Stars and the Solar System · Weeks 10-18

The Force of Gravity

Investigating the gravitational force exerted by Earth on objects directed toward the center of the planet.

About This Topic

Gravity is the force that gives the solar system its structure. Under NGSS standard 5-PS2-1, fifth graders investigate the gravitational force Earth exerts on objects , always directed toward the planet's center. This center-seeking quality of gravity is the key concept: it explains why dropped objects fall, why the atmosphere stays around Earth, and why the oceans remain on the surface as the planet spins.

Students also consider how gravity governs planetary orbits. The same force that holds students to the ground holds the moon in orbit and keeps the planets in their paths around the sun. This connection between everyday gravity and astronomical gravity bridges the physical science and Earth science strands of the NGSS framework.

A common challenge is helping students overcome their intuitive sense that down is a universal direction. Using globes to show that a person in Australia falls toward Earth's center , not upward by Northern Hemisphere intuition , is a productive disruption. Active learning approaches that require students to revise their concept of direction and test their predictions build the evidence-based argumentation central to this standard.

Learning Objectives

• Explain that gravity is a force that pulls objects toward the center of the Earth.
• Analyze how gravity influences the motion of objects on Earth and celestial bodies in the solar system.
• Compare the effect of Earth's mass on an object's weight using a hypothetical scenario.
• Predict how gravity would affect an object's motion if Earth's mass were different.

Before You Start

Forces and Motion

Why: Students need a basic understanding of forces as pushes or pulls to grasp the concept of gravity as a specific type of force.

Properties of Objects

Why: Understanding that objects have mass is foundational to understanding how gravity acts upon them.

Key Vocabulary

gravity	A natural force of attraction that exists between any two objects with mass. On Earth, it pulls objects toward the planet's center.
mass	The amount of matter in an object. More mass means a stronger gravitational pull.
weight	The measure of the force of gravity on an object. It changes depending on the strength of the gravitational field.
orbit	The curved path an object takes around a star, planet, or moon, due to the force of gravity.

Active Learning Ideas

Think-Pair-Share: Which Way is Down?

Show students a globe with small figures attached at multiple locations: the US, Australia, Brazil, and Norway. Partners draw arrows showing the direction gravity pulls each figure and discuss what they notice about all four arrows. The class shares and identifies that every arrow points toward the center of the globe , not 'down' as a floor reference.

⏱ 20 min·Pairs

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Inquiry Circle: Gravity Drop Tests

Groups simultaneously drop identical objects from the same height: a tennis ball, a crumpled paper ball, and a flat sheet of paper. They record results and work to separate the effect of air resistance from gravity, building a data-based argument about whether gravity pulls harder on heavier objects.

⏱ 40 min·Small Groups

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Structured Discussion: If Earth Were More Massive

Give students the prompt: Earth's mass doubles overnight. What happens to your weight? To the moon's orbit? To the atmosphere? Small groups build a cause-and-effect chain using what they know about gravitational force and orbital motion, then share their reasoning with the class for peer questioning.

⏱ 30 min·Small Groups

Generate mission

Real-World Connections

Astronauts training at NASA facilities must understand how gravity affects their bodies and equipment differently in space compared to Earth's surface.

Civil engineers designing bridges and tall buildings must account for gravitational forces to ensure structural stability and safety.

Farmers use tractors that are designed with specific tire treads and weight distribution to maintain traction on sloped fields, a direct application of understanding gravity's pull.

Watch Out for These Misconceptions

Common Misconception

Gravity pulls things down , and down is toward the floor.

What to Teach Instead

Students growing up with flat floors don't naturally think of down as toward the Earth's center. Globe activities that attach figures on multiple continents and ask students to draw gravity arrows help them reconstruct their mental model of direction from the ground up, literally.

Common Misconception

Heavier objects fall faster because gravity pulls harder on them.

What to Teach Instead

This is one of the most persistent misconceptions in elementary science. Drop tests comparing crumpled versus flat paper help students identify air resistance as the cause of different fall speeds. Two balls of different weight dropped simultaneously provide the key evidence: gravity gives every object the same acceleration.

Assessment Ideas

Exit Ticket

Ask students to draw a picture showing an object falling towards Earth. Have them label the direction of the gravitational pull and write one sentence explaining why the object falls.

Quick Check

Pose the question: 'Imagine you are on the Moon, which has less gravity than Earth. Would you weigh more, less, or the same? Explain your answer using the terms mass and gravity.'

Discussion Prompt

Facilitate a class discussion using the key question: 'What would happen to an object's weight if Earth's mass doubled? How might this change affect things on Earth?' Encourage students to support their ideas with reasoning about mass and gravity.

Frequently Asked Questions

What keeps the oceans from falling off the Earth?

Gravity does. Gravity pulls all the ocean water toward Earth's center, keeping it on the surface. From any point on Earth, down means toward the center , so there is no direction for the oceans to fall except onto the planet, which they are already doing. This is the same reason people in Australia stay on the ground just as firmly as people in North America.

How does gravity affect the movement of planets in our solar system?

Gravity from the sun continuously pulls each planet toward it. Each planet is also moving sideways at high speed, however. The combination of the inward gravitational pull and the sideways momentum produces a curved orbital path. Without gravity, the planets would fly off in straight lines into space. Without their sideways speed, they would fall directly into the sun.

What would happen to an object's weight if Earth's mass doubled?

The object's weight would double. Weight is the measure of gravitational force on an object, and gravitational force increases as mass increases. The object's own mass would stay the same , mass doesn't change with location , but the force pulling it toward a more massive Earth would be twice as strong, so it would weigh twice as much.

How does active learning help students understand the force of gravity?

Predictive experiments where students state their expectation before the drop test and then compare their prediction to the data are essential for correcting deeply held misconceptions. When students see that a crumpled and flat sheet of paper fall at different speeds but two different-weight balls fall together, they have to revise their model. That productive revision, worked out collaboratively with peers, is what makes the corrected understanding stick.