Using the Spring Balance
Now that we have a spring balance, let's create a system to see how to show a comparison between coins.
1. First, set your scale so that the pointer aims at, but does not touch your graph. When the bottle cap is empty, record the number the pointer aims at. In the example below, the empty reading is "10"
2. Now add a recent penny (one with the shield design) to the bottle cap. Record the number on the graph paper where the pointer now aims.
3. Repeat this for two, three and four pennies. Record your data as shown.
4. When your data is complete, graph it as shown below:
This graph shows a "linear" relationship (a straight line between the points) between the weight of the penny and how far the plastic spring balance moves. This is a physical principle discovered by Robert Hooke in 1678. Now called Hooke's Law, it provides the basis for how commercial spring scales operate. The principle allows us to predict the movement of the spring with the weight of each additional penny.
Now that we have a spring scale that we can measure the effect of our shield penny, let's compare it to the pre 1982 penny.
5. Repeat the process we used with the shield penny only this time use pennies with dates before 1982.
2. As before, collect the data for 1, 2, and 3 coins to see where the pointer aims. Graph the data on the same graph using a different color pen
Here again we see the same linear relationship. This time, however, the line shows a steeper drop. The plastic spring moves further with the pre 1982 penny.
Question 1. What can we logically conclude about the weight of pre-1982 pennies compared with shield pennies?
Discussion
Older pennies are made of copper. Newer pennies are made of zinc. Take a look at this chart:
Question 2. Why do you think the US government changed from using copper in pennies to mostly zinc?
Extensions.
If you have access to some old quarters or dimes you can try this experiment comparing silver with modern nickel/copper coins. Considering the information in the chart above, why is copper/nickel the combination being used now?