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So intrinsically we can imagine the bullet (a cluster of particles) moving through a medium (a sea of particles), and as the cluster moves, it bumps into lots of particles within the sea and imparts some kinetic energy to each of the sea's particles as the cluster moves through the sea. As the kinetic energy is moved outside of the cluster, the cluster slows down.

My question is: Could we imagine the air immediately in front of the bullet as having a higher pressure than the rest of the air, thus slowing the bullet? Or could we also imagine the air behind the bullet as being more vacuum-like than the rest of the air? This pressure difference between the front and back of the bullet would explain why the bullet slows down. Or is the only actual answer that of, the air particles gliding over the bullet are "scraping" away some of the kinetic energy?

I guess my question may be getting a little theoretical, but I've been obsessing over this for a bit now and I just can't talk myself into one reasoning or the other as to why the bullet slows down.

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    $\begingroup$ Is there a reason you think it has to be one or the other? Because certainly both happen at the same time, and compressing air molecules causes there to be more of them closer together to steal KE from the bullet through direct collision. $\endgroup$
    – Triatticus
    Commented Dec 30, 2020 at 15:53
  • $\begingroup$ @Triatticus I guess I have pondered that it could be a combination of the two effects. I could imagine a higher pressure gradient being nothing more than more particles in place for the direct collision to occur more often. If it is a combination of the two, I guess that is the answer that i am after $\endgroup$
    – Tyler M
    Commented Dec 30, 2020 at 15:55
  • $\begingroup$ Triatticus is correct, also there is lower air pressure behind a moving object so all of your assumptions are true. $\endgroup$
    – Adrian Howard
    Commented Dec 30, 2020 at 15:58
  • $\begingroup$ The flow situation you are describing is typical of hypersonic flows (Mach number >5). Most bullets are somewhat slower, where continuum mechanics is needed instead. $\endgroup$
    – D. Halsey
    Commented Dec 30, 2020 at 20:03
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    $\begingroup$ As has already been explained, both skin friction and pressure gradient have distinct slowing effects on a projectile. Here is a cool, quite tangible implication: why a golf ball has dimples but an airplane wing doesn't. $\endgroup$
    – jnez71
    Commented Dec 30, 2020 at 22:01

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All of the reasons you say are correct.

The bullet bumps into air molecules, speeding them up. This slows the bullet.

The air molecules are bumped forward, crowding into air already present. More molecules are now in the region than would normally be. The density goes up. Since that region has extra molecules bumping into a neighboring region, the force exerted on the neighbor is above normal. The pressure goes up.

As the bullet passes, it leaves a region where molecules have all been pushed away. Air rushes in because neighbors bump it in that direction. There are fewer molecules than normal. Density and pressure are low. Note that this means fewer molecules bump into the bullet from behind.

So there are two ways of saying this.

Lots of molecules bump into the bullet from the front, slowing it down. Few molecules bump into it from the front, which means not many bumps speed it up. This slows the bullet.

Or you can say air pressure is high in front of the bullet and low behind. These exert a greater force backward than forward. This slows the bullet.

This mechanism is dominant in almost all normal fluid flow. Friction also plays a role, but it dominates only in small, slow motion or when the fluid is extra viscous. The Reynolds number helps figure out which mechanism is dominant. It is the ratio of two kinds of forces: the force created by pushing air around and forces of friction.

A rule of thumb: in air at usual speeds, the Reynolds number is large unless the system is insect size or smaller. A bullet is heavier than a typical insect and travels much faster.

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    $\begingroup$ No mention of Mach numbers? $\endgroup$
    – D. Halsey
    Commented Dec 30, 2020 at 20:04
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    $\begingroup$ Another important mechanism forgotten here is flow separation, which causes a dramatic increase in adverse pressure gradient drag, and occurs at lower Reynolds numbers. This counters your notion that lower Reynolds numbers always mean skin friction dominates drag. See: why a golf ball has dimples but an airplane wing doesn't. Airfoils at high speeds care more about skin friction drag than they do about adverse pressure gradient. $\endgroup$
    – jnez71
    Commented Dec 30, 2020 at 22:25

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