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Take a look at the Saturn V rocket, the vehicle that powered the Apollo Moon exploration program in the 1960s and 1970s. It rose more than a football field in height, was 33 feet in diameter at the base and weighed over 6,000,000 pounds (mass = 2.7 million Kg). It's not difficult to imagine that it would be immensely difficult to move the Saturn V, and that it would take a tremendous amount of rocket force in order to get it to move upward, against gravity, fast enough to escape into orbit.

The Saturn V had a lot of inertia. The Latin root of the word means "laziness." Inertia is the tendency of all objects to keep doing just what they're doing at any given instant; stationary things tend to keep still and moving objects, in the absence of any external force, tend to keep on moving in the same direction.

In the universe, there are a few kinds of forces. Whether you're standing on the ground or sitting in a chair right now, the reason you don't just sink right into the matter of the floor or chair is that there are repulsive forces at work between the negatively-charged electrons of the atoms that make up everything.

There are other forces at work deep inside the nuclei of atoms, and there are the familiar forces we feel all day long, like when you are jostled in a crowd, or the force of gravity that always brings you back to Earth when you jump.

In the absence of any forces, two things will happen:

  1. Things that aren't in motion will continue on that way.

  2. Things that are in motion will continue in motion in the same direction with the same speed.

These tendencies are called inertia. Inertia can't be measured. It has no units and cannot be calculated. it's just a concept — but one of the most important in the physical sciences.

saturn v image

Image: NASA

Modern definition of inertia

The velocity vector of an object will remain constant unless the object is subjected to an unbalanced (net) force.

Note that most objects that appear to be moving without the influence of a net force are actually subject to some forces. The universe is big and we might not be aware of all of these forces.

Inertia is proportional to mass and to speed

Imagine how easy it is to alter the course of a blowing leaf with your hand, but how difficult it would be to change the direction of a freight train moving at the same speed. It would take an immense amount of force. The train has more inertia, whether sitting still or moving.

In general, things that have more mass have more inertia, whether they are stationary or

moving. The greater the inertia, the more difficult it is to get a stationary object to move, or a moving object to change direction, speed up or slow down. Momentum is proportional to both the mass and speed of a moving object; it is a good surrogate for the inertia of moving objects.

The law of inertia (traditional)

Objects at rest tend to remain at rest, and objects in motion tend to remain in motion, unless acted upon by an outside force.

What is mass?

Image: Julian Herzog

Mass is often confused for weight. Mass isn't weight, it's just a measure of the quantity of matter. It's proportional to the number of atoms in a sample. An object has the same mass on Earth and its Moon, but not the same weight.

If there is any satisfying explanation of the property of inertia, it is probably tied to an explanation of what mass is. But we don't yet really understand what gives objects mass. For now, that seems to be the domain of the physics of subatomic particles, and there seems to be some recent glimmer of hope with news (2012) of the possible discovery of the Higgs boson particle. Part of that work is being done at the CERN particle accelerator/collider in Switzerland (left).

Everyday experience with inertia: Cars

Consider the feeling you get when you slow down or speed up rapidly in a car. As you speed up, you feel like you're being pressed back into your seat. What's really happening is that your inertia to remain still or at the speed you were going tends to keep you still or at that speed. What you feel on your back is the car exerting a force on you to get you moving faster Slowing down is analogous. This time you have a certain forward inertia and the car and seat belt slow down as your inertia tends to keep you at the speed you were going. The result is a backward pulling force by the seat belt.


You've probably experienced going around a sharp corner in a car a little too fast. What happens?

In a left turn you feel like if it weren't for the seat belt and the door itself, you'd go flying right out the passenger side of the car. The illusion is that there is a force pushing you to the right when you're going around a left-hand corner (←) or to the left as you turn to the right.

What's actually happening in a left curve is that your body has inertia, more or less in the direction of the headlights - straight ahead relative to the car. But the car is being forced (by the turning of the wheels and their friction with the road) to the left. Thus, the car is pushing on you from the right side as your inertia keeps your body moving forward.

Inertia (and some gravity) keeps planets in orbits

When people first began to discover that planets orbit the sun and the moon orbits Earth, they looked for ways to explain it. An early theory proposed that invisible angels pushed the moon around Earth in a circle. Hey, it kind of made sense, right — for something to move in a direction (around a circle), there should be a force pushing from behind.

That turned out to be wrong. Isaac Newton showed that the only force needed was the centripetal ("center-seeking") force of gravity between the two bodies. It is the inertia of the moon that keeps it moving, and the gravitational force that simply bends its path.

If we could cut gravity off instantly, the moon would continue on a straight line (by its inertia) on a path that is tangent to the orbit at the point of switching-off.

That's not all there is to orbits, but it's pretty close. Actually, the moon and Earth orbit one another, about their center of mass, which lies about a fourth of the way into Earth. One body never orbits another, they alwas orbit one another.

Practice questions

Here are some questions to help you organize your thinking about inertia.


The tendency of an object to resist any change to its motion is called __________, and this quantity depends on the amount of __________ that the object has.


The tendency of an object to resist any change to its motion is called inertia, and this quantity depends on the amount of mass that the object has.


What force pulls Earth toward the moon (or equivalently, the moon toward Earth)?


The gravitational force (gravity) pulls Earth and its moon together. The inertia of the planets keeps them in motion in their orbits.

See gravity.


Two objects have masses of 200 Kg and 240 Kg. Which has more inertia?


The object with more mass always has the greater inertia.


The greater an object's inertia, the greater its

  1. gravitational force.
  2. speed.
  3. mass.
  4. acceleration.

Mass is the thing that's proportional to inertia.


When a spacecraft operating outside of the gravitational force of stars or planets shuts off its engines, in which direction will it move?


Absent any other forces, the spacecraft will continue in the same direction at which it was traveling when the engines cut off, and at the same speed.


When a car stops moving suddenly, in which direction do the people and untethered objects in it move, and why?


Anything not tied down (including people) in the car will continue to travel in the direction that the car was moving when it stopped. That's why, without a seat belt, passengers could fly right through the front windshield in a forward collision.

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