# Modeling Gravity

## Summary

6th-8th
Group Size
Entire class (two groups for large class)
Active Time
40 minutes
Total Time
40 minutes
Area of Science
Astronomy
Physics
Key Concepts
Mass, gravity, orbit
Credits
Ben Finio, PhD, Science Buddies

## Overview

Why can we feel gravity pull us down towards the Earth, but not sideways towards other big objects like buildings? Why do the planets in our solar system orbit the sun instead of flying off into space? In this lesson plan your students will develop a model for gravity and use it to explore answers to these questions.

## Learning Objectives

• Explain why, on Earth, we can only feel gravity pulling us down, and not sideways towards other objects.
• Explain how gravity influences the motion of planets in our solar system.
• Understand the usefulness of models as well as their limitations.

## NGSS Alignment

This lesson helps students prepare for these Next Generation Science Standards Performance Expectations:
• MS-PS2-4. Construct and present arguments using evidence to support the claim that gravitational interactions are attractive and depend on the masses of interacting objects.
• MS-ESS1-2. Develop and use a model to describe the role of gravity in the motions within galaxies and the solar system.
This lesson focuses on these aspects of NGSS Three Dimensional Learning:

 Science & Engineering Practices Disciplinary Core Ideas Crosscutting Concepts Science & Engineering Practices Developing and Using Models. Develop and use a model to describe phenomena. Evaluate limitations of a model for a proposed object or tool. Disciplinary Core Ideas PS2.B: Types of Interactions. Gravitational forces are always attractive. There is a gravitational force between any two masses, but it is very small except when one or both of the objects have large mass—e.g., Earth and the sun. ESS1.B: Earth and the Solar System. The solar system consists of the sun and a collection of objects, including planets, their moons, and asteroids that are held in orbit around the sun by its gravitational pull on them. Crosscutting Concepts Scale, Proportion and Quantity. Time, space, and energy phenomena can be observed at various scales using models to study systems that are too large or too small. Systems and System Models. Models can be used to represent systems and their interactions. Models are limited in that they only represent certain aspects of the system under study.

## Materials

These materials will be enough to set up the activity for the entire class. For a large class, you may want to split into two groups, so you will need twice as many materials. The following supplies are available from Amazon.com:

• Large sheet of stretchy fabric (polyester/spandex/lycra etc.), approximately 2 yards by 2 yards.
• Marble set that includes larger "shooter" marbles in addition to regular marbles.
• At least one pool ball, which you can purchase as a set or individually. Other heavy, round objects like oranges or grapefruit will also work.
• Lots of duct tape or masking tape, or spring clamps (at least twice as many as you have chairs). Make sure the jaws of the spring clamps open wide enough to clip onto the back of your chairs.
• 8–10 chairs (more if you have a bigger piece of fabric)

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## Background Information for Teachers

This section contains a quick review for teachers of the science and concepts covered in this lesson.

In everyday language, we describe gravity as the thing that pulls us down toward the surface of the Earth. It's the reason behind the old saying "what goes up, must come down." Experiencing it becomes a regular part of our lives from an early age. For example, as a child you quickly learn that if you trip, you will fall down (and it will hurt!). If you drop a ball, it will fall down to the ground—not go sideways or up.

In classical (or "Newtonian") physics, we describe gravity as an attractive force that acts between any two objects with mass (Note: this lesson assumes your students are already familiar with the word mass and understand that it is different from weight. See the Additional Background section for a reference on this topic.) However, in order for the gravitational force to be noticeable, at least one of the masses has to be extremely large. The average human has a mass of somewhere around 60 or 70 kilograms (kg). The Earth has a mass of about 5.97×1024 kg. If you write out all the zeroes, that's 5,970,000,000,000,000,000,000,000 kg, about as much as 85 sextillion people (more than ten trillion times the world's population). This explains why we can only feel the force of gravity pulling us down towards the Earth, and not sideways towards other people; or even larger, heavier objects like cars or buildings. Their masses are much too small for you to notice the gravitational force they exert on you. The strength of the gravitational force between two objects also depends on the distance between them—the force gets stronger as the objects get closer together.

What about the rest of our solar system? It contains other planets that, like the Earth, all orbit around the Sun in nearly circular (technically elliptical) paths. What makes the planets orbit the Sun instead of either flying off into space, falling into the Sun, or staying motionless? Imaging cutting a rubber band, tying a ball to one end, and twirling it around, as shown in Figure 1. The force in the rubber band (tension) makes the ball move in a circle. If you cut the rubber band, there is no more force to keep the ball moving in a circle, and it will fly off in a straight line. If you stop twirling the ball, the rubber band will pull it back in toward your hand. The Earth's motion around the Sun is similar (Figure 1). If there was no gravitational force between the Earth and Sun, the Earth would just fly off into space. If the Earth had no sideways motion, it would be pulled into the Sun by the gravitational force.

Figure 1. The motion of a twirling ball attached to a rubber band (left) is similar to the motion of the planets in our solar system around the Sun (right). (diagram not to scale)

In this lesson plan, your students will create a model for gravity and our solar system using pool balls, marbles, and a sheet of stretchy fabric. The following video provides an excellent demonstration of the activity, and we recommend that you watch it before you start*:

Video: Gravity Realized

*Note: the video starts out by discussing "warped (or curved) spacetime," which is a concept from Einstein's theory of relativity. This is a different explanation for gravity that can be rather difficult to grasp (even for adults with scientific backgrounds!). For purposes of this lesson plan, you can stick with the Newtonian explanation for gravity as a force that acts between two objects with mass. See the Variations section for more resources about the relativistic explanation.

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