[ExI] from Quora - long answers to friction and moon questions
William Flynn Wallace
foozler83 at gmail.com
Sat Jan 28 15:13:18 UTC 2023
What are some examples of things that sound right but are actually wrong?
I can think of two things that sound right but are wrong.
A) Friction with the air is responsible for meteors and spacecraft burning
up and disintegrating in the sky.
B) The Moon’s gravity lifts up a bulge of water in the oceans and is
responsible for the tides.
Both of these things are wrong. To be clear, Things reentering the
atmosphere do burn up. The Moon’s gravity is responsible for the tidal
bulges. It is the explanation as to how they do it is wrong.
Let’s start with reentry into the atmosphere.
Friction does cause heating. It’s how early man mastered fire by rubbing
two sticks together really fast. Rub your palms together really fast and
you hands warm up. Aircraft that fly at very high supersonic speeds do get
affected by extreme heating, which is why the SR-71 was made of titanium.
So it makes sense that the heating caused by reentry and speeds even faster
than supersonic is caused by the extreme friction.
But it isn’t. At such extreme speeds (we are talking mach 25, not the
SR-71’s Mach 3.2) The air cannot get out of the way fast enough and forms a
separated compression bow shock ahead of the object. the air in the bow
shock is extremely compressed and it is this compression, not the friction
of air moving past the spacecraft, that creates the extreme heat.
You can see the densely compressed bow shock as the the very dark curve
formed ahead of the capsule in this wind tunnel test. note that the bow
shock is ahead of and not in direct contact with the craft.
It is within this bow shock where the greatest heating occurs, that heat is
then reradiated into the spacecraft as the shock heating creates ionized
So it is compression of the air ahead of the object, not friction of the
air moving past the object, which creates the tremendous heat of reentry.
There is friction heating going on as well, but it is nothing compared to
the heat generated by compression.
Speed regimes are broken into several different categories.
Subsonic: 0 to Mach 1
Transonic: a special region that is actual in the subsonic but with local
airflows reaching supersonic flow, about mach 0.95 to mach 1
Supersonic: mach 1 to mach 4.9
Hypersonic: mach 5 to mach 9.9
High Hypersonic: mach 10 to mach 19.9
Reentry velocities: above mach 20
The difference in these speeds are not a fixed value. They depend on air
pressure and temperature among other things. The differences are defined by
the flow characteristics and other phenomena that happen during those
regimes. At supersonic speeds, no bow shock is formed and the primary
heating source is friction. into hypersonic regimes and higher, a bow shock
forms and compression is the dominant source of heating.
But still you will hear a lot of people talk about things burning up on
reentry due to friction with the air.
The above is fairly easy to understand once explained. Though it is easy to
see why the wrong explanation is common, you can still see the truth after
The Tidal force of the Moon on Earth is much more difficult to grasp, even
after explaining. Many stubbornly refuse to accept because they can’t grasp
it as easily as they can the explanation they’ve heard wrong all their life.
Tidal force is the gradient in gravity felt across an object. Your feet are
closer to the earth than your head. As a result, your feet are being pulled
more strongly by gravity than your head is. There is a tidal force between
your feet and head as a consequence. But Earth’s gravity field is so large
and the distance between your feet and head so small that the gradient is
almost nothing. Even if you stood as tall as the ISS Orbits (250 miles
tall), there would only be a 10% difference between your head and feet.
Yes… the ISS experiences about 90% of the same gravity you feel standing on
The next thing to understand is that gravity works both ways between to
things. Earth pulls the moon towards it as much as the Moon pulls the Earth
So using the Tidal force, we often get this explanation…
Considering the tidal force of the moon across the Earth, the water closest
to the moon has a stronger pull of the Moons gravity than the earth itself.
and the Earth itself has a stronger pull towards the moon than does the
water opposite the moon. So the closest water gets pulled the most, the
Earth somewhat less, and the Far waters the least. The near waters is
pulled up away from Earth. and the Earth is pulling away from the far
waters which lags behind.
Now this tidal force explanation is true. There really is a difference felt
between the near waters, the Earth, and the far waters. But it isn’t
causing the tidal bulges. Think about it. The Moon’s surface gravity is
only a sixth of Earths surface gravity and it is over a quarter of a
million miles away. That means the Moons gravity is trying to pull water up
against Earths gravity pulling it down and Earths gravity at the waters
surface is over 10,000 times stronger. You are not going to pull anything
upwards against a force weighing it down that is ten thousand times
stronger. And if the Moon’s gravity could pull something upwards against
Earth gravity pulling it down, it wouldn't stop. Nor would it just be water
affected You would fly off the Earth’s surface and into space. That doesn’t
happen. So what is happening?
Tidal force is real, it’s just not the explanation for the Tidal bulges.
Something far more subtle is at work, and Yes… it is because of the Moon’s
gravity, just not through tidal force.
For that explanation you need to understand force vectors and vector
A vector is a combination of both direction and magnitude of change.
For example. Speed, expressed in some units like Miles per Hour (MPH) is
just a magnitude of change. It tells you how fast you are going, but
North is a direction. but nothing more.
You moving 30 mph to the north is a Vector. It tells you both where you are
going and how fast you are going in that direction.
Force vectors are often expressed as arrows or lines. the orientation of
the arrow gives you direction, the length of the arrow give you the
magnitude of change. The magnitude doesn't have to be speed. it can be
anything, Acceleration, Gravity.. etc.
Now think of Gravity as force vectors. You are standing on the Surface of
Earth. Earth’s gravity is this huge arrow pointing down. The moon is
directly overhead and it’s gravity is this tiny little arrow pointing up.
the tiny little arrow pointing up is not going to overcome the massive
arrow pointing down.
Now we need to understand vector addition.
Vector addition can be done graphically. Draw two arrows of different
magnitudes originating from the same point. A Red Arrow and a Blue arrow.
These are two different forces acting in two different direction at
different strengths on the same object.
The sum of those two forces is found by moving the base of one of those
arrows to the tip of the other, keeping its length and direction the same.
Then draw a new arrow from the object to the tip of the moved arrow. This
new arrow is the sum of the other two and it is in this vector the object
When these two vectors being added are 180 degrees apart, pulling in
opposite directions, the smaller just cancels out a part of the larger, but
the direction doesn't change.
Back you you and the Earth and Moon. Adding those two vectors together
still results in a vector straight down to Earth, just slightly less so by
the amount cancelled out by the Moons smaller arrow.
The Earth still pulls you down, just less so by the amount the Moon’s
gravity was pulling you. But it is still down. You are not lifting a bulge
of water against that.
But now lets look at all the vectors of all the water not directly on the
On the Earth-Moon line, the Vector is still straight down to Earth. But off
it… it results in a very slightly offset from straight down to Earth. The
Vector strength is still down to Earth but its direction is slightly to the
side of straight down, in the direction towards the Earth-Moon line.
This slightly sideways force from straight down results in a very tiny side
force on every parcel of water not directly on the Earth-Moon line. This
side force is extremely tiny, but over vast distances and lots and lots of
water does add up.
The bulges are not lifted up by the Moon pulling up on them. They are
squeezed up by the Hydraulic action of a ocean of side force from the
summed vectors of the Earth and Moon’s combined gravities.
The sum of all this hydraulic side force ends up something like this…
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