Also called the law of inertia, this is the most important thing to realize about motion.

Why do objects slow down?

Before Galileo and Newton, many people thought objects slowed down because they had a natural built in tendency to do so. But those people weren't taking into account the many forces—e.g., friction, gravity, and air resistance—here on Earth that cause objects to change their velocity. If we could observe the motion of an object in deep interstellar space, we would be able to observe the natural tendencies of an object's motion free from any external influences. In deep interstellar space, we would observe that if an object had a velocity, it would continue moving with that velocity until there was some force to cause a change in the motion. Similarly, if an object were at rest in interstellar space, it would remain at rest until there was a force to cause it to change its motion.
In the video below, we can see that objects in the international space station either remain at rest or continue with constant velocity relative to the space station until acted upon by a force.
Yes, the force of gravity at the International Space Station (ISS) is almost as large as on the surface of Earth. But since the space station is in free fall orbiting around the Earth, from inside the ISS, it's as if there were no gravitational force at all.
There is air in the ISS—since astronauts have to breathe—and so there would be some air resistance. However, since everything is in free fall, objects don't immediately plummet and make contact with the floor, so the force of friction is much less of a distracting influence on the motion of an object.
The idea that objects only change their velocity due to a force is encapsulated in Newton's first law.
Newton's first law: An object at rest remains at rest, or if in motion, remains in motion at a constant velocity unless acted on by a net external force.
Note the repeated use of the verb remains. We can think of this law as preserving the status quo of motion. Newton’s first law of motion states that there must be a cause—which is a net external force—for there to be any change in velocity, either a change in magnitude or direction. An object sliding across a table or floor slows down due to the net force of friction acting on the object. But on an air hockey table, where air keeps the puck from touching the table, the air hockey puck continues moving with a roughly constant velocity until a force acts on it—like when it bumps into the side of the table.
Well, there would be basically zero friction since air hockey pucks aren't touching the table, but there would still be a small amount of air resistance. So, the puck wouldn't make it all the way around the Earth. We can't get rid of the air since that is the only thing keeping the puck lifted off the table—it is called air hockey after all.
Unfortunately this means that playing air hockey with people in other countries on a table thousands of miles long doesn't seem possible. This is probably for the better since even if there were no air resistance or friction, it would take hundreds of hours for your air hockey serve in London, England to get returned by someone in Beijing, China.

What do force, external force, and net force mean?

A force is a push or a pull exerted on one object by another object. The units of force FF are called Newtons or simply N\text{N}.
An external force is a force originating from outside an object rather than a force internal to an object. For instance, the force of gravity that Earth exerts on the moon is an external force on the moon. However, the force of gravity that the inner core of the moon exerts on the outer crust of the moon is an internal force on the moon. Internal forces within an object can't cause a change in that object's overall motion.
The net force, written as ΣF\Sigma F, on an object is the total force on an object. If many forces act on an object, then the net force is the sum of all the forces. But be careful—since force FF is a vector, to find the net force ΣF\Sigma F, the forces must be added up like vectors using vector addition.
In other words, if a box of frozen burritos had a force of magnitude 45 Newtons exerted on it to the right and a force of magnitude 30 Newtons exerted on it to the left, the net force in the horizontal direction would be
ΣFhorizontal=45 N30 N\Sigma F_{\text{horizontal}}=45\text{ N}-30\text{ N}
ΣFhorizontal=15 N\Sigma F_{\text{horizontal}}=15\text{ N}
Assuming rightward is the positive direction.
Newton's first law says that if the net force on an object is zero (ΣF=0\Sigma F=0), then that object will have zero acceleration. That doesn't necessarily mean the object is at rest, but it means that the velocity is constant. In other words, constant zero velocity—at rest—or constant non-zero velocity—moving with a constant velocity.
For the box of frozen burritos, if the rightward force had a magnitude of 45 Newtons and the leftward force had a magnitude of 45 Newtons, the net force would be zero. The box of burritos would either continue moving with a constant velocity—if it started with a velocity before the forces were applied—or stay at rest—if it was already at rest before the forces were applied.

What does mass mean?

The property of a body to remain at rest or to remain in motion with constant velocity is called inertia. Newton’s first law is often called the law of inertia. As we know from experience, some objects have more inertia than others. It is obviously more difficult to change the motion of a large boulder than that of a basketball, for example.
The inertia of an object is measured by its mass. Mass can be determined by measuring how difficult an object is to accelerate. The more mass an object has, the harder it is to accelerate.
Also, roughly speaking, the more “stuff”—or matter—in something, the more mass it will have, and the harder it will be to change its velocity, i.e., accelerate.

What do solved questions involving Newton's first law look like?

Example 1: space probe drift

A space probe is drifting to the right at a constant velocity in deep interstellar space—far from any influence due to planets and stars—with its rockets off. If two rocket thrusters both turn on simultaneously exerting identical forces leftward and rightward in the directions shown, what would happen to the motion of the rocket?
a. The space probe would continue with constant velocity.
b. The space probe would speed up. c. The space probe would slow down and eventually stop.
d. The space probe would immediately stop.
The correct answer is a. According to Newton's first law, a non-zero net force is required to change the velocity of an object. The net force on the space probe is zero—since the forces on it cancel—so there is no change in the velocity of the probe.

Example 2: elevator lift

An elevator is being pulled upward at a constant velocity by a cable as seen in the diagram below. While the elevator is moving upward at constant velocity, how does the magnitude of the upward force exerted on the elevator by the cable—Fc\redD{F_c}—compare to the magnitude of the downward force of gravity—Fg\greenD{F_g}—on the elevator?
a. Fc\redD{F_c} is greater than Fg\greenD{F_g}.
b. Fc\redD{F_c} is equal to Fg\greenD{F_g}.
c. Fc\redD{F_c} is smaller than Fg\greenD{F_g}.
d. Fc\redD{F_c} could be larger or smaller than Fg\greenD{F_g} depending on the mass of the elevator.
The correct answer is b. If the elevator is moving with constant velocity, the net force must be zero. In order for the net force on the elevator to be zero, the upward and downward forces must cancel exactly.

Example 3: space probe path

A space probe is drifting to the right with constant velocity in deep interstellar space—far from any influence due to planets and stars. If a rocket thruster turns on and then off for a short burst of force in the direction shown, what would best represent the path traveled by the rocket after the thruster turns off?
a. Path a
b. Path b
c. Path c
d. Path d
The correct answer is c. After the rocket thruster turns off, there will be no net force on the space probe. Once the net force is zero, the velocity—both magnitude and direction—must be constant. Because of Newton's first law, the space probe moves in a straight line at constant speed. The fact that there was a vertical force on the space probe does not affect the horizontal velocity of the space probe, it only changes the vertical velocity. A constant vertical and horizontal velocity yields a diagonal straight line through space.
This article was adapted from "Newton's First Law" from Openstax College Physics. Download the original article free at http://www.cnx.org/contents/031da8d3-b525-429c-80cf-6c8ed997733a@9.1:24/Newtons-First-Law-of-Motion-In.
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