Laws of Inertia: Definition & Formula

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  • 0:05 Definition
  • 0:42 Newton's First Law
  • 2:21 Effects of Inertia
  • 4:31 Examples
  • 8:43 Lesson Summary
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Lesson Transcript
Instructor: Elena Cox
Inertia is the tendency of an object to stay at rest or in motion. Learn more about Newton's first law as it explains how forces acting on an object are related to the object's mass and motion through several examples, and test your knowledge with quiz questions.


Place a book on your desk. Does the book move? Unless you move the book, it will remain where you put it without moving.

Imagine a spacecraft moving through space. When the engines are turned off, the spacecraft will coast through space at the same speed and in the same direction.

The book and the spacecraft have inertia. Inertia is the property of an object that resists changes in its motion. Because of inertia, an object at rest tends to stay at rest. An object in motion tends to keep moving at a constant speed in a straight line.

Newton's First Law

Newton's First Law of Motion explains how inertia affects moving and non-moving objects. Newton's first law states that an object will remain at rest or move at a constant speed in a straight line unless it is acted on by an unbalanced force. Inertia comes from mass. Objects with more mass have more inertia.

To understand inertia, imagine moving a bowling ball and a golf ball that are at rest. The golf ball has a mass of 0.05 kilogram, and the bowling ball has a mass of five kilograms. The bowling ball has 100 times more mass than the golf ball, so it has 100 times more inertia, too.

Now ask yourself, which needs more force to start moving? If you push for the same distance, the bowling ball takes much more force to get it moving the same speed as the golf ball. The bowling ball needs more force because a bowling ball has more inertia than a golf ball. The greater an object's inertia, the greater the force needed to change its motion.

According to Newton's first law, an unbalanced force is needed to move the book on your desk. You could supply the force by pushing the book.

An unbalanced force is needed to change the speed or direction of the spacecraft. This force could be supplied by the spacecraft's engines.

Because of inertia, an object at rest will remain at rest until something causes it to move. Likewise, a moving object continues to move at the same speed and in the same direction unless something acts on it to change its speed or direction.

Effects of Inertia

You can feel the effects of inertia every day. Suppose you are riding in a car. What happens if the car comes to a sudden stop? Your body has inertia. When the car stops, you keep on moving forward. What happens when the car starts moving? Because of inertia, your body tends to stay at rest when the car moves forward.

In baseball, inertia tends to keep a player running in a straight line. So, base runners have to 'round' the bases instead of making sharp turns.

An oscillator is a physical system that has repeating cycles (harmonic motion). A child on a swing is an oscillator, as is a vibrating guitar string. A wagon rolling down a hill is not an oscillator. Systems that oscillate move back and forth around a center, or equilibrium, position. You can think of equilibrium as the system at rest, undisturbed, with zero net force. A wagon rolling down a hill is not in equilibrium because the force of gravity that causes it to accelerate is not balanced by another force. A child sitting motionless on a swing is in equilibrium because the force of gravity is balanced by the tension in the ropes.

A restoring force is any force that always acts to pull a system back toward equilibrium. Restoring force is related to the force of gravity or weight and the lift force (or tension) of the string of a pendulum. If a pendulum is pulled forward or backward, gravity creates a restoring force that pulls it toward equilibrium. Systems with restoring forces become oscillators.

Inertia causes an oscillator to go past equilibrium. The motion of an oscillator is the result of an interaction between a restoring force and inertia. For example, the restoring force pulls a pendulum toward equilibrium. But, because of Newton's first law, the pendulum does not just stop at equilibrium. According to the first law, an object in motion tends to stay in motion. The pendulum has inertia that keeps it moving forward so it overshoots its equilibrium position every time.


Cars and planes with more inertia take more force to accelerate. Since inertia is related to mass, in order to reduce inertia you must reduce mass. The mass of a car or plane is a trade-off between inertia and the strength of materials of the car or plane. You want strong materials, but you don't want them so heavy that it takes too much energy (fuel) just to get the car or plane moving!

Seat belts could be called 'anti-inertia' belts. A car tends to keep moving in a straight line, even if the driver's foot is not on the gas pedal. Everyone inside the car also has inertia. They are moving at the same speed as the car.

Suppose you are riding in a car. The driver is forced to step on brakes suddenly. Inertia keeps you moving forward at the same speed as the car was moving. You will keep moving until something stops you. This might be the car's steering wheel, dashboard or windshield. You might be hurt if you hit these parts of the car, unless you are wearing a seat belt. A seat belt keeps you from moving forward when the car stops suddenly. Seat belts can prevent serious injuries. You should always remember to 'buckle up' when you get into a car.

If you suddenly apply the brakes to your rapidly moving bicycle, you have to brace yourself to keep from falling forward. If you are on a ride at an amusement park in which you suddenly accelerate forward, your body seems to push back against the seat.

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