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Subject: Physics 
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 What is Newton's first law?
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 are causing 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. [Wait, isn't there still
gravity at the space station?]
Space Station Astronauts Eat a Floating Dinner | ISS Science Video
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).
[What if we made an air hockey table that stretched around the Earth?]
What does "force", "external force", and "net force" mean?
A force is a push or a pull exerted on one object from another object. The units of force F are called
Newtons or simply N.
An external force is a force originating from outside of an object rather than being 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 objects overall
motion.
The net force, written as ΣF , on an object is the total force on an object. If many forces act on an object,
the net force is the sum of all the forces. But be careful, since force F is a vector, to find the net force ΣF
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 N − 30 N
ΣFhorizontal = 15 N
Assuming rightward is the positive direction.
Newton's first law says that if the net force on an object is zero (ΣF = 0) then that object will have zero
acceleration. That doesn't necessarily mean that 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").
So for the box of frozen burritos, if the rightward force had a magnitude of 45 N and the leftward force
had a magnitude of 45 N the net force would be zero. So the box of burritos would either continue
moving with a constant velocity, if it started with a velocity before the forces were applied. Or the box of
burritos would stay at rest, if they started 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 compare to the magnitude of the downward force of gravity Fg on the
elevator?
a. Fc is greater than Fg
b. Fc is equal to Fg
c. Fc is smaller than Fg
d. Fc could be larger or smaller than Fg 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. So, 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.
[References and Attributions]
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