The discussion centers on the mechanics of a glass of water sliding outward when a refrigerator door is opened quickly. Participants clarify that the glass is subject to contact forces with the door and shelf, which dictate its motion. The concept of centrifugal force is debated, with consensus that it does not exist in an inertial frame; instead, the glass's outward motion results from tangential acceleration and the door's rotation. The glass's trajectory is influenced by the interplay of these forces as the door opens.
PREREQUISITESPhysics students, mechanical engineers, and anyone interested in understanding the dynamics of motion and forces in everyday scenarios.
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.ALBAR said:I agree that opposing and equal forces do not always cancel. Squeezing something in a vice proves that.
Fixed.jbriggs444 said:Please be selective and non-duplicative with the quoting.
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.ALBAR said:Yes, there are two bodies, but each is captive of those perplexing third law pairs!
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.ALBAR said:Does it seem like either one should be moving?
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".sophiecentaur said:Just stressing the better (proper?) model louder and louder really doesn't 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.A.T. said:But stressing wrong ideas does help?
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 said:Putting oneself in the shoes of another person can often allow to help them effectively.
Oh! Could you quote what I wrote that could give that impression? Maybe I was too subtle?A.T. said: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.
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.A.T. said: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.
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.A.T. said: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.
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.ALBAR said:Do separate bodies such as earth and moon exert reaction forces on each other?
No.ALBAR said: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.
Yes. The gravitational force from the Earth on the Moon is the third law pair of that from the Moon on the Earth.ALBAR said:Do separate bodies such as earth and moon exert reaction forces on each other?
This is nonsense. (My emphasis)ALBAR said: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.
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.ALBAR said: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.
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?jbriggs444 said: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.
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.ALBAR said: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.ALBAR said: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?
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.ALBAR said:Could any case of cause and effect--action and reaction
Gravity is a good example why the "action/reaction" terminology is bad and confusing.ALBAR said:I don't believe that force of gravity should be designated as reactive.
You mean only one force exists at each body? Can't be! No force can be exerted without resistance. Try pushing on nothing.Ibix said: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.
In the inertial frame of reference, yes.ALBAR said:You mean only one force exists at each body?
That's called inertia, and is represented by the mass of a body, not by some resistive force.ALBAR said: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.ALBAR said:You mean only one force exists at each body? Can't be! No force can be exerted without resistance. Try pushing on nothing.
Do you think the other planet is massless and has no inertia?ALBAR said:No force can be exerted without resistance.
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.PeroK said:You mean motion is impossible? Can't be. A force can result in acceleration! Try kicking a football.
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.ALBAR said:The acceleration triggers the resistive force per the second law
But your foot accelerates, so that must be the effect and the ball the cause!ALBAR said:The acceleration triggers the resistive force per the second law resulting in a symmetric pair of opposing forces.
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 said:If the gravitational forces are reacting to something, then what are the other two forces that the third law requires doing?
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.Ibix said:But your foot accelerates, so that must be the effect and the ball the cause!
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.ALBAR said: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.
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.ALBAR said:Actually the foot decelerates ....
The foot decelerates because the ball exerts a force on it. That's Newton's second law.ALBAR said:Actually the foot decelerates due to the counter force it needs to exert any force at all on the football.
Please, PLEASE let's stay in the good old Newtonian inertial reference frame when discussing the most basic principles of mechanics.A.T. said: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.