Science · 6th Grade · Earth's Place in the Universe · Quarter 4

Gravity: The Universal Force

Understand how the force of gravity governs the motion of everything from falling apples to orbiting planets and the structure of entire galaxies.

TL;DR:Get ready to explore gravity, the invisible force that shapes everything from a falling apple to the structure of the entire universe. These activities will take your students on a conceptual journey to understand this fundamental force.

• Common Core State Standards
• NGSS: MS-ESS1 - Earth's Place in the Universe

About This Topic

This topic delves into gravity, one of the four fundamental forces of nature, aligning with the Next Generation Science Standards (NGSS) performance expectation MS-PS2-4, which focuses on gravitational interactions. For sixth graders, the goal is to move from a simple understanding of gravity as 'what makes things fall' to a more nuanced view of it as a universal force of attraction between any two objects with mass. The curriculum should emphasize the relationship between mass, distance, and gravitational force, as described by Newton's Law of Universal Gravitation in a conceptual, non-mathematical way. Students will explore how this single force is responsible for phenomena on vastly different scales, from keeping our feet on the ground to holding the entire Milky Way galaxy together. This foundational knowledge is crucial for future studies in physical science, Earth and space science, and physics, providing the conceptual framework for understanding planetary motion, tides, and the life cycle of stars.

Key Questions

1. Explain how gravity keeps planets in orbit around the Sun and moons in orbit around planets.
2. Analyze the relationship between mass, distance, and the strength of gravitational force.
3. Compare the effects of gravity on Earth with its effects on other celestial bodies like the Moon.

Learning Objectives

• Describe gravity as a universal, attractive force that exists between any two objects with mass.
• Explain the relationship between the strength of gravity, the mass of the objects, and the distance between them.
• Model how gravity and inertia work together to keep satellites, moons, and planets in orbit.
• Differentiate between the concepts of mass and weight.
• Compare the effects of gravity on Earth to its effects on other celestial bodies.

Key Vocabulary

Gravity	The universal force of attraction acting between all matter.
Mass	A measure of the amount of matter in an object, typically measured in kilograms or grams.
Weight	The force of gravity acting on an object's mass.
Orbit	The curved path of a celestial object or spacecraft around a star, planet, or moon, especially a periodic elliptical revolution.
Force	A push or a pull upon an object resulting from the object's interaction with another object.

Watch Out for These Misconceptions

Common Misconception

There is no gravity in space.

What to Teach Instead

Gravity is everywhere in the universe. Astronauts on the International Space Station feel 'weightless' because they are in a constant state of freefall around the Earth, not because there is no gravity.

Common Misconception

Heavier objects fall faster than lighter objects.

What to Teach Instead

In the absence of air resistance, all objects fall at the same rate of acceleration regardless of their mass. Air resistance can make lighter, less dense objects fall slower, but the force of gravity pulls on them with the same acceleration.

Common Misconception

The Sun's gravity just pulls planets in; something else pushes them out to keep them in orbit.

What to Teach Instead

There is no outward push. A planet's orbit is a balance between its forward motion (inertia) and the inward pull of the Sun's gravity. The planet is constantly falling toward the Sun, but its sideways motion is fast enough that it continually misses.

Active Learning Ideas

Simulation Game

Gravity Drop Challenge

Students are given two objects of different masses but similar shapes (e.g., a golf ball and a ping-pong ball) and predict which will hit the ground first when dropped from the same height. They then perform the experiment to observe that they land simultaneously, challenging the misconception that heavier objects fall faster.

20 min·Small Groups

Simulation Game

Orbiting Objects Model

Students use a central object (like a student) and a smaller object tied to a string (like a soft ball) to model an orbit. By swinging the ball around, they can feel the tension (gravity) and see how forward motion (inertia) combine to create a stable orbit.

25 min·Whole Class

Simulation Game

Weighing In Around the Solar System

Students use their own mass to calculate what their weight would be on the Moon, Mars, and Jupiter. This activity uses a simple formula (Weight = mass × local gravity) to reinforce the difference between constant mass and variable weight.

30 min·Individual

Real-World Connections

• The functioning of GPS satellites, which require precise calculations of Earth's gravitational field to work correctly.
• The rising and falling of ocean tides, caused primarily by the gravitational pull of the Moon and Sun.
• The design of roller coasters and water slides, which use gravity to create thrilling acceleration.
• The trajectory of a thrown baseball or a launched rocket, both of which are governed by Earth's gravity.
• The formation of planets, stars, and galaxies from cosmic dust and gas clouds that collapsed under their own gravity.

Assessment Ideas

Exit Ticket

Use an exit ticket where students draw a diagram of the Earth-Moon system, using arrows to represent the force of gravity and the Moon's direction of motion to explain its orbit.

Peer Assessment

Students create a short comic strip or story about an astronaut's journey to Mars, explaining how and why their weight would change during the trip while their mass stays the same.

Quick Check

Students complete a K-W-L (Know, Want to Know, Learned) chart about gravity at the beginning and end of the unit to reflect on their own learning.

Frequently Asked Questions

What is the difference between mass and weight?

Mass is the amount of 'stuff' or matter in an object, and it's the same everywhere. Weight is the measure of the force of gravity on an object, so your weight changes depending on where you are, for example, on Earth versus the Moon.

If gravity pulls everything, why doesn't the Moon crash into the Earth?

The Moon is moving very fast sideways. This forward motion is balanced with Earth's gravitational pull, causing the Moon to constantly 'fall' around the Earth in a stable orbit instead of falling into it.

Why can't we feel the gravitational pull from a person standing next to us?

Gravity is a very weak force unless one or both of the objects have a huge amount of mass. The gravitational force between two people is so incredibly tiny that it's impossible to feel, while the pull from the massive Earth is very obvious.