Energy changes that result when something drops from a height

[kenny1999]

Energy is always conserved, when something drops to the ground, potential energy changes to kinetic energy and as soon as it hits the ground it changes to another form of energy. If it's an elastic collision, from what I have learnt, the kinetic energy on colliding with the ground will be probably changed to sound energy, energy lost as friction to the air, which is easy to understand.

However, how about inelastic one, when you can't hear any sound, and it doesn't bounce back from the ground, where is the energy lost? For example in our daily life if we drop something rigid from height, we can usually hear some sound. If we drop something like clothing, most of the time there is little or no sound but the initial potential energy could be the same in both cases
because PE=mgh which is independent of the types of materials. By common sense we know that if we drop 1kg of wool from the same height as 1kg of metal, usually there is no sound on collision for the case of wool but we must be hearing some sound for the metal, in both cases they could have the same initial potential energy... where is the energy in the case of wool lost??

 

[PeroK]

Energy is always conserved, when something drops to the ground, potential energy changes to kinetic energy and as soon as it hits the ground it changes to another form of energy. If it's an elastic collision, from what I have learnt, the kinetic energy on colliding with the ground will be probably changed to sound energy, energy lost as friction to the air, which is easy to understand.

However, how about inelastic one, when you can't hear any sound, and it doesn't bounce back from the ground, where is the energy lost? For example in our daily life if we drop something rigid from height, we can usually hear some sound. If we drop something like clothing, most of the time there is little or no sound but the initial potential energy could be the same in both cases
because PE=mgh which is independent of the types of materials. By common sense we know that if we drop 1kg of wool from the same height as 1kg of metal, usually there is no sound on collision for the case of wool but we must be hearing some sound for the metal, in both cases they could have the same initial potential energy... where is the energy in the case of wool lost??

Most energy for less dense objects is dissipated in the air in collisions with air molecules. For denser objects that don't bounce well it will be heat, both internal and on the ground where it lands.

 

[Lnewqban]

The package of wool absorbes some of the impact noise, which is itself smaller because it produces much less vibration than in the case of metal (imagine the different sounds produced by two strings of a guitar, each with very different degree of pre-tension).

 

[A.T.]

However, how about inelastic one, when you can't hear any sound, ...

1) Not all energy is converted to sound. Some goes directly to heat

2) Most of the sound stays in the material. Only some of it travels though air to your ear (might be too little to notice)

3) Humans don't hear all frequencies of sound

 

[Mister T]

However, how about inelastic one, when you can't hear any sound, and it doesn't bounce back from the ground, where is the energy lost?

Deformation. The objects involved are deformed during the collision. The mechanical energy is not "lost", it's converted to internal energy of the colliding objects.

 

[kenny1999]

Deformation. The objects involved are deformed during the collision. The mechanical energy is not "lost", it's converted to internal energy of the colliding objects.

When internal energy is given to the colliding objects, what will this energy be further converted to ?

does the particles/atoms/molecules/electrons of the colliding objects just keep vibrating at higher speed or pulled farther apart forever?

 

[jbriggs444]

When internal energy is given to the colliding objects, what will this energy be further converted to ?

does the particles/atoms/molecules/electrons of the colliding objects just keep vibrating at higher speed or pulled farther apart forever?

If the object is in a perfectly insulating container, yes. It'll stay warmer forever.

In practice, no. The brake pads on your car that were warm when you pulled into the garage last night will be perfectly cool by morning.

 

[Merlin3189]

Energy is always conserved, when something drops to the ground, potential energy changes to kinetic energy and as soon as it hits the ground it changes to another form of energy.
If it's an elastic collision, ... , the kinetic energy on colliding with the ground will be probably changed to sound energy, energy lost as friction to the air, which is easy to understand.

If it is perfectly elastic, then energy isn't lost. It is retained in the motion of the bodies after collision, on rebound.
In practice macro objects don't undergo perfectly elastic collisions, so some energy is lost. I think sound in air is rarely significant.

However, how about inelastic one, when you can't hear any sound, and it doesn't bounce back from the ground, where is the energy lost?

Inelastic collisions can produce sound, (eg car shunt, breaking window), but most energy goes into plastic deformation, breakage and other inelastic losses that I might characterise as internal friction. It ends up mainly as random vibration of atoms, heat, though may have spread this vibration to other parts of the objects via pressure waves.

I can understand inelastic losses by thinking of the force - elongation graph of a spring or wire. Up to a point stretching is elastic and linear(ish), Hookes law. If you deform it, that takes force and energy, which is stored as internal energy in streched bonds (largely electrostatic forces) and that energy is returned to you when you remove the deforming force.
If you deform it beyond the "elastic limit", you will get permanent deformation. When you remove the force, it does not return to its original length, so you get back less energy than you put in (Force x Distance.) Some of the lost energy may go into changes in internal energy when bonds are broken and reformed, but in metals the new bonds are much like the old ones, so the loss is again vibrations as atoms spring into their new positions.
Eventually it snaps and bonds are broken, requiring force and work, but returning nothing but heat macroscopically.

...if we drop something rigid from height, we can usually hear some sound. ... something like clothing, ... there is little or no sound but the initial potential energy could be the same in both cases ...
... if we drop 1kg of wool from the same height as 1kg of metal, usually there is no sound on collision for the case of wool but we must be hearing some sound for the metal, ... where is the energy in the case of wool lost??

I could be wrong about this, but I'd really forget about audible sound in air as a considerable amount of energy. It's usually a brief transient. Small objects don't shift much air.

In your specific example, although the metal and wool start with equal PE, they may not land with the same KE, as Perok may have said. The less dense and more irregular object creates more turbulence and loses heat to the air as it falls.
The wool is made of many interwoven fibrs. When it compresses on landing, these move relative to each other and dissipate energy in many small fricional losses. Swift compression also forces air to move between the fibres. Turbulence warms the air.

In short, most KE ends up as heat by one mechanism or another. Sound, mainly through solids, is a mechanism for spreading some of it around, but you are not generally aware of it. Even in permanent deformation, most of the absorbed energy ends up as heat.

 

[kenny1999]

If it is perfectly elastic, then energy isn't lost. It is retained in the motion of the bodies after collision, on rebound.
In practice macro objects don't undergo perfectly elastic collisions, so some energy is lost. I think sound in air is rarely significant.

Inelastic collisions can produce sound, (eg car shunt, breaking window), but most energy goes into plastic deformation, breakage and other inelastic losses that I might characterise as internal friction. It ends up mainly as random vibration of atoms, heat, though may have spread this vibration to other parts of the objects via pressure waves.

I can understand inelastic losses by thinking of the force - elongation graph of a spring or wire. Up to a point stretching is elastic and linear(ish), Hookes law. If you deform it, that takes force and energy, which is stored as internal energy in streched bonds (largely electrostatic forces) and that energy is returned to you when you remove the deforming force.
If you deform it beyond the "elastic limit", you will get permanent deformation. When you remove the force, it does not return to its original length, so you get back less energy than you put in (Force x Distance.) Some of the lost energy may go into changes in internal energy when bonds are broken and reformed, but in metals the new bonds are much like the old ones, so the loss is again vibrations as atoms spring into their new positions.
Eventually it snaps and bonds are broken, requiring force and work, but returning nothing but heat macroscopically.I could be wrong about this, but I'd really forget about audible sound in air as a considerable amount of energy. It's usually a brief transient. Small objects don't shift much air.

In your specific example, although the metal and wool start with equal PE, they may not land with the same KE, as Perok may have said. The less dense and more irregular object creates more turbulence and loses heat to the air as it falls.
The wool is made of many interwoven fibrs. When it compresses on landing, these move relative to each other and dissipate energy in many small fricional losses. Swift compression also forces air to move between the fibres. Turbulence warms the air.

In short, most KE ends up as heat by one mechanism or another. Sound, mainly through solids, is a mechanism for spreading some of it around, but you are not generally aware of it. Even in permanent deformation, most of the absorbed energy ends up as heat.

I pretty much get what you are saying.

I see you stated the term -> macroscopically

Can I understand that Physics is about "macroscopically" while Chemistry is about "Microscopically"?

By the way, can you give me a brief elaboration on what you have replied lately in the Chemisty's sense? Thanks...

 

[Merlin3189]

Can I understand that Physics is about "macroscopically" while Chemistry is about "Microscopically"?

I wouldn't think so. Both physics and chemistry consider both scales. For almost anything observable we are looking at very large numbers of atoms, whether in chem or physics, but in both we often go back to consideration of these atoms (/molecules/particles) for understanding of mechanisms.

I'm not sure why I tagged a macroscopically on the end of that sentence. I do a lot of editing/ rewriting in my posts (I'm a bit verbose and repetitive), so it could be a remnant of a longer sentence. I think I was just trying to cover my back about any internal energy involved in rearranged atomic structure, when all we notice is the heat.
I'm not sure what you're getting at in your last sentence about elaboration? Perhaps you can ask about specific points.

 

[kenny1999]

I wouldn't think so. Both physics and chemistry consider both scales. For almost anything observable we are looking at very large numbers of atoms, whether in chem or physics, but in both we often go back to consideration of these atoms (/molecules/particles) for understanding of mechanisms.

I'm not sure why I tagged a macroscopically on the end of that sentence. I do a lot of editing/ rewriting in my posts (I'm a bit verbose and repetitive), so it could be a remnant of a longer sentence. I think I was just trying to cover my back about any internal energy involved in rearranged atomic structure, when all we notice is the heat.
I'm not sure what you're getting at in your last sentence about elaboration? Perhaps you can ask about specific points.

For example, what could happen to the atoms/molecules/electrons from the when something collide / drop / touch / slide / hit etc

 

[Mister T]

When internal energy is given to the colliding objects, what will this energy be further converted to ?

That depends on the circumstances. The possibilities are endless

does the particles/atoms/molecules/electrons of the colliding objects just keep vibrating at higher speed or pulled farther apart forever?

I don't know how to answer this question. You can't just treat particles, atoms, molecules, and electrons the same by placing slashes between them.

 

[Merlin3189]

I'm not sure I should try to say anything about this. I've stuck my neck out a long way with my previous incomplete and sketchy answers. What we need is a proper materials scientist to give some authoritative detail.
I can only tell you my own very limited, hazy and fragmented ideas (I can't call it knowledge) about this. I actually have many questions of my own about some of this.

When solids touch, collide, slide I would generally assume that the atoms/molecules remain bound, but their positions are temporarily moved slightly away from their normal equilibrium position: bonds are stretched or compressed, rather than broken, Electric PE is returnable when the atoms move back.

Where contact forces are high enough, maybe large deceleration or contact over a small area (sharp point), then atoms can be permanently moved from their original place in the solid structure. In metals, atoms often have no difficulty fitting in a bit further along, making them maleable: in non-metals it is more likely to result in breaks or cracks. Energy used in deformation is non-returnable.

Electrons do get removed or added to a material fairly easily, but maybe not due to KE of collision? There is the triboelectric effect, where two different materials in contact can gain and lose electrons, leaving each with the opposite charge. Electrons in higher energy levels in one material may jump to a lower energy level available in the other. AFAIK this can only affect the electrons in atoms on the surface which come into close contact with the other material. I think the rubbing is more about maximising contact than doing useful work on electrons. The energy input is in moving the objects apart against electrostatic attraction, rather than KE of impact.

I have no idea about chemical reaction in impacts. I can't think of any reactions that occur solid with solid. I suppose you could think about striking a match. In the terms you originally asked, I don't think chemistry comes into the sort of collisions you were thinking of.