B Can anyone explain following scenarios in Force-Reaction

[Velocity2D]

1. What happens when bigger mass object (with zero net force) collides into smaller massed object in vacuum, if both are "monolithic" and indestructible?
My theory is, that they just bounce off of each other with equal force.

2. Bigger mass object collides into smaller mass object and both are destructible, in vacuum?
My theory is, that higher mass object goes through the smaller mass object because of following reason: Both objects receive same amount of force, which is goes into deforming them. The force vector is mostly sideways, since the objects are getting "flattened" or breaking up and have no way to go but sideways because of inertia (the still object is trying to stay still and the impacting object is trying to move forward). Only part of the force is vectored towards forward in the object getting impacted, thus only part of the reaction force pushes the colliding object backwards. For an example the object may hit something with force of 100N and only gets part-reaction of 50N so it still has 50N forward force which keeps it going.

 

[berkeman]

1. What happens when bigger mass object (with zero net force) collides into smaller massed object in vacuum, if both are "monolithic" and indestructible?
My theory is, that they just bounce off of each other with equal force.

2. Bigger mass object collides into smaller mass object and both are destructible, in vacuum?
My theory is, that higher mass object goes through the smaller mass object because of following reason: Both objects receive same amount of force, which is goes into deforming them. The force vector is mostly sideways, since the objects are getting "flattened" or breaking up and have no way to go but sideways because of inertia (the still object is trying to stay still and the impacting object is trying to move forward). Only part of the force is vectored towards forward in the object getting impacted, thus only part of the reaction force pushes the colliding object backwards. For an example the object may hit something with force of 100N and only gets part-reaction of 50N so it still has 50N forward force which keeps it going.

Welcome to the PF.

Is this for schoolwork?

 

[BvU]

Hello Velocity2D ( )

1. Easiest to consider this in the center-of mass frame where the sum of momenta is zero. Both approach (with different speed) and bounce back with the same speeds but in the opposite directions -- conservation of momentum. "With equal force" you could say: the force one exerts on the other is equal in magnitude but opposite to the force the other exerts on the one (Newton 3)

2. Same frame of reference again. What happens in reality depends on how the objects respond to the pressure the impact causes. They could fragment, melt, lump together, etc. Again, the total momentum in the frame is and remains zero, the total momentum of the system is constant.
You really don't want to reason about this in terms of force: Newton 3 remains valid. So talk "momentum"

Here and here are a few gory simulations of what could happen... don't watch them unless you're in a good mood.

 

[Velocity2D]

Hello Velocity2D ( )

1. Easiest to consider this in the center-of mass frame where the sum of momenta is zero. Both approach (with different speed) and bounce back with the same speeds but in the opposite directions -- conservation of momentum. "With equal force" you could say: the force one exerts on the other is equal in magnitude but opposite to the force the other exerts on the one (Newton 3)

2. Same frame of reference again. What happens in reality depends on how the objects respond to the pressure the impact causes. They could fragment, melt, lump together, etc. Again, the total momentum in the frame is and remains zero, the total momentum of the system is constant.
You really don't want to reason about this in terms of force: Newton 3 remains valid. So talk "momentum"

Here and here are a few gory simulations of what could happen... don't watch them unless you're in a good mood.

Hello Velocity2D ( )

1. Easiest to consider this in the center-of mass frame where the sum of momenta is zero. Both approach (with different speed) and bounce back with the same speeds but in the opposite directions -- conservation of momentum. "With equal force" you could say: the force one exerts on the other is equal in magnitude but opposite to the force the other exerts on the one (Newton 3)

2. Same frame of reference again. What happens in reality depends on how the objects respond to the pressure the impact causes. They could fragment, melt, lump together, etc. Again, the total momentum in the frame is and remains zero, the total momentum of the system is constant.
You really don't want to reason about this in terms of force: Newton 3 remains valid. So talk "momentum"

Here and here are a few gory simulations of what could happen... don't watch them unless you're in a good mood.

I don't quiet understand.

 

[Velocity2D]

Welcome to the PF.

Is this for schoolwork?

I hard trouble understanding this concept, so I invented imaginary scenarios and tried to explain them the way I can come up.

In particular, I have trouble understanding why for an example train hitting car doesn't bounce backwards off of car but instead rams through it? Force and reaction would imply otherwise.

 

[berkeman]

In particular, I have trouble understanding why for an example train hitting car doesn't bounce backwards off of car but instead rams through it? Force and reaction would imply otherwise.

https://en.wikipedia.org/wiki/Momentum#Conservation

 

[Velocity2D]

My earlier intepretation of destructible objects colliding, how the "counter force" that the impacting object experiences as pushing it backwards is only part of the total counter force, should be correct since most of the stuff in collision is going sideways?

 

[jbriggs444]

My earlier intepretation of destructible objects colliding, how the "counter force" that the impacting object experiences as pushing it backwards is only part of the total counter force, should be correct since most of the stuff in collision is going sideways?

No. Not correct. The force of train on car and the force of car on train are equal and opposite. The car is small. A large force on a small car produces a large change in velocity. The train is large. A large force on a large train produces a smaller change in velocity.

 

[Velocity2D]

No. Not correct. The force of train on car and the force of car on train are equal and opposite. The car is small. A large force on a small car produces a large change in velocity. The train is large. A large force on a large train produces a smaller change in velocity.

But let's say the train is coming at the car with force of 400, (lets assume 400 is force required to keep the train at the speed its coming). So if train receives 400N force, it should stop since its equal to force that accelerated it to the original speed to begin with, and thus reverse its direction and the car should be intact.

 

[BvU]

Once again, do not talk in terms of force. Talk in terms of momentum.

I don't quite understand

does not help me or anyone else to improve on my reply. I tried hard to explain -- what exactly is it that you do not understand ?

 

[Velocity2D]

I tried reading the wiki article, but i don't really see how its related to this.

 

[Velocity2D]

So if the train is going steady speed, with zero net force, it has a lot of momentum. The car at the tracks is still, so its momentum is zero. When the train hits the car, the car get pushed by train and for short period of time (impulse of force) the train receives huge amount of force opposite direction of its velocity direction, after that the car and the train become one single object and no reaction force is present anymore, which is the reason the train doesn't stop? If the opposite force was constant, the train would stop and eventually start going backwards.

 

[ogg]

It is better to think of collisions happening on a scale from completely elastic to completely inelastic. Neither is actually possible in the real world, but inelastic collisions are "easier" to imagine since when two objects interact, there's likely (almost certainly) to be changes (possibly sub-atomic) in the energy of both. But when the objects are thought of as super-soft-and-super-bouncy, so that all kinetic energy turns into potential energy for an instant and then returns (bounces) back, the two objects will move away from the collision with the same speeds they started with, but with opposite direction (opposite velocities) --a perfect bounce. As stated, in your 2nd example, "destructable" doesn't have a clear meaning. Super soft objects will hit and turn all of that kinetic energy into heat. In the real world, a bullet hitting most (soft) objects will turn some of its K.E. into P.E. and tear through the soft object. Force is always equal and opposite! But there are two skills in beginning Physics: knowing what coordinates to use, and knowing whether to use energy, momentum, or force to solve a particular problem. Force isn't so useful here. (also note that many problems will involve combinations of energy, momentum and force to solve). The "reason" why force might not be useful is that in many problems, kinetic and potential energy are not individually conserved (the total energy is always conserved (except when considering cosmological distance scales)) and tracing the way an initial force is distributed into the various parts of the system becomes highly convoluted and complex. Oh, I should have also said that picking out what the "System" is that the problem is addressing is also extremely important. The "system" is the parts that "matter", not a very clear definition! LOL! (see Thermodynamic Systems for more clarity) {matter = of interest, of importance, of relevance}

 

[jbriggs444]

So if the train is going steady speed, with zero net force, it has a lot of momentum. The car at the tracks is still, so its momentum is zero. When the train hits the car, the car get pushed by train and for short period of time (impulse of force) the train receives huge amount of force opposite direction

This is wrong. The force on the car by the train is exactly equal to the force on the train by the car. That's Newton's third law. There is no "huge force" here. Nor is there a constant force. There is one force pair. Train on car and car on train, equal and opposite. The car gains a quantity of momentum. The train loses an identical quantity of momentum. The change in speed of the car is large, the change in speed of the train is small.

 

[Velocity2D]

This is wrong. The force on the car by the train is exactly equal to the force on the train by the car. That's Newton's third law. There is no "huge force" here. Nor is there a constant force. There is one force pair. Train on car and car on train, equal and opposite. The car gains a quantity of momentum. The train loses an identical quantity of momentum. The change in speed of the car is large, the change in speed of the train is small.

Isnt that what i said? i said the train receives the "huge force" which is the force it hits the car. Also I said there is no constant force, hence the force impulse. If the force was constant, the train would stop because of that.

 

[jbriggs444]

Isnt that what i said? i said the train receives the "huge force" which is the force it hits the car.

No. That is the opposite of what you said.

You called the train force huge but did not call the car force huge. You called the car force brief and considered the train force to be possibly constant.

 

[Velocity2D]

No. That is the opposite of what you said.

You called the train force huge but did not call the car force huge. You called the car force brief and considered the train force to be possibly constant.

Thats what i meant, the car inflicts "huge force" on the train, since train hits the car with "huge force". I mentioned the force impulse

 

[jbriggs444]

Thats what i meant, the car inflicts "huge force" on the train, since train hits the car with "huge force". I mentioned the force impulse

If a large mass is subject to a force and a small mass is subject to the same force, both for the same length of time, which do you think will experience a greater change in velocity?

 

[Velocity2D]

If a large mass is subject to a force and a small mass is subject to the same force, both for the same length of time, which do you think will experience a greater change in velocity?

The car. But to clarify, is it correct to assume that the car and the train becomes one single object after that period of force?

 

[jbriggs444]

The train.

Can you explain that answer? A train and a car are subject to the same force and you expect the train to change speed more rapidly as a result?

 

[Velocity2D]

Can you explain that answer? A train and a car are subject to the same force and you expect the train to change speed more rapidly as a result?

I meant the car, but is that interpretation correct?

Also another question: When a human pushes off of the ground making a jump, is this physically explained by that the body creates chemical reaction based force, by driving energy through the body to the ground, which in return gives the body force equal to the pushing power because of ground reaction force exceeding gravitational forces giving the body kinetic energy and thus momentum?

 

[jbriggs444]

I meant the car, but is that interpretation correct?

Yes, car it is.

Also another question: When a human pushes off of the ground making a jump, is this physically explained by that the body creates chemical reaction based force, by driving energy through the body to the ground

Chemical reactions create tensions and stresses (a pattern of forces) within the body. This causes a tiny bit of downward motion by the feet. But the feet are in contact with the ground. The contact force prevents the feet from penetrating into the ground. So there is an upward force of the ground on the feet and a downward force of the feet on the ground.

None of this involves any significant energy. In principle, the amount of energy involved in the process of initially tensing the muscles and starting to jump can be arbitrarily small even though the force can be arbitrarily large.

The ground does not move [significantly] as a result of the jump. That means that no work is done on the ground by the jumper. No energy is transferred to the ground.

which in return gives the body force equal to the pushing power because of ground reaction force exceeding gravitational forces giving the body kinetic energy and thus momentum?

When you say "gives the body force", what do you mean? Force is not an attribute that a body can possess. Accordingly, it is not something that can be "given".

 

[Velocity2D]

What else "force" is than energy going somewhere? I am confused. Doesnt the Earth receive some amount of energy? The ground beneath can have some kind of "leave crater"

 

[jbriggs444]

Force is force. It is not a transfer of energy. It can be viewed as a rate of transfer of momentum over time, but that is not at all the same thing as a transfer of energy.

Edit to add...

A book sitting on a table is not transferring energy to the table. Nor is the table transferring energy to the book. Both are just sitting there, passive and static. Nonetheless, there is a non-zero contact force between the two. The table is pushing upward on the book and the book is pushing downward on the table.

 

[Velocity2D]

When object falls through air and reaches terminal velocity, where does the falling object get its "force that's equal to gravity"? The force applied to the object is still the same that is caused by gravity. Simply, if the object doesn't already push the air with same force the moment it starts falling, where does it get it later?

 

[jbriggs444]

When object falls through air and reaches terminal velocity, where does the falling object get its "force that's equal to gravity"? The force applied to the object is still the same that is caused by gravity. Simply, if the object doesn't already push the air with same force the moment it starts falling, where does it get it later?

Air resistance. The faster you push air out of the way, the more force it takes to do so.

 

[Velocity2D]

Air resistance. The faster you push air out of the way, the more force it takes to do so.

Was asking, how exactly does this work? Where the object gets more force to push air more? What is force?

 

[jbriggs444]

What is force?

When you pull on a rope, your pull is a force. When you lean against the wall, your push on the wall is a force. When a book lies motionless on a table, there is a downward force from the book on the table and an upward force from the table on the book.

Here is the first hit I found on Google: http://www.physicsclassroom.com/class/newtlaws/Lesson-2/The-Meaning-of-Force

 

[sophiecentaur]

Post no6 has a link that has all the maths. Without the maths, a topic like this is just arm waving and very much prone to misunderstanding and confusion (just read some of the above).
The OP has to read that wiki article and consider what it says about collisions. It really is the only way. Would our finances make any sense without some accurate arithmetic? Same thing applies here.

 

[sophiecentaur]

Was asking, how exactly does this work? Where the object gets more force to push air more? What is force?

You either have to provide a force against the air resistance or the vehicle will slow down due to the resistance force. (Losing Energy)

 

[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.