Why Does a Glass Move Outward When Opening the Fridge Door?

[berkeman]

Thread closed temporarily for Moderation and cleanup...

 

[Mark44]

I agree that opposing and equal forces do not always cancel. Squeezing something in a vice proves that.

No it doesn't. The force that the vise (not vice, that's completely different) exerts on the object is equal to the force that the object exerts on the vise, but in the opposite direction.

Please be selective and non-duplicative with the quoting.

Fixed.
Thread is still closed for moderation and cleanup.

 

[berkeman]

Okay, after the cleanup and a Mentor discussion, the thread is reopened provisionally.

 

[A.T.]

Yes, there are two bodies, but each is captive of those perplexing third law pairs!

Only one force of a 3rd law pair acts on a body. The other force of that 3rd law pair always acts on another body.

Does it seem like either one should be moving?

How they are moving is a matter of reference frame and of what other forces act on them (2nd law). For the 3rd law motion is irrelevant.

 

[A.T.]

Just stressing the better (proper?) model louder and louder really doesn't help.

But stressing wrong ideas does help? I don't see how you are helping anybody, by being/acting just as confused as the person you are trying to help, in the name of "picking them up where they are".

Stating the correct model is the basis. Making it more intuitive is a matter of going through many examples, and applying it, rather than "explaining it in the right way".

 

[sophiecentaur]

But stressing wrong ideas does help?

Is that what I wrote? I am suggesting that acknowledging that the 'wrong' ideas exist and pointing out why will give an opportunity to reconcile the right and the wrong. When the people who 'know' cannot see how the people are thinking who "don't know" then they are not likely to point them in the best direction for revised thinking. You may be forgetting that questions like the ones in this thread are very common and they must be arising for a reason - not just ignorance.

Putting oneself in the shoes of another person can often allow to help them effectively. People who don't get that could maybe limit their (accurate and well informed) responses to the better informed threads in which they can contribute on a more level playing field; a win-win situation. I suspect that idea may be in quadrature with people who are arguing against me here.

 

[A.T.]

Putting oneself in the shoes of another person can often allow to help them effectively.

The problem with your posts is that it's not clear if you are putting yourself into the shoes of a confused person, or if you are confused yourself. Maybe you can make it more clear when you are merely channeling some common misconception.

 

[sophiecentaur]

The problem with your posts is that it's not clear if you are putting yourself into the shoes of a confused person, or if you are confused yourself.

Oh! Could you quote what I wrote that could give that impression? Maybe I was too subtle?

 

[rcgldr]

Only one force of a 3rd law pair acts on a body. The other force of that 3rd law pair always acts on another body.

It may help with a simple example, like the spinning space station example posted earlier. The "floor" of the space station exerts a centripetal force on the astronaut, and the reaction to that centripetal acceleration results in a reactive centrifugal force exerted onto the "floor" of the space station.

As explained earlier, this is a different scenario than the glass sliding to the outside of an opening door, or a bead sliding outwards on a rotating rod, which is due to the fact that once the velocity is non-zero, the direction of force is inwards of the direction of velocity, resulting in a curved path, but not curved enough to prevent outwards movement relative to the axis of rotation of the door | bead. For the bead sliding on a rod, there could be an initial linear impulse, and then the angular velocity of the rod adjusted so that it no longer exerts any force on the bead, in which case the bead moves in a linear path and outwards on the rod. The rod would asymptotically approach the direction of the initial linear impulse.

 

[ALBAR]

Only one force of a 3rd law pair acts on a body. The other force of that 3rd law pair always acts on another body.

How they are moving is a matter of reference frame and of what other forces act on them (2nd law). For the 3rd law motion is irrelevant.

Do separate bodies such as earth and moon exert reaction forces on each other? I believe that the reactions of two isolated bodies to gravitational attraction, either pulling them together or holding them in orbit, stay local at the masses that generate them rather than act or react on other mass. Reaction forces can't be exerted over empty distance as gravity can. Reaction forces pair up in opposition, reacting to each other when objects make contact as in a collision or are physically joined, as by a rope, or are parts of a single object. Example: A twirled baton is thrown up in the air. The centrifugal force (or whatever the modern term) of each of its halves is linked to the other half and serves as its centripetal force. These two force mutually support each other because they act on the same object, the baton.

 

[jbriggs444]

Do separate bodies such as earth and moon exert reaction forces on each other?

Yes. In the Newtonian model, gravity is a force-at-a-distance. The gravitational force of moon on earth is the third law reaction force that goes with the gravitational force of earth on moon.

In the model of General Relativity, gravity is not a force at all and there is no non-locality problem. But let us not go there.

Reaction forces pair up in opposition, reacting to each other when objects make contact as in a collision or are physically joined, as by a rope, or are parts of a single object. Example: A twirled baton is thrown up in the air. The centrifugal force (or whatever the modern term) of each of its halves is linked to the other half and serves as its centripetal force.

No.

 

[Orodruin]

Do separate bodies such as earth and moon exert reaction forces on each other?

Yes. The gravitational force from the Earth on the Moon is the third law pair of that from the Moon on the Earth.

I believe that the reactions of two isolated bodies to gravitational attraction, either pulling them together or holding them in orbit, stay local at the masses that generate them rather than act or react on other mass. Reaction forces can't be exerted over empty distance as gravity can.

This is nonsense. (My emphasis)
What you are describing here are contact forces - not action/reaction pairs in general.

Reaction forces pair up in opposition, reacting to each other when objects make contact as in a collision or are physically joined, as by a rope, or are parts of a single object. Example: A twirled baton is thrown up in the air. The centrifugal force (or whatever the modern term) of each of its halves is linked to the other half and serves as its centripetal force. These two force mutually support each other because they act on the same object, the baton.

Wrong. They do not act on the same object in the classicsl mechanics sense of the word. One acts on one half of the baton and the other on the other half. You chose to split the baton as far as making FBDs go and then the forces act on different parts.

The baton as a whole has internal forces and strains, but those are irrelevant as far as a FBD for the entire baton would be concerned.

 

[ALBAR]

Yes. In the Newtonian model, gravity is a force-at-a-distance. The gravitational force of moon on earth is the third law reaction force that goes with the gravitational force of earth on moon.

In the model of General Relativity, gravity is not a force at all and there is no non-locality problem. But let us not go there.

No.

I don't believe that force of gravity should be designated as reactive. The first law clarifies the roles of forces, but is hardly needed to explain what is instinctive knowledge. Release a mass in a gravitational field and everybody knows what happens and why. Could any case of cause and effect--action and reaction--be more obvious? But set this issue aside. If the gravitational forces are reacting to something, then what are the other two forces that the third law requires doing?

 

[Ibix]

If the gravitational forces are reacting to something, then what are the other two forces that the third law requires doing?

The gravitational force of A on B and the gravitational force of B on A are a third law pair, which is why the joint center of mass of the two bodies doesn't move. You don't need two more forces.

 

[Orodruin]

I don't believe that force of gravity should be designated as reactive. The first law clarifies the roles of forces, but is hardly needed to explain what is instinctive knowledge. Release a mass in a gravitational field and everybody knows what happens and why. Could any case of cause and effect--action and reaction--be more obvious? But set this issue aside. If the gravitational forces are reacting to something, then what are the other two forces that the third law requires doing?

Again, you are confusing action/reaction with contact forces.

 

[jbriggs444]

Could any case of cause and effect--action and reaction

Action/reaction forces are NOT cause and effect. The third law is a statement that interaction forces come in pairs. Neither force is designated as cause. Neither force is designated as effect. It is correlation, not causation.

"the mutual actions of two bodies upon each other are always equal, and directed to contrary parts"

 

[A.T.]

I don't believe that force of gravity should be designated as reactive.

Gravity is a good example why the "action/reaction" terminology is bad and confusing.

All that Newton's 3rd says is that there are equal but opposite forces, and that also applies to Newtonian gravity as well: Each mass exerts a force on the other mass, and those two forces are equal but opposite.

 

[ALBAR]

The gravitational force of A on B and the gravitational force of B on A are a third law pair, which is why the joint center of mass of the two bodies doesn't move. You don't need two more forces.

You mean only one force exists at each body? Can't be! No force can be exerted without resistance. Try pushing on nothing.

 

[A.T.]

You mean only one force exists at each body?

In the inertial frame of reference, yes.

Can't be! No force can be exerted without resistance. Try pushing on nothing.

That's called inertia, and is represented by the mass of a body, not by some resistive force.

 

[PeroK]

You mean only one force exists at each body? Can't be! No force can be exerted without resistance. Try pushing on nothing.

You mean motion is impossible? Can't be. A force can result in acceleration! Try kicking a football.

 

[Ibix]

No force can be exerted without resistance.

Do you think the other planet is massless and has no inertia?

 

[ALBAR]

You mean motion is impossible? Can't be. A force can result in acceleration! Try kicking a football.

This is exactly why we need to differentiate cause from effect. The football must push back 100% at every instant force is applied, but it undergoes acceleration as an effect of the force being applied. The acceleration triggers the resistive force per the second law resulting in a symmetric pair of opposing forces.

 

[jbriggs444]

The acceleration triggers the resistive force per the second law

Correlation, not causation. Whether the ball strikes your foot or your foot strikes the ball, a strain arises, a stress arises and a pair of forces arises.

Normally we consider that the relative motion of ball and foot is the "cause" and the ensuing contact deformation, force pair and acceleration are "effects".

However, the second law is neutral on the matter. It only assures us that when a net force exists, so will an acceleration. And conversely, if there is an acceleration, there must be a net force.

Same for the third law. It only assures us that a force from A on B in one direction will be accompanied by an equal force of B on A in the opposite direction. It is silent about the question of causation.

 

[Ibix]

The acceleration triggers the resistive force per the second law resulting in a symmetric pair of opposing forces.

But your foot accelerates, so that must be the effect and the ball the cause!

 

[sophiecentaur]

If the gravitational forces are reacting to something, then what are the other two forces that the third law requires doing?

Whaaaat? You now have three forces?? Going completely classical, there's a force acting on the CM of Earth - just like a piece of string and a ball whirling round . The string pulls against the ball and the ball pulls against the string. Also the string pulls on your hand as you pull it. That's N3 happening at each end of the string. Connect the ball directly to your hand (throw away the string) and the direct force is the same as the tension in the string. The gravitational attraction between Earth and Moon is the equivalent to the tension in the string.

 

[ALBAR]

But your foot accelerates, so that must be the effect and the ball the cause!

We are getting into collision physics now. Actually the foot decelerates due to the counter force it needs to exert any force at all on the football.

 

[jbriggs444]

We are getting into collision physics now. Actually the foot decelerates due to the counter force it needs to exert any force at all on the football.

The decelleration of the foot (times the mass of the foot) matches the contact force of ball on foot. That's Newton's second law.

No need to invoke magical counter forces. Or anthropomorphic "needs".

 

[A.T.]

Actually the foot decelerates ....

Whether the speed to the foot increases or decreases is just a matter of the chosen reference frame. If your "causation logic" depends on that arbitrary choice, then it is equally arbitrary, and not physically relevant.

 

[Ibix]

Actually the foot decelerates due to the counter force it needs to exert any force at all on the football.

The foot decelerates because the ball exerts a force on it. That's Newton's second law.

 

[ALBAR]

Whether the speed to the foot increases or decreases is just a matter of the chosen reference frame. If your "causation logic" depends on that arbitrary choice, then it is equally arbitrary, and not physically relevant.

Please, PLEASE let's stay in the good old Newtonian inertial reference frame when discussing the most basic principles of mechanics.