Inertia: Newton's First Law of Motion

Lesson Plan - Get It!

Audio:

Changes happen all the time.

It could be a change in the weather, your schedule, the type of food in the cafeteria, or anything else that is not in your plan for the day. You may deal with changes like these well, or you may resist accepting them.

• Did you know that everyone - and everything - has the natural tendency to resist change?
• How does that affect us?
• Do certain things have a higher tendency to resist change than others?

To answer these questions, you need to learn about an object's resistance to change in its state of motion!

What Is Inertia?

Inertia is the resistance of an object to any change in its state of rest or state of motion.

It is the tendency of an object to keep doing what it is doing. That is, if the object is at rest, it will remain at rest. If the object is in motion, it will keep moving.

In the first example above, you were a passenger in a car at rest in front of a red traffic light. Your body was also in a state of rest, so it would resist any motion. Therefore, when the car suddenly moved forward, your body moved backward in its attempt to keep you at rest.

This can be seen in the diagram below.

Conversely, the second example described you as a passenger in a moving car. Your body was in a state of motion and would resist anything that would stop you from moving. Therefore, when your driver stopped the car quickly, your body moved forward in its attempt to continue moving in the same direction.

This can be seen in the diagram below.

Mass is a measure of inertia. The amount of an object's resistance to any change in its state of rest or motion depends on its mass.

Heavier objects have a higher tendency to resist this change than lighter objects.

A car, for example, has more inertia than a bicycle. You can push a bicycle and change its state of rest into motion. A car requires a lot more force to change its state of rest into motion.

Forces and Newton's First Law of Motion

Pushing a car into motion is much more challenging than pushing a bicycle because it requires a greater amount of force. When you add forces to the explanation of inertia, you define Newton's first law of motion.

An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

• What is an unbalanced force?
• What happens to an object if the forces acting on it are unbalanced?

Let's continue using a car to demonstrate Newton's first law of motion. Look at the diagram below showing two forces acting on a car.

Both people are pushing on the car with the same level of force in opposite directions.

• Do you think that the car will move as a result of these forces?

Absolutely not!

In this case, the forces acting on the car are balanced forces. When forces are balanced, an object at rest will remain at rest, and an object in motion will move at the same speed all the time.

Now, suppose you ask your friend to help push the car in the same direction as shown in the diagram below.

This combines the forces and applies them to the car in the same direction.

• Is there a possibility that the car will move?

Absolutely yes!

In this case, the forces acting on the car are unbalanced forces. When the forces are unbalanced, an object will start moving in the direction of the applied force. The unbalanced force will make the car speed up in a particular direction.

In summary, Newton's first law of motion adds the special condition unless acted upon by an unbalanced force to inertia.

When an object is at rest, it will remain at rest unless an unbalanced force - such as a push - is applied to start its motion. When the object is moving at a certain speed, it will continue to move unless there is an unbalanced force - such as a wall - that would stop it from moving.

Watch the video below to review this concept.

Applications of Newton's First Law of Motion

You experience inertia when you ride in a car. Unfortunately, not everyone uses the safety features in their car, which can lead to serious injuries in the event of accidents.

If a moving car collides with an object head-on, the passengers will continue to move forward even as the car stops. Safety features, such as airbags and seatbelts, are designed to stop their inertia.

• What if a passenger is not wearing a seatbelt?

Watch the following video to see what happens to the pretend passengers in this test crash.

• Have you ever heard of whiplash?

Whiplash is a neck injury caused by a rapid forward and backward movement of the head and neck. It happens often in car accidents when one vehicle is hit from behind.

To prevent this from happening, car seats have headrests that keep your head and neck from moving rapidly. Some seats even have special shock absorbers to keep you safer.

Watch a portion of this next video to see these car safety features.

Inertia is an important physics concept that has been widely used not only in developing safety features for cars but also in other industries such as sports.

In the Got It? section, you will discover many other applications of Newton's first law of motion.