Does "internal force" have 2 meanings in physics?

[pkc111]

Homework Statement

Its not really a homework question but a question at a high school homework level.

Relevant Equations

W=mg

My understanding is that gravitational fields produce an "internal force" (weight) on objects and hence gravity cannot change the total mechanical energy of an object.
But in a free falling situation there is no other internal force cancelling it out, so it does not seem to be an internal force according to this definition at https://www.embibe.com/study/work-done-by-internal-force-concept?entity_code=KTWP26

 

[Office_Shredder]

Gravity is not generally considered an internal force, unless you are considering the two bodies as one system. Why don't you post what you think the definition is?

 

[pkc111]

I read it here

https://www.physicsclassroom.com/cl...only type of,the object's energy changes form.

 

[pkc111]

and on same site

 

[Orodruin]

I read it here

View attachment 283206

https://www.physicsclassroom.com/class/energy/Lesson-2/Internal-vs-External-Forces#:~:text=When the only type of,the object's energy changes form.

Stop reading that site immediately.

 

[Delta2]

Its not completely wrong (usually internal forces are conservative and external forces non conservative) but a more accurate description would be to say conservative force instead of internal force.

 

[pkc111]

So what is an "internal force" in physics? Is it any force that is on the inside of a boundary that you define as a system? Is it always on the inside of an object? Does it alway an opposite that cancels it out if it is inside an object as the first reference states? Its term that seems to pop up a lot.

 

[pkc111]

OK so does this one sound better? It defines an external force as being on a body if there is an external body causing it...and vice versa...sound legit? Nothing about being inside or outside a system and nothing about being always associated with conservative or nonconservative forces. (From Lecture 5, Purdue University Physics).
But my problem is that makes gravity (weight) an external force and that makes statements about the association between it being conservative and internal wrong.

 

[Delta2]

So what is an "internal force" in physics? Is it any force that is on the inside of a boundary that you define as a system? Is it always on the inside of an object? Does it alway an opposite that cancels it out if it is inside an object as the first reference states? Its term that seems to pop up a lot.

Internal force is about as you say. More precisely if we have a system of bodies (for example two bodies connected by a spring) then any force between any two bodies of the system is an internal force. From Newton's 3rd law, if body 1 exerts a force $F_{12}$ to body 2, then body 2 exerts an opposite and equal force $F_{21}$ to body 1 and it is $F_{12}=-F_{21}$. So when we take the sum of internal forces inside a system it always equal zero.

 

[Delta2]

Deep down what is internal force and what is external it depends on how you define your system of bodies. In post #8 we take as system all the different parts of an object (could be all the protons and all the neutrons of the object for example).

 

[Office_Shredder]

I think maybe the point from that website is that only conservative forces are capable of being internal forces. Gravity is an internal force in the system that consists of the two bodies - the two bodies when considered as a single thing are not affected by the gravity between them.Friction doesn't work like this. When heat is released into the environment by friction, the two objects that are moving against each other are incapable of treating that force between them as internal, because they are losing energy to the environment.

 

[Orodruin]

I think maybe the point from that website is that only conservative forces are capable of being internal forces. Gravity is an internal force in the system that consists of the two bodies - the two bodies when considered as a single thing are not affected by the gravity between them.

This is not completely accurate. An internal force is a force between the components of an internal system. It is perfectly possible for a non-conservative force to be internal. However, such a force will redistribute internal energy from kinetic to heating the components. Regardless of the internal forces, the system’s momentum and energy due CoM motion will remain the same because no momentum or mass is transferred to/from the system.

 

[Frigus]

Friction doesn't work like this. When heat is released into the environment by friction, the two objects that are moving against each other are incapable of treating that force between them as internal, because they are losing energy to the environment.

I think we can take two objects as a system For e.g:-while solving inelastic Collision problems we use conservation of momentum equation which works For a System.

 

[Orodruin]

I think we can take two objects as a system For e.g:-while solving inelastic Collision problems we use conservation of momentum equation which works For a System.

Yes, this is a good example. Take the example of two equal masses m that undergo an inellastic collision where one of the masses has initial velocity v and the other is at rest. The momentum before and after the collision is mv and the CoM velocity before and after is v/2, meaning that the kinetic energy due to CoM motion is (2m)(v/2)^2/2 = mv^2/4 both before and after the collision. There remains another mv^2/4 kinetic energy before the collision due to the movement of the constituents relative to the CoM (2*m(v/2)^2/2) that is converted to other forms of internal energy by heating the constituents.

 

[Herman Trivilino]

Homework Statement:: Its not really a homework question but a question at a high school homework level.
Relevant Equations:: W=mg

But in a free falling situation there is no other internal force cancelling it out, so it does not seem to be an internal force

The equal-but-opposite internal forces discussed in your example (the cable and pulley) never cancel each other out. The phrase "cancel each other out" in this context refers to a pair of equal but opposite forces that act on the same object. These forces act on different objects. One force acts on the pulley, the other force acts on the cable. In the case of an object in free fall, one force acts on the object, the other force acts on the Earth. (Note that these forces are equal and opposite in accordance with Newton's Third Law.)

Your efforts at distinguishing internal forces from external forces has led you down a rabbit hole. It's not the forces themselves that make the distinction, it's the circumstances. For example, for a freely falling object the force of gravity acting upon it is an external force. But if you include planet Earth in your system then that very same force is an internal force. It's all about what you decide to include in your system.

 

[Steve4Physics]

Start waffle.

May I add an additional way of thinking about this...

A system can be a single object or collection of objects.

With respect to a particular system, an external force is one capable of changing the system’s momentum, i.e. accelerating* the system’s centre of mass. An external force can be:
- conservative (e.g. if the system is a falling ball, gravity) or
- non-conservative (e.g. for the falling ball, air resistance).

With respect to a particular system, an internal force is one not capable of changing the system’s momentum, Note, an internal force can be:
- conservative (e.g. if the system is an ideal spring, the interatomic force between any two atoms) or
- non-conservative (e.g. if the system is a lump of rubber (deformation causes heating) the intermolecular forces).

Not the best examples maybe, but I hope you get the idea.

And, while I’m waffling, I’m reminded of my old physics teacher pointing out that when a stationary bomb explodes, the vector-sum of the fragments’ momenta is exactly zero. That’s quite a nice illustration of the fact that internal forces do not change a system’s momentum.

Of course, changing the definition of what constitutes a ‘system’, may change a force from being internal to being external, or vice versa.

*I use ‘accelerating’ in a general sense, i.e. changing velocity.

End waffle.

 

[pkc111]

Thanks everybody that helps a lot!