# Can anyone explain following scenarios in Force-Reaction

• B
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 eachother 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.

Parshant

berkeman
Mentor
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 eachother 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
Homework Helper
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.

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 dont quiet understand.

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 doesnt bounce backwards off of car but instead rams through it? Force and reaction would imply otherwise.

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
Homework Helper
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.

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 lets 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
Homework Helper
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 ?

I tried reading the wiki article, but i dont really see how its related to this.

So if the train is going steady speed, with zero net force, it has alot 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 doesnt stop? If the opposite force was constant, the train would stop and eventually start going backwards.

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
Homework Helper
So if the train is going steady speed, with zero net force, it has alot 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.

Abscissiesque
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
Homework Helper
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.

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
Homework Helper
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?

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
Homework Helper
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?

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
Homework Helper
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".

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
Homework Helper
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.

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.

When object falls through air and reaches terminal velocity, where does the falling object get its "force thats equal to gravity"? The force applied to the object is still the same that is caused by gravity. Simply, if the object doesnt already push the air with same force the moment it starts falling, where does it get it later?

jbriggs444
Homework Helper
When object falls through air and reaches terminal velocity, where does the falling object get its "force thats equal to gravity"? The force applied to the object is still the same that is caused by gravity. Simply, if the object doesnt 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.

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
Homework Helper
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
Gold Member
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