B Can anyone explain following scenarios in Force-Reaction

[Velocity2D]

Yeah, but what is force? If its not transmission of energy? Then how do objects get their "kinetic energy" from getting hit by something etc?

Also, I still don't understand where does falling object get more force to hit the air molecules? The object still has the same gravity force applied to it, it doesn't change at any point. It has to do something with the momentum, and the more momentum something has the force force it applies to the air molecules, but how?

Those wiki article are very little use to me, since I am not PhD of physics, those equations and whatnot arent easy to understand.

 

[sophiecentaur]

Also, I still don't understand where does falling object get more force to hit the air molecules?

Whilst you continue to use your own terms to describe the situation, I don't think you will ever 'understand'. Objects don't "get force"; they acquire or lose momentum and kinetic energy as a result of a force acting on them. We use a certain vocabulary because it works and when you only use your own terminology, things will fall apart for you.
Have you actually read through (properly) that wiki article? If it means nothing to you then you will need to go back further to basics. Try this link.
I would advise that you aim at a better understanding of simple one to one collisions before moving on to air resistance. That is a much harder subject.

 

[Velocity2D]

How can swinging balls conserve momentum? The balls will have less momentum every moment, since energy dissipates? If the momentum was the same in the balls, the balls would never stop?

 

[jbriggs444]

How can swinging balls conserve momentum? The balls will have less momentum every moment, since energy dissipates? If the momentum was the same in the balls, the balls would never stop?

First, momentum is a vector quantity. It has not only magnitude but also direction. The first question you should be asking is how swinging balls can conserve momentum when they swing back and forth, changing direction.

Momentum changes when a net force is applied. There are several forces being applied to swinging balls. Can you identify some?

 

[Velocity2D]

I don't really understand, since momentum is less each moment because if dissipating energy. The balls slow down and eventually stop since they lose momentum, otherwise they would stay forever at the same velocity.<

Also another question I am thinking is, that how can gravity be Newton force pair? It is not really the same thing, when Earth pulls moon its not really "opposite" force when moon pulls Earth in the same sense that bouncing objects collide from each other.

Also, how can moon and Earth pull each other equally, when at the same time moon has less gravity and is said to apply less force on let's say human on the surface of the moon? Moon gives acceleration of 1,635ms2, so it has to apply different force on Earth than Earth to moon.

Lets assume Earth mass is 81kg and Moon 1kg. Moon applies force of 81kg*1,1635ms2 to Earth which is 132N. Earth applies force of 9.81ms2*1 which is 9,81N.

132N != 9,81N

So Moon pulls Earth more than Earth pulls Moon.

 

[jbriggs444]

I don't really understand, since momentum is less each moment because if dissipating energy.

See post #32 and #34. You have to do your homework.

 

[sophiecentaur]

How can swinging balls conserve momentum? The balls will have less momentum every moment, since energy dissipates? If the momentum was the same in the balls, the balls would never stop?

Here you go again. You can't make up your own rules. You need to believe that, in these classical matters, in particular. The system works. Use the (complete) system and don't try to invent your own system which you seem to have based on some but not all of the basics. As jbriggs says "You have to do your homework" and stop arguing that the system doesn't agree with you. It's the other way round.

 

[Velocity2D]

Well, as far as I understand this classical mechanics, the balls itself do lose their momentum but it stays the same in the "system", but when looking the single ball it does lose momentum. Momentum = v*m, and the velocity of the balls will drop over time because nothing can move forever, so it loses momentum.

 

[jbriggs444]

Well, as far as I understand this classical mechanics, the balls itself do lose their momentum but it stays the same in the "system",

The "system" has boundaries where you define them to be. If you draw the boundaries so that the balls are in the system and the air and the frame and the strings and the Earth's gravity are outside the system then you do not have a "closed system" that is subject to zero net external force. Momentum is not guaranteed to be conserved if you do not have a closed system subject to zero net external force.

 

[Velocity2D]

The "system" has boundaries where you define them to be. If you draw the boundaries so that the balls are in the system and the air and the frame and the strings and the Earth's gravity are outside the system then you do not have a "closed system" that is subject to zero net external force. Momentum is not guaranteed to be conserved if you do not have a closed system subject to zero net external force.

I have trouble understanding, why talk about "systems"? Its not possible to include everything, because the system is bound to have boundaries and it loses momentum unless the whole universe is the "system", in which case momentum is constant like energy.

 

[jbriggs444]

I have trouble understanding, why talk about "systems"? Its not possible to include everything, because the system is bound to have boundaries and it loses momentum unless the whole universe is the "system", in which case momentum is constant like energy.

Please do your homework.

 

[Velocity2D]

Those "Newtons balls" is just theoretical model to illustrate certain equations etc. But in reality there are no "closed circuits", everything affects evertything. Whenever we define any systems, there appears to be leakage of momentum, energy etc.

In case of Newtons balls-device, the balls lose their momentum to surrounding environment in forms of heat dissipation from friction between balls and air molecules, elastic damage to the balls etc.

 

[A.T.]

In case of Newtons balls-device, the balls lose their momentum to surrounding environment in forms of heat dissipation from friction between balls and air molecules, elastic damage to the balls etc.

Kinetic energy is dissipated as heat, but not momentum. If momentum is transferred to the air, it stays momentum.

 

[Velocity2D]

I just don't understand, why heat isn't kinetic energy? Heat is movement of particles. If collision causes molecules to vibrate, isn't the kinetic energy conserved?

 

[A.T.]

I just don't understand, why heat isn't kinetic energy? Heat is movement of particles.

That's why you can convert macroscopic KE (bulk movement) into microscopic KE (heat), or some other energy form. But there is only one form of linear momentum, which is also conserved, yet cannot be converted into something non movement related.

 

[Velocity2D]

That's why you can convert macroscopic KE (bulk movement) into microscopic KE (heat), or some other energy form. But there is only one form of linear momentum, which is also conserved, yet cannot be converted into something non movement related.

But if momentum is conserved because the total mass and movement amounts to the momentum before and after, why isn't kinetic energy of moving particles in form of heat account for total kinetic energy the same way?

 

[jbriggs444]

But if momentum is conserved because the total mass and movement amounts to the momentum before and after, why isn't kinetic energy of moving particles in form of heat account for total kinetic energy the same way?

Can you define momentum for us?

 

[Velocity2D]

Can you define momentum for us?

Momentum = mv

I understand that kinetic energy is scalar instead of linear, but shouldn't it still amount the same if billions of molecules/atoms are vibrating at very high speed?

 

[sophiecentaur]

There is an essential and very practical difference between the KE of coherent motion and the KE of thermal motion. If any of the initial mechanical KE of the system gets transferred to thermal energy (possibly KE but not necessarily all KE), you can't get it back without the use of a heat engine. It is usual to treat mechanical situations like collisions by just sticking with the mechanical energy and the momentum and then regarding any deficit in KE as 'loss'.
The effect of drag in the air is most conveniently dealt with by introducing a drag force into the equations.
How would you justify a different approach? You seem to be suggesting that your idea of adding complexity is absolutely necessary? It would not be practical in most situations.

 

[Velocity2D]

I was trying to think, that there must be total amount of "kinetic energy" in form of general vibrations of particles that remains conserved, just like momentum. But I have no way to calculate and check if that is true so I am stuck with this hypothesis.

I have this problem also, that whenever I don't quiet understand something I feel I am stupid. This issue is perfect example of this, it is stated everywhere that kinetic energy isn't conserved but I fail to see the fundamental difference in how it is just not spread over large quantitis like momentum.

 

[A.T.]

I understand that kinetic energy is scalar instead of linear, but shouldn't it still amount the same if billions of molecules/atoms are vibrating at very high speed?

It's not the same, because equal but opposite momenta cancel to zero, while kinetic energies from such motion don't.

 

[A.T.]

there must be total amount of "kinetic energy" in form of general vibrations of particles that remains conserved..

No, because it can be converted into some form of potential energy.

 

[Velocity2D]

No, because it can be converted into some form of potential energy.

But in the end, everything in this universe is movement of particles. Particles are never still, so all energy that there is, should be ultimately tied to the vibrations of particles/waves.

 

[A.T.]

all energy that there is, should be ultimately tied to the vibrations of particles/waves.

No, because potential energy is not tied to movement.

 

[Velocity2D]

No, because potential energy is not tied to movement.

But let's go deeper, what is potential energy? Since the very nature of wave-particles is the vibration (nothing is never at rest and there are particles transmitting the "potential energy"), the energy of the whole universe must therefore be kinetic by nature. I think classical mechanics isn't adequate enough to define world, and saying that kinetic energy isn't conserved is flawed view.

Also, forces are made of carrier particles, so when something is "at rest" they are actually communicating with other particles through thise force carriers. https://en.wikipedia.org/wiki/Force_carrier

 

[A.T.]

saying that kinetic energy isn't conserved is flawed view.

If you redefine "kinetic energy" to mean total energy, then it will be conserved classically.

 

[johns1]

Big mass hits small mass ... F = m1 x A1 = m2 x A2 good grief you guys !