# How an external force causes a body to move?

## Main Question or Discussion Point

Hi there.
I was just wondering how does a force actually casue motion of body .. and what are the factors that determine whether the force will cause change in bodies state of motion or direction or its shape?

This is what i was thinking about the action of force:
A solid body has matter in the form of atoms consisting of charges. In equilibrium state when there is no external force acting on the system, the system has balanced forces. In the bulk of the body where the atoms are surrounded by similar atoms everywhere the forces of repulsion between the electrons and that between electron and protons is balanced, i.e. the atom experiences no net force and hence tends to remain in its state of rest (so assumed). But the atoms at the surface of the body are not surrounded by similar atoms; hence they have net unbalanced force, which should mean that the atoms on surface should move.(Well, here i took it for granted that unbalanced force causes a body to move, assuming this is true for one particle body like electron). But may be because they are bound to the atoms inside, they tend to remain stressed but at their own place.

Hence when an external force (as in a body coming closer to this body and electrostatic interactions coming into play) acts on the body the atoms use their unbalanced force and the extra energy to overcome the repulsion from this body. And when the external force becomes greater than that extra force of the surface, the atoms around the force applying body get repelled, disturbing their neighbour in the bulk and the wave of disturbance travels through the medium till the last layer of the atoms. When the effect reaches the last layer of atoms, the magnitude of force acting on individual atom must have decreased such that the force cannot make the last layer move beyond a certain extent. This way the body drifts by a small amount (this has been assumed that the friction exists otherwise also it can be thought of) and executes motion. And the wave travels to and fro again and again ensuring that the motion continues…..

Is this in any form a sensible description of how force causes motion?

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mfb
Mentor
In the bulk of the body where the atoms are surrounded by similar atoms everywhere the forces of repulsion between the electrons and that between electron and protons is balanced, i.e. the atom experiences no net force and hence tends to remain in its state of rest (so assumed). But the atoms at the surface of the body are not surrounded by similar atoms; hence they have net unbalanced force, which should mean that the atoms on surface should move.
No. Every interaction with its neighbours has a net force of about 0. If the atom is in a position with net force, it moves towards a position without net force (as this is the point of minimal energy). Temperature can lead to oscillations around that point, but that is a different issue.

Hence when an external force (as in a body coming closer to this body and electrostatic interactions coming into play) acts on the body the atoms use their unbalanced force and the extra energy to overcome the repulsion from this body.
No.

[...] the atoms around the force applying body get repelled, disturbing their neighbour in the bulk and the wave of disturbance travels through the medium till the last layer of the atoms. When the effect reaches the last layer of atoms, the magnitude of force acting on individual atom must have decreased such that the force cannot make the last layer move beyond a certain extent.
I would use other words, but the concept is right if the external force is a surface force (and therefore affects only atoms near the surface of the object).

And the wave travels to and fro again and again ensuring that the motion continues…..
That is not required.

Imagine a sponge which you accelerate: With your initial hit, you accelerate one side of it and compress it. This compression is away from the stable configuration, therefore the other side will accelerate away (to come closer to the equilibrium position). If you keep accelerating the object, it will stay compressed a bit, which drives the acceleration of the whole object.
Solid objects are a bit stronger than sponges, but the concept is the same.

This compression is away from the stable configuration, therefore the other side will accelerate away (to come closer to the equilibrium position).
i am not able to make out this statement of yours. so can you please explain it little more elaborately.
If the other side of the body accelerates away, it will pull the first part with it and hence the first part will not reach its equilibrium position again.

No. Every interaction with its neighbours has a net force of about 0. If the atom is in a position with net force, it moves towards a position without net force (as this is the point of minimal energy). Temperature can lead to oscillations around that point, but that is a different issue.
.
what i am asking is not directly related to my first question; yet i am left wondering if the forces get balanced even at the surface, then where does the idea of surface tension come from for fluid?
What i thought was that the forces between similar atoms will be different in magnitude as compared to the force between two dissimilar atoms... Does this dissimilarity not affect the balancing action of force on a surface atom?

mfb
Mentor
i am not able to make out this statement of yours. so can you please explain it little more elaborately.
If the other side of the body accelerates away, it will pull the first part with it and hence the first part will not reach its equilibrium position again.
As long as you keep pushing, the side you push feels the force due to compression and the force from your hand. The latter one is a bit bigger and the difference accelerates this side.

then where does the idea of surface tension come from for fluid?
If you try to remove particles from the surface (or modify the surface in other ways), the forces are not balanced any more.

What i thought was that the forces between similar atoms will be different in magnitude as compared to the force between two dissimilar atoms
In general, the forces between atoms depends on the atoms (and their environment).

Simple thing: If something is not in a stable position, it will move towards one. As the timescales involved are usually very small (picoseconds), all microscopic objects you normally have are in the equilibrium or very close to it.

Fundamentally, solid bodies collide because of two things: the electromagnetic force which binds the molecules in each object into a solid structure, and the Pauli exclusion principle which says that two fermions, such as electrons, cannot occupy the same state (such as in an atomic bond) at the same time. When the atoms of my foot come very near to the atoms of a football, their electron wavefunctions start to overlap. Due to Pauli exclusion, the atoms of the football cannot mesh with my atoms, so they must instead get pushed out of the way. If the atoms were not bound, they would simply scatter, like kicking sand. A solid object, however, has intermolecular bonds that are fundamentally electrodynamic in nature. You can think of them as springs. When two bound molecules are stretched apart gently, their chemical bond pulls them back together. When two molecules are pushed together, their bond pushes back. In the football, the first few atoms are accelerated by direct contact with my foot. These atoms then collide into the atoms in front of them and accelerate them because of Pauli exclusion, whereas they also drag atoms to their sides with them because of the binding forces.

Fundamentally, solid bodies collide because of two things: the electromagnetic force which binds the molecules in each object into a solid structure, and the Pauli exclusion principle which says that two fermions, such as electrons, cannot occupy the same state (such as in an atomic bond) at the same time. When the atoms of my foot come very near to the atoms of a football, their electron wavefunctions start to overlap. Due to Pauli exclusion, the atoms of the football cannot mesh with my atoms, so they must instead get pushed out of the way. If the atoms were not bound, they would simply scatter, like kicking sand. A solid object, however, has intermolecular bonds that are fundamentally electrodynamic in nature. You can think of them as springs. When two bound molecules are stretched apart gently, their chemical bond pulls them back together. When two molecules are pushed together, their bond pushes back. In the football, the first few atoms are accelerated by direct contact with my foot. These atoms then collide into the atoms in front of them and accelerate them because of Pauli exclusion, whereas they also drag atoms to their sides with them because of the binding forces.
I wanted to hear someone speak in the same terms as i am asking.
As you have mentioned, according to Pauli's exclusion principle fermions not occupying same place, hence causing a force to the particles seems like a nice way of looking at the issue..
also you have mentioned about this being any electrodynamic issue, which is what i was thinking about. so how do these electrodynamic forces between the atoms (or molecules) ensure that the body as a whole moves when another electrodynamic force acts on the system externally? Also there must be certain conditions for the magnitude of this force which would cause motion as a whole, or change in its shape. And what are these conditions?

thanks

It is definitely Pauli exclusion at work. Particles and systems that do not obey Pauli exclusion (bosons) do not collide, they just pass through each other. For example, two photons do not collide (yes, I know there are strange quantum effects where photons can indirectly collide, but these effects are small). That is why if I shine a laser beam at you and you shine one at me, the beams do not hit and send light splattering everywhere.

If I kick an object and it retains its shape, it just means that the forces binding the molecules together are stronger than the force I supply with the kick, so I cannot break any bonds. I may induce vibrations in the bonds, such that they stretch and shrink, stretch and shrink, which will manifest itself visually as the object deforming. But over time, these vibrations will tend to damp out and the object will spring back to its original shape.

If I kick the object hard enough, I can break molecular bonds (or even atomic bonds, or even nuclear bonds with a strong enough "kick"). The object either breaks apart into pieces or becomes permanently deformed, or both. Ultimately you are asking why objects retain their shapes. If I connect four balls with rigid rods in a square shape, I can still deform the square. If I connect three balls in a triangle shape, I cannot. Building a structure out of many triangles and it will retain its shape. In a solid, typically the molecules are bound in a repeating lattice that gives it rigidity.

Particles and systems that do not obey Pauli exclusion (bosons) do not collide, they just pass through each other. For example, two photons do not collide (yes, I know there are strange quantum effects where photons can indirectly collide, but these effects are small). That is why if I shine a laser beam at you and you shine one at me, the beams do not hit and send light splattering everywhere.
So here, you are trying to say that photons can cross each other... Just like the so called supernatural things or ghosts can!(that sounds like interesting science fiction...) But on a serious note, is it really the case? Can particles of matter do the same somehow?(sorry i am getting off track but this idea sounds like really interesting).

If I kick an object and it retains its shape, it just means that the forces binding the molecules together are stronger than the force I supply with the kick, so I cannot break any bonds. I may induce vibrations in the bonds, such that they stretch and shrink, stretch and shrink, which will manifest itself visually as the object deforming. But over time, these vibrations will tend to damp out and the object will spring back to its original shape.

If I kick the object hard enough, I can break molecular bonds (or even atomic bonds, or even nuclear bonds with a strong enough "kick"). The object either breaks apart into pieces or becomes permanently deformed, or both. Ultimately you are asking why objects retain their shapes. If I connect four balls with rigid rods in a square shape, I can still deform the square.
i got what you were saying till this point..
If I connect three balls in a triangle shape, I cannot. Building a structure out of many triangles and it will retain its shape. In a solid, typically the molecules are bound in a repeating lattice that gives it rigidity.
But the idea that triangular shape cannot be deformed, is something i didn't get.. Can you please explain this stuff again.

Thanks..

jtbell
Mentor
Why are we talking about the Pauli exclusion principle in a classical physics forum?

oh.. i am quite sorry about that.
(If this is not the right part please tell me where it should be moved?)

mfb
Mentor
So here, you are trying to say that photons can cross each other... Just like the so called supernatural things or ghosts can!(that sounds like interesting science fiction...) But on a serious note, is it really the case? Can particles of matter do the same somehow?(sorry i am getting off track but this idea sounds like really interesting).
About 65 billion neutrinos per second pass every square centimeter of you (and everything else on earth) perpendicular to the sun. Most of them go through the whole earth without any interaction.
Using neutrino detectors, it is possible to see the sun at night - via neutrinos which come from the other side of the earth.

A bit more abstract: Suprafluids and electrons in superconductors can pass through material without (effective) interactions, too.

that's amazing!