Science Lesson Plan · Grades 4-8

Law of Motion Lesson

Teach students Newton’s First Law of Motion and the concept of inertia. The lesson traces Galileo’s experiments on friction and motion and shows how Newton built on that work to define the law of inertia: an object at rest stays at rest and an object in motion stays in motion unless acted on by an outside force. Includes comprehension questions and answer key.

Lesson Overview

Newton’s First Law and the Law of Inertia

Before Newton, Galileo demonstrated that objects moving on a frictionless surface would continue moving indefinitely. Newton formalized this observation as his First Law of Motion: an object at rest remains at rest and an object in motion remains in motion at the same speed and direction unless acted upon by an unbalanced force. This property — the tendency of objects to resist changes in motion — is called inertia. Heavier objects have greater inertia and require more force to change their motion.

The four-page lesson pairs a reading passage on Newton’s First Law with comprehension questions that test students’ understanding of inertia, friction, and the relationship between mass and motion. A complete answer key is included.

How to Use This Lesson

1. Read the lesson passage together or have students read silently (page 1). Ask students to give everyday examples of inertia — a book sliding off a table, a car stopping suddenly.

2. Discuss how Galileo’s ramp experiments led to Newton’s First Law. Have students explain in their own words what ‘inertia’ means.

3. Assign the comprehension questions (pages 2–3) independently. Review answers using the key on page 4.

Printable Resource

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