Understanding the Concept of Recoil in Relation to Momentum and Force

[MrDickinson]

TL;DR Summary

I would like to know how to understand "recoil", especially in terms of forces, momentum, and energy.

I am having some difficulty understanding what "recoil" really is with respect to momentum, force, and and time.

On the one hand, momentum is considered to be the product of mass and velocity or perhaps the product of the sum of masses and some velocity, or some variant of P=mv, where P is a vector indicating momentum, m is a scalar, and v is a vector indicating velocity.

I am unclear as to momentum defined through integration, but it is my understanding that the change in the momentum of a system can be expressed as the integral of F=M(dv/dt) with respect to time, or that momentum can be expressed as the product of the net force acting on the system over the period of time that the net force acts upon said system...

But what is "recoil". Recoil has properties of force, momentum, displacement, and the like.For example, in an idealized situation, perhaps there is a cannon, initially at rest, that fires a cannonball completely parallel to the ground that the cannon sits upon, and there is no friction. Plus, the ground is completely flat.

The system has properties of force (mass and acceleration), momentum (velocity & mass), displacement, and perhaps more.

But what is recoil? Even the recoil appears to have properties of force, momentum, displacement, and the like (perhaps I am wrong).

But what exactly is recoil?

Thanks

 

[russ_watters]

But what exactly is recoil?

I don't think it is explicitly defined, and can either refer to motion or momentum.

 

[phinds]

But what exactly is recoil?

Do you understand Newton's Third Law?

 

[russ_watters]

Do you understand Newton's Third Law?

I don't think you are understanding the question...

The OP appears to understand multiple different ways to describe "recoil" and is asking which one is the actual definition.

 

[phinds]

I don't think you are understanding the question...

The OP appears to understand multiple different ways to describe "recoil" and is asking which one is the actual definition.

Ah. Well, it seems he is over-complicating the issue. Wouldn't Newton say that it's just the opposite of what sends the cannonball out of the barrel of the cannon according to the 3rd law?

 

[russ_watters]

Ah. Well, it seems he is over-complicating the issue. Wouldn't Newton say that it's just the opposite of what sends the cannonball out of the barrel of the cannon according to the 3rd law?

I don't know - did Newton ever use the word "recoil"? Did he ever fire a gun/cannon?

 

[MrDickinson]

Do you understand Newton's Third Law?

I have a basic understanding of Newton's Third Law. This definition from KhanAcademy seems sufficient.

"If an object A exerts a force on object B, then object B must exert a force of equal magnitude and opposite direction back on object A."

An object of some mass that exerts force an some other object will experience equal and opposite force exerted by the "other object." However, I do not have a deep understanding of this Third Law.

 

[Ibix]

Isn't recoil basically "how hard a gun kicks when you fire it"? Which is likely to be subjective, and going to depend on all sorts of things - notably momentum and energy of the gun, but also things like how much time it takes to transfer the energy to you and the contact area between you and the gun.

Basically, I suspect that if you define it clearly then the answer in physics terms will be obvious. But whatever definition you pick, 90% of gun users will tell you that's not what they mean by recoil.

 

[MrDickinson]

Isn't recoil basically "how hard a gun kicks when you fire it"? Which is likely to be subjective, and going to depend on all sorts of things - notably momentum and energy of the gun, but also things like how much time it takes to transfer the energy to you and the contact area between you and the gun.

Basically, I suspect that if you define it clearly then the answer in physics terms will be obvious. But whatever definition you pick, 90% of gun users will tell you that's not what they mean by recoil.

Thank you for your response.

I am considering "recoil" more generally. I used the cannon example because it is one that I happened upon and it was fresh in my memory. But if "recoil" appears to have properties of force, momentum, displacement, and the the like, then can recoil be defined and understood more generally, but also deeply.

What does it mean for some particle to experience "recoil."

 

[Ibix]

What does it mean for some particle to experience "recoil".

Just that it's gone backwards because something hit it (or otherwise applied a force to it) or because it emitted something, as far as I'm aware. It's a consequence of the conservation of momentum but I don't think it's got a precise technical meaning.

 

[MrDickinson]

Just that it's gone backwards because something hit it (or otherwise applied a force to it) or because it emitted something, as far as I'm aware. It's a consequence of the conservation of momentum but I don't think it's got a precise technical meaning.

Thank you for your response

Is it possible to have recoil without conservation of momentum? It appears possible to me upon my first consideration, but I could not necessary precisely prove it.

Perhaps, what I mean to say, is this: Is a ΔF or a ΔP or the Law of Conservation of Momentum an essential property of "recoil"

As a side note, I am interested in this question because I try to gain insights into each physics problem that I encounter, and I have encountered a lot of momentum problems dealing with "recoil." I wish to understand the deeper insights that problems offer as well as how to quickly solve problems. Perhaps I am wrong, but "recoil" appears to be more than a trivial word. It appears to describe some set of properties of deeper properties or some consequence of various properties, as you described. But if it is a consequence of properties, can it be described deeply, yet generally. For example, if we have two properties--mass and acceleration--then the consequence of the product of those two properties yields a force.

I have thought to describe "recoil" as a sum of properties, but that won't work for obvious reasons (the properties contain different units in the first place).

Thanks again

 

[phinds]

Is it possible to have recoil without conservation of momentum? It appears possible to me upon my first consideration, but I could not necessary precisely prove it.

Be hard to prove that momentum is not conserved, since it IS conserved.

https://www.physicsclassroom.com/class/momentum/u4l2b.cfm

 

[jrmichler]

Recoil, as I know it, is a term used by shooters that applies to guns. Guns are a good example of Newton's Third Law, with the condition that the force is applied for a very short time. The force is applied for one or two milliseconds, while the shooter's reaction time is over 100 milliseconds. As far as the shooter is concerned, the force is effectively an infinitely short impulse, so the gun's acceleration is effectively infinite. The gun is stationary, then the gun is moving with a velocity. The shooter stops the gun's motion by holding on to it.

On a microsecond time scale, the gun accelerates to a velocity, maintains that velocity, then is slowly decelerated by the shooter. On a 100 msec time scale, the gun instantaneously acquires a velocity (with a loud noise), then decelerates while pushing the shooter back.

I think of recoil as a shooting specific term, used by people who know that their shoulder got hammered, and don't really care about the physics. But yes, momentum is conserved.

 

[MrDickinson]

Be hard to prove that momentum is not conserved, since it IS conserved.

https://www.physicsclassroom.com/class/momentum/u4l2b.cfm

https://www.physicsclassroom.com/class/momentum/u4l2b.cfm

Thank you for your response.

To some extent, I agree with you, but to some extent I disagree. But perhaps I am wrong, I cannot say for now because I do not entirely understand the topic.

Precisely speaking, the momentum of a system is conserved so long as there are no external forces acting upon the system. But it is entirely possible to have a situation where momentum is not conserved. Suppose we have Box A and Box B, the two are placed next to each other, with Box A being first from left to right. We define our system to be (Box A and Box B). Now, suppose a bat hits Box A perpendicular to the left, vertical surface of box A, then another Bat hits the first bat, then a projectile hits the other two bats. In such a situation, Momentum is not conserved; however, we can make our system sufficiently large so that momentum is conserved (our system might be the entire universe).

But the question, then, is this: Can recoil exist in the situation where our system is not sufficiently large--the system consists of only Box A and Box B, and in what form would "recoil" be present and what objects would experience "recoil"

Although, this seems difficult without a precise definition of "recoil."

Thank you

 

[phinds]

To some extent, I agree with you, but to some extent I disagree.

You are not agreeing or disagreeing with me but with Newton's 3rd Law. Disagreeing is not really an option.

Precisely speaking, the momentum of a system is conserved so long as there are no external forces acting upon the system.

Yes

But it is entirely possible to have a situation where momentum is not conserved.

No, not if you have a closed system.

Suppose we have Box A and Box B, the two are placed next to each other, with Box A being first from left to right. We define our system to be (Box A and Box B). Now, suppose a bat hits Box A perpendicular to the left, vertical surface of box A, then another Bat hits the first bat, then a projectile hits the other two bats. In such a situation, Momentum is not conserved

In each of the strikes, momentum is conserved. I don't understand why you think momentum is not conserved.

 

[russ_watters]

Is it possible to have recoil without conservation of momentum? It appears possible to me upon my first consideration, but I could not necessary precisely prove it.

Perhaps, what I mean to say, is this: Is a ΔF or a ΔP or the Law of Conservation of Momentum an essential property of "recoil"

I would say conservation of momentum is the most important component, and everything else follows from it. As @Ibix said, if you are trying to minimize the effects of recoil, the typical way to do it is by increasing the distance and time over which the momentum is absorbed. That way a gun doesn't punch you in the shoulder and a cannon doesn't knock over/back or break its mount.

 

[anorlunda]

Perhaps I am wrong, but "recoil" appears to be more than a trivial word. It appears to describe some set of properties of deeper properties or some consequence of various properties, as you described.

I think that might be the source of your confusion. Fundamental words to use in physics would include force, mass, position, velocity, acceleration, energy, momentum. Recoil is not in that list. The same applies to the words push and pull. They have simple meanings in natural language, but we don't use them in physics. Force (plus or minus) is a more general and useful word.

Physicists are interested in things that are useful, not true/false right/wrong better/worse. The general word force is more useful than push/pull/recoil/thrust specialized terms for force.

If a cannon was very securely tied down, then the recoil force would not make the cannon move, and without motion, it has no momentum. (But be careful, the Earth-cannon system does have momentum. You need to be extremely careful where to draw the boundaries of "the system.")

 

[hutchphd]

Perhaps I am wrong, but "recoil" appears to be more than a trivial word. It appears to describe some set of properties of deeper properties or some consequence of various properties, as you described. But if it is a consequence of properties, can it be described deeply, yet generally

Consider the the case of "free recoil" where the bullet and gun are free to move:

1. The gun converts nearly all of the chemical energy of the powder into kinetic energy of the bullet and gun
2. At separation the gun and bullet have exactly equal and opposite momenta. This means their speeds differ exactly inversely according to their masses: the bullet is faster by the ratio m_gun/m_bullet
3. The kinetic energy of the bullet is therefore larger by the ratio m_gun/m_bullet
4. Whether the gun recoil causes injury depends upon exactly how you pass the momentum to your shoulder (which is the same magnitude as that of the bullet from (2)
5. A heavier gun will deliver the same momentum but gets less kinetic energy and so delivers less "kick" to your shoulder

That is all the basics I believe. (I have assumed the momentum of the gas from the powder to be negligible)
Incidentally this is rocket science.

 

[jrmichler]

I have assumed the momentum of the gas from the powder to be negligible

I recall reading something in Hatcher's Notebook about what he called the "rocket effect". The mass of the gunpowder adds significantly to the recoil. My hazy memory thinks up to 30% or so. That's the reason for muzzle brakes, even though they add to the muzzle blast noise. I see the book, originally published in 1948, is still in print with five star average reviews: https://www.amazon.com/dp/1614272832/?tag=pfamazon01-20.

 

[hutchphd]

Yeah it looks like the mass of the charge is typically ~half that of the bullet and I bet the average ejection velocity is likely ~ half the muzzle speed so probably your memory is accurate.