# Inertia Lesson for Kids: Definition, Law & Examples

Instructor: Sandra Van Fleet
Have you ever heard the phrase, 'An object in motion will stay in motion'? This phrase refers to inertia, which we will define in this lesson. We'll explore the scientific law of inertia, and you can see some real-world examples.

## What is Inertia?

You've overslept, so your mother has to drive you to school. Since you're late, she doesn't seem to be taking the usual care in driving. She drives quickly through the neighborhood, and with every turn she takes, you feel yourself being pulled. As she approaches the school, she slams on the breaks, narrowly missing an oncoming train. Thankfully, you are wearing your seat belt.

You finally arrive at school, and you run up and push the door open. But the door is locked, and the force of your push knocks you down to the ground. This wild adventure was made possible by Newton's law of inertia.

Inertia is a property of an object, and it refers to the resistance of change in motion. As you rode in the car to school, your body traveled in the same direction and speed as the car. When your mom turned the corner, your body resisted against that change in direction--it wanted to keep going straight. That's why you felt a slight pull as she made a sharp turn--because of inertia.

## Newton's Law of Inertia

17th-century physicist Sir Isaac Newton came up with three laws of motion, but we will focus on his first law: the law of inertia. It states that an object at rest will remain at rest, and an object in motion will remain in motion, unless some outside force starts or stops the object.

Let's think about when your mom slammed on the brakes. When the car stopped, your body continued to move forward until your seat belt pulled you back into your seat. Without the seat belt, you would continue to move at the same rate of the car and in the same direction until some outside force slowed or stopped your motion. The seat belt is the outside force that Newton refers to in the law of inertia.

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