| I don't understand how nothing can
stop instantly. I was told that if a bullet going
1000 mph was fired at a wall, even when it hit the
wall, it would have to go through all the speeds
from 1000-0. It couldn't instantly go from
1000->0. What really trips me out is how can the
bullet be going 999 mph if, in fact, the bullet is
not moving (since it hit the wall)?|
|Question Date: 2016-12-22|
I understand the misconception, however many times
with physics, what one observes is not actually
what happens in nature.
This case is answered by the fact that
objects in motion MUST BE SLOWED DOWN TO A
STOP. Things have to get slowed down, or
accelerated, which brings them to a stop, whether
it is a bullet hitting a wall or a car's brakes.
Consider a bullet, as you mentioned, being shot
at a thick wall of metal. When we observe this, we
conclude the bullet will do one of three things.
The bullet will either hit the wall and bounce
off, go completely through the wall, or, as you
suggested, get stuck in the wall. For our example
we will consider the third case.
If a bullet is shot into a wall and stops,
we know it cannot stop instantly. Once the
bullet enters the wall, a large force pushes
against the bullet in the opposite way. This force
accelerates the bullet in the OPPOSITE DIRECTION,
making the bullet slow down until it stops.
This force is the reason the bullet gets stuck
in the wall.
What if the bullet went completely through
the wall? The bullet experiences a force in
the opposite direction in the same way, slowing
the bullet down. However, what is different about
this case is that the wall is not as strong, and
doesn't accelerate the bullet enough to stop its
motion. There is nothing that says the bullet HAS
TO STOP, as it could just be slowed down a little
bit. This is why a bullet would not possibly go
from its initial speed to a stop instantly. Since
acceleration is applied over time, there is no way
to accelerate something in an instant, which is
one moment of time. Hope this answers your
Physically it doesn't make sense if the speed of
the bullet goes from 1000 to 0 directly. When
the bullet hits the wall, the velocity will drop
instantly. The whole process could take less
than half a second, or even shorter. It is the
scale that you should keep in mind. Suppose you
have a supercamera that can record 10000 frames
per second, when you play back the process slowly,
then you should be able to resolve the drop of
velocity of the bullet, going from 1000 to 999,
and then eventually to 0.
I like to think about this problem in two ways.
First, when the bullet hits the wall, either the
bullet, the wall, or both, deform. For
example, if the bullet is harder than the wall, it
will penetrate into the wall, and slow to a stop
somewhere inside. If the wall is harder than the
bullet, the bullet will be squished as it hits the
wall, so even after the tip of the bullet hits,
the rest of the bullet is still moving toward the
wall, but quickly slows and stops. In both of
these cases, the bullet does not instantly stop.
However, what if neither the bullet nor the
wall can deform? Or, in the simplest case,
what happens when two atoms collide? The
key idea here is that when all matter (i.e.,
atoms) is pushed close enough together, it
repels. And, the closer together the atoms
are, the more strongly they repel. So, when an
atom collides with another atom, they don’t
immediately “hit” each other. Instead, they begin
to softly repel, followed by a stronger
repulsion as they get closer, as though there was
a spring being compressed between the atoms. So,
since any collision can be thought of as a
compression of an atomic spring between atoms, no
collision occurs instantaneously, but instead over
some (very small) time period.
The bullet does not stop the instant it hits
the wall. If you look at the wall after the
bullet has hit it, you will notice that there is a
bullet hole in the wall. This is because the
bullet continued some distance into the wall (the
depth of the hole) before stopping. While the
bullet was forcing its way into the wall, it was
being subjected to tremendous force that was
slowing the bullet to a stop. In the extreme
case, the wall actually stretches slightly as does
the bullet, resulting in the bullet ricocheting
like a bouncing object (which is exactly what it
Click Here to return to the search form.
Copyright © 2020 The Regents of the University of California,
All Rights Reserved.