminology when describing processes related to force, weight, and mass.

Spring scales are key to a number of activities, including those teaching students about the newton as it is used to measure force and how to identify the direction forces are acting. Initially, give students a spring scale and several objects to be measured. Ask them to measure the weight of each object and note it. When they are finished, measure each object in front of the class and have students compare your results with their own. Then, discuss how students may have come up with results different from yours and what strategies are effective in ensuring accurate results.

Students can also use a spring scale to find the weight of an object, and, from there, students can use Newton's Second Law to work backward to find the mass of the object. This should give students the opportunity to see that mass and weight are different, but that they are proportional here on Earth.

Although other types of scales are known to be more accurate, spring scales are useful due to their simplicity, cheapness, and robustness, which make them an ideal addition to any classroom—or home—laboratory. Since it is such a versatile piece of equipment, there are many options to choose from. Check out a few of hBARSCI’s spring scales here