Professor Dave here, let's talk about
Newton's first law.
In Newton's Principia he describes three
laws of motion, and the first of these
three was revolutionary. It can be
summarized as follows. An object at rest
will remain at rest and an object in
motion will remain in motion unless
acted upon by some external force. The
first part seems to make sense, since
motionless objects typically don't just
start moving, but the second part
contradicts much of our everyday
experience. When you throw a ball
it doesn't keep moving forever, it
eventually stops. For a wheelbarrow to
roll or for your car to move or for a
plane to stay in the air, a constant
force must be applied. So where did Newton
come up with this stuff? To understand
Newton's first law of motion we have to
understand all of the forces that act on
a body in motion.
Our planet is a complicated system and
when we move around in our atmosphere we
experience things like wind resistance
and other forms of friction, which is a
force that resists motion.
This is what slows us down, requiring the
application of constant force to keep
moving. But think of something like a
skating rink.
The ice is very slippery. It is a surface
with very little friction. If you gently
push a hockey puck it will glide rather
far, much farther than if you pushed it
on a table or on the ground. But there is
still some friction so it eventually
does stop. Now imagine, as Galileo and
Newton did, that there is a surface that
is completely free of friction. If you
push the hockey puck on this surface,
whatever velocity it has when it leaves
the stick would be retained indefinitely,
and it would glide forever, unless some
other force were to act upon it.
This is the kind of motion that can be
seen in the vacuum of space.
Stars and planets and asteroids and
satellites, these objects have roughly
constant velocity and they never stop
moving.
As it happens, most of the universe is in
space and very little of it is on earth,
so we should consider motion in space
where there is no friction and no wind
resistance to be normal motion. It is
motion within Earth's atmosphere that
is the special case, where objects are
subjected to a variety of variables.
This is why a car needs an engine that
is always running, because there is
resistance from the atmosphere and
friction with the road. But a spaceship
can just accelerate to a certain
velocity and then turn the engine off
and it will cruise at that final
velocity without end. The capacity of an
object to resist changes in motion is
called its inertia. Force must be applied
to get an object moving and again to
slow it down to a stop.
Only motion with constant velocity
including zero velocity, requires no
application of force. An object will have
some amount of inertia that is
proportional to its mass, a scalar value
measured in kilograms. More massive
objects have greater inertia or greater
resistance to motion. Think of a tiny
sailboat versus a huge luxury cruise
ship. A strong gust of wind is enough to
get the sailboat going, and if it hits a
rock it will stop on impact. But the
cruise ship needs huge engines to
accelerate from rest. A lot of force is
needed to get it going and a lot of
force is needed to get it to stop.
Similarly, it is easy to set a ping-pong
ball in motion.
Just give it a flick. But a bowling ball
will need quite a bit more force, and
more force to stop as well, as any
bowling pin will tell you. In this way
mass is essentially a quantitative
measure of an object's inertia. As we said,
this applies to objects that are already
in motion as well. If you get into a car
accident, the force of impact will stop
the car, but your inertia dictates that
your body will tend to continue moving,
and this is why we wear seatbelts,
because without them we might continue
moving right through the windshield
which wouldn't be very fun. So Newton's
first law can be summarized as the law
of inertia, and it tells us that in
absence of a net force an object will
preserve its state of motion, wether at
rest or moving with constant velocity. We
must understand what net force means
because there will often be multiple
forces operating on an object, and the
sum of the vectors representing these
forces is called the net force. If the
net force is not zero, there will be some
acceleration. If the net force is zero
there will be no acceleration. Now we can
move on to the second law of motion, but
first let's check comprehension.
Thanks for watching, guys. Subscribe to my channel
for more tutorials, support me on patreon
so I can keep making content, and as
always, feel free to email me: