Physics doesn't care what makes sense to you.
:-)
Kinetic energy is one half the mass times velocity squared.
If the rifle doesn't move, it has no kinetic energy.
If it only moves slowly, then it has little kinetic energy compared to the
high velocity bullet. The ratio of the half masses is low compared to the ratio
of the squared velocities.
By your intuition, a ball that hits the ground and bounces back up would be
giving half it's energy to the sidewalk.
In your second intuition failure, you are confusing inelastic collisions with
elastic collisions. The energy of a bullet hitting something like an armoured
box or a car will be mostly spent damaging the box and the bullet, generating
heat. That is an inelastic collision. The same amount of energy applied in a
slower collision, say one BSC colliding with another, will cause the second one
to move much like a cue ball causes the eight ball to move.
Before replying with more intuition, consider reading about the subject:
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Get a free science project every week! "http://scitoys.com/newsletter.html"On Mon, Mar 5, 2012 at 9:45 AM, Phil <philgoetz@gmail.com> wrote:
> Well, no.
You seem to be saying that the fraction of energy going into the
> In the case of a 2.25 kg pound rifle firing a 9.7 gram bullet, less than
> half a
> percent of the energy goes into the rifle, and more than 99.5% goes into the
> bullet.
bullet is equal to the fraction of total mass made up by the rifle.
This makes no sense to me.
An explosion between the bullet and the rifle should transfer about as
much energy into the bullet as into the rifle.
A person in body armor can be shot with a shotgun at close range and
not knocked down; and the kick from firing a shotgun is enough to
knock someone down who isn't braced for it; so the magnitude of these
forces must be similar.
No, a second is a very long time for these things. I would be
>Let's say it came to rest in a single second, so the speed is 0.1 meters
per second.
surprised if the speed was less than 10 meters per second. If you
knew the coefficient of friction you could compute the minimum force
needed to make the thing start moving and the speed at which it would
move.
Put it this way: If you fired your 420j .38 into an armoured BSC, I'd
be very very surprised if it moved at all. So it does not make sense
to say it moved 10cm after absorbing 3 joules.
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