Hands-on Activity Hanging Around:
Gravity and Slinky Spring Scales

Learning Objectives

After this activity, students should be able to

• Explain that weight is a comparison of the force of gravity pulling on objects with different masses.
• Collect and analyze data.
• Make predictions from observed data.
• Describe why engineers must balance the relationship between weight of materials and gravity in their designs.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

Common Core State Standards - Math

International Technology and Engineering Educators Association - Technology

State Standards

Materials List

Each group needs:

• 1 mini slinky or other lightweight spring, available online
• 1 paper cup
• 25 cm of string
• tape
• 2 rulers
• a few books or another heavy object to hold 1 ruler down on a table or desk
• 6 small objects with identical weights, such as pennies, quarters or marbles

Introduction/Motivation

The force of gravity acts upon all matter, everywhere. However, gravity pulls more strongly on an object that has more mass than on an object with less mass. We call the force of gravity acting upon mass, weight. Weight is a comparison of how strongly gravity pulls on one object or another. For example, when we say that something weighs 2 pounds or 2 kilograms, it means that gravity is pulling on that object two times as strongly as it does on something that ways 1 pound or 1 kilogram. "Pound" and "kilogram" are just the unit names we give to certain amounts of gravitational pull. Measuring weight, then, is simply a matter of comparing the pull of gravity on different objects.

Historically, people have compared the weight of unknown things to different specific objects known as standard weights. People have used special rocks or stones as standard weights to compare to heavy objects, and special seeds or nuts as standard weights to compare to lighter objects. Today, we compare weights to a standard kilogram and a standard pound, which are special pieces of metal kept in the U.K. Copies of the standard kilogram and the standard pound are kept at different places all over the world, including at the National Institute of Standards and Technology in the U.S.

Comparing the force of gravity on different objects (weighing things) is a really useful thing to do. Aeronautical engineers understand how the force of gravity works so that they can design airplanes and rockets; Civil engineers understand gravity so that they can design tall buildings and bridges that will not fall down; and environmental engineers understand gravity so that they can analyze the way water flows down a river canyon or the forces pushing against a storage tank's wall. In fact, every kind of engineer uses an understanding of gravity to do their work! Because understanding gravity is so important to engineers, engineers have devised many interesting ways to measure the force of gravity acting upon different objects. In this activity, we will engineer a tool to measure the force of gravity, and use that tool to examine gravity's pull on small objects.

Procedure

Before the Activity

• Gather materials.
• Read through all the activity steps and prepare a few relevant questions.

With the Students

Direct each student team to make its own spring scaby le following these steps:

1. Place a ruler on the edge of a table or desk so that it extends past the edge. Stack enough books on the ruler to hold it down securely.
2. Tape the end coil of a plastic slinky or other lightweight spring to the end of the ruler extending over the edge of the tabletop. It is best if the slinky is not too far away from the edge of the table.
3. Punch holes in opposite sides of a 5-ounce paper cup near the top edge of the cup.
4. Tie each end of a short piece of string (25 cm) to one of the holes, making a string handle for the cup.
5. Hang the string handle onto the last coil at the bottom of the spring.
6. Tape a sheet of plain white paper to the side of the table near the bottom of the cup.
7. Tape a short pencil (about 5 cm) to the side of the cup so that the point faces the paper.
8. Using a marker, make a short, horizontal line on the paper at the place that the pencil is pointing to, and label it "0."
9. Drop one of the light objects (such as a penny, marble, quarter) into the cup. After the cup stops bouncing, make a line to show the pencil's new position and label it "1."
10. One by one, drop each of six differing objects into the cup. Each time, mark the position of the pencil and label the mark with the number of objects.
11. Measure and record the distance between each mark and the starting position.
12. Predict the distance to the next mark if you were to drop a seventh heavier object.
13. Conduct the post-activity toss a question assessment activity as described in the Assessment section.

Assessment

Pre-Activity Assessment

Brainstorming: In small groups, have students to engage in open discussion. Remind them that no idea or suggestion is "silly." All ideas should be respectfully hears. Ask students to think of examples that show how different objects are affected by gravity. (Possible examples: a baseball, a building being demolished, a high jumper, etc.)

Activity Embedded Assessment

Data Recording: As directed in the Procedure section, each time a new object is dropped in the cup, have students mark the new position of the pencil, label the mark with the number of objects, and measure the distance from the starting position to the new mark. 

Prediction: Ask students to predict where the pencil would have pointed if they had added a seventh heavier object into the cup. (Answer: It would have been below the last mark.)

Post-Activity Assessment

Graphing: Have students create a plot showing the distance that each object pulled the spring. Discuss why certain objects pull the spring further than others. (Answer: Weight differences.)

Toss-a-Question: Provide students with a list of questions (see below) without answers. Have them work in groups and toss a ball or wad of paper back and forth. The student with the ball asks a question and then tosses the ball to someone else to answer. If a student does not know the answer, s/he tosses the ball onward until someone gets it. Review the answers at the end. Possible questions/answers:

• What does this activity teach you about weight? (Answer: Weight is a comparison of the force of gravity pulling on objects with different masses.)
• What can you conclude about how a spring scale works? (Answer: a spring scale is stretched more by objects with larger masses. We can compare the amount it stretches when the mass is a "standard weight" with the amount it stretches when the weight is unknown to measure the weight.)
• What kind of objects would your scale be good for measuring? (Answer: Similar, lightweight objects.)
• Would the scale work on other planets? (Answer: Yes) How would it be different? (Answer: The force of gravity is different on other planets [see the Activity Extension section], so the spring would not be pulled by the same amount. For a given object, the spring will stretch more on some planets [such as Jupiter] and less other places [such as the moon].)

Troubleshooting Tips

The scale works best in the middle of the spring's expansion range. So, choose objects to weigh that are heavy enough to stretch the spring a little bit, but not so heavy that the spring is stretched out all the way.

If the spring is too long for the table and the cup hits the ground, shorten it by putting the ruler through the middle of the spring rather than taping the end of the spring to the ruler.

It is important that the spring is attached to the ruler at the same place for all the measurements. Do not change the spring midway through measuring the six items.

Activity Extensions

Weight is not the same everywhere! Because different planets have more or less mass than Earth, they have different gravity forces. So, weight (the measurement of the force of gravity pulling on mass) changes from one planet to another. If you visited another planet, your size would not change because your body mass (your skin, bones, and all the other particles that make up your body) would still be the same. However, you would not weigh the same, because the force of gravity would be pulling on your mass by a different amount than it does on Earth! Find your weight on other planets at: https://www.exploratorium.edu/explore/solar-system/weight

See a history of standardized weights and measures at: https://nvlpubs.nist.gov/nistpubs/bulletin/01/nbsbulletinv1n3p365_A2b.pdf

References

Copyright

Contributors

Supporting Program

Acknowledgements

The contents of this digital library curriculum were developed under grants from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education, and National Science Foundation (GK-12 grant no 0338326). However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.