Can a bullet knock someone back as hard as a semi truck?

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    Bullet Impact Law
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Discussion Overview

The discussion revolves around the physics of bullets and their potential to knock a person back upon impact, particularly in the context of a hydroshock bullet. Participants explore concepts related to Newton's third law, energy transfer, and the misconceptions often portrayed in media regarding the effects of gunfire on targets.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the validity of the claim that a bullet could impart a force equivalent to being hit by a semi truck, suggesting that this idea is exaggerated.
  • Another participant asserts that the notion of a target flying backwards is a common error in movie physics, referencing external material for further context.
  • A participant shares a personal anecdote about a demonstration involving rifle recoil, noting that while bullets like hydroshock may have significant "knockdown power," this does not equate to knocking someone back.
  • Some participants mention that novices can be knocked over when firing a shotgun, attributing this to their posture and reaction to the loud noise rather than the force of the shot itself.
  • One participant compares the energy of a shotgun's kick to that of a major league fastball, suggesting a difference in energy transfer that may influence perceptions of being knocked back.

Areas of Agreement / Disagreement

Participants generally agree that the idea of being knocked back by a bullet is a misconception, but there are differing views on the factors that contribute to this perception, such as posture and the effects of sound. The discussion remains unresolved regarding the specifics of energy transfer and the physics involved.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about energy, momentum, and force, as well as the definitions of "knockdown" versus "knock back." Participants do not fully explore the mathematical aspects of these concepts.

jeremyearle5
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A friends father told him that a hydroshock bullet (a type of hollowpoint bullet that spreads out on impact and ussually doesn't exit the target) shot into a mans palm from ten feet away would impart a force on the rest of his body equal to being hit by a semi truck. I know this is ridiculous, but it got us to talking about Newton's 3rd law and whether or not a bullet could ever knock someone back. In order for a bullet to knock someone back wouldn't it also have to knock the shooter back an equal amount? I guess I'm just asking for someone to give me a good physics overview of this whole scenario, please.
 
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jeremyearle5 said:
A friends father told him that a hydroshock bullet (a type of hollowpoint bullet that spreads out on impact and ussually doesn't exit the target) shot into a mans palm from ten feet away would impart a force on the rest of his body equal to being hit by a semi truck. I know this is ridiculous, but it got us to talking about Newton's 3rd law and whether or not a bullet could ever knock someone back. In order for a bullet to knock someone back wouldn't it also have to knock the shooter back an equal amount? I guess I'm just asking for someone to give me a good physics overview of this whole scenario, please.

A common demostration in boot camp long, long ago (back about the time water was invented) was for a drill sergeant to hold an M1 rifle with the butt resting on his nose (sometimes other parts of the anatomy) and fire it. The secret, of course, was the recoil spring. The reverse of the same principle gives bullets like the hydroshock a tremendous knockdown power because the bullet decelerates so quickly. But, knockdown is not the same as knock back.
 
mgb_phys said:
You are exactly correct - the target flying backwards is a typical movie-physics error.
See http://www.intuitor.com/moviephysics/mpmain.html#glass for this and other movie physics fun.

I have heard of many novices knocked over by firing a shotgun. Your article was quite edifying as to the differences of energy, momentum and force involved.
 
That's because they stand leaning backwards like they were holding a camera and fall over when they hear the load bang!
 
mgb_phys said:
That's because they stand leaning backwards like they were holding a camera and fall over when they hear the load bang!

I have no idea why i found that funny irl ^^
 
Loren Booda said:
I have heard of many novices knocked over by firing a shotgun. Your article was quite edifying as to the differences of energy, momentum and force involved.

I've seen people knocked over, but I think they were flinching from the expected "BANG". A typical shotgun has a "kick" of 20-30 ft-lb. By way of comparison, a major league fastball has an energy of about 150 ft-lb.
 

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