What is the difference between momentum and kinetic energy?

[aloshi]

Hello!
yes is from Sweden and my English is not good, but will try to do as best as possible. My question is:
why can not consider the momentum as the acceleration energy?

I know that:
Impulse is change in momentum which is not the same as energy
Impulse can be expressed either as F * (delta) t or m * (delta) v since it is the same thing. That the expressions are as follows from Newton's 2nd Kraftlag together with the definition of acceleration: (delta) v / (delta) t

but as we move into the energy we see that energy is defined as:
W = mv ^ 2 / 2

if we compare the energy between, thus förendringen Middle two speeds we get that the change in energy is:
http://www.pluggakuten.se/wiki/image...itled11111.jpg
I can not see a big difference between them, the only thing that separates them is that we have abbreviated removed (delta)stretch

but what is the difference between rörelsenergi and momentum:

I couple of things:
An important difference is that momentum is always kept in a collision between two or more objects. The kinetic energy conservation is generally not in a collision.
Another difference between kinetic energy and momentum is that kinetic energy is a scalar (ie, has size but not direction) while the momentum is a vector (ie, both the size and direction)

but I can not really understand what the difference between momentum and kinetic energy (accelerating energy)

 

[aloshi]

Hello!
yes is from Sweden and my English is not good, but will try to do as best as possible. My question is:
why can not consider the momentum as the acceleration energy?

I know that:
Impulse is change in momentum which is not the same as energy
Impulse can be expressed either as F \cdot \Delta t or m * (delta) v since it is the same thing. That the expressions are as follows from Newton's 2nd Kraftlag together with the definition of acceleration: (delta) v / (delta) t

but as we move into the energy we see that energy is defined as:
W = mv ^ 2 / 2 mv ^ 2 / 2

if we compare the energy between, thus förendringen Middle two speeds we get that the change in energy is:
http://www.pluggakuten.se/wiki/image...itled11111.jpg
I can not see a big difference between them, the only thing that separates them is that we have abbreviated removed (delta)stretch

but what is the difference between rörelsenergi and momentum:

I couple of things:
An important difference is that momentum is always kept in a collision between two or more objects. The kinetic energy conservation is generally not in a collision.
Another difference between kinetic energy and momentum is that kinetic energy is a scalar (ie, has size but not direction) while the momentum is a vector (ie, both the size and direction)

but I can not really understand what the difference between momentum and kinetic energy (accelerating energy)

 

[aloshi]

Hello!
yes is from Sweden and my English is not good, but will try to do as best as possible. My question is:
why can not consider the momentum as the acceleration energy?

I know that:
Impulse is change in momentum which is not the same as energy
Impulse can be expressed either as F * (delta) t or m * (delta) v since it is the same thing. That the expressions are as follows from Newton's 2nd Kraftlag together with the definition of acceleration: (delta) v / (delta) t

but as we move into the energy we see that energy is defined as:
W = mv ^ 2 / 2

if we compare the energy between, thus förendringen Middle two speeds we get that the change in energy is:
http://www.pluggakuten.se/wiki/image...itled11111.jpg
I can not see a big difference between them, the only thing that separates them is that we have abbreviated removed (delta)stretch

but what is the difference between rörelsenergi and momentum:

I couple of things:
An important difference is that momentum is always kept in a collision between two or more objects. The kinetic energy conservation is generally not in a collision.
Another difference between kinetic energy and momentum is that kinetic energy is a scalar (ie, has size but not direction) while the momentum is a vector (ie, both the size and direction)

but I can not really understand what the difference between momentum and kinetic energy (accelerating energy)[/QUOTE]

Hello!
yes is from Sweden and my English is not good, but will try to do as best as possible. My question is:
why can not consider the momentum as the acceleration energy?

I know that:
Impulse is change in momentum which is not the same as energy
Impulse can be expressed either as F \cdot \Delta t or m \cdot \Delta v since it is the same thing. That the expressions are as follows from Newton's 2nd Kraftlag together with the definition of acceleration: (delta) v / (delta) t

but as we move into the energy we see that energy is defined as:
mv ^ 2 / 2

if we compare the energy between, thus förendringen Middle two speeds we get that the change in energy is:
http://www.pluggakuten.se/wiki/image...itled11111.jpg
I can not see a big difference between them, the only thing that separates them is that we have abbreviated removed (delta)stretch

but what is the difference between rörelsenergi and momentum:

I couple of things:
An important difference is that momentum is always kept in a collision between two or more objects. The kinetic energy conservation is generally not in a collision.
Another difference between kinetic energy and momentum is that kinetic energy is a scalar (ie, has size but not direction) while the momentum is a vector (ie, both the size and direction)

but I can not really understand what the difference between momentum and kinetic energy (accelerating energy)

 

[tiny-tim]

Welcome to PF!

Hello! Hello! Hello! Welcome to PF!

I'm not sure what you mean by "acceleration energy" or "rörelsenergi", and your link isn't working, but anyway …

From the chain rule, d(mv)/dt = d(mv)/ds ds/dt = d(mv)/ds v = d(1/2 mv²)/ds.

Does that help?

 

[aloshi]

In order to acceleration, there must be a job. that is what I call the acceleration energy thus acceleration work. acceleration of work is F\cdot \Delta s and this means increasing the kinetic energy. therefore increases speed.

what would be linked are:
http://img195.imageshack.us/img195/4176/40958030.jpg



what I can not understand the difference between kinetic energy and momentum is not so great. we have only shortened away \Delta s[/QUOTE]

 

[aloshi]

This link shows how I have been derived momentum and nothing else-:
http://img195.imageshack.us/img195/4176/40958030.jpg

 

[tiny-tim]

Hi aloshi!

(have a delta: ∆ and try using the X² tag just above the Reply box )

Yes, I can see your link now.

I'll rewrite it so that it's easier to read …

If we have a very small increase in speed, from v to v + ∆v,

then ∆KE = 1/2 m ((v + ∆v)² - v²) = mv ∆v = m ∆s/∆t ∆v = m ∆s (m∆v/∆t) = ∆s F,

which is the standard work-energy theorem, change in KE = work done by force F moving through distance s.

(I've used Newton's second law : impulse = force x time = change of momentum, or F ∆t = ∆(mv))​

After that, I don't follow what your objection is.

Are you confusing impulse with work done ?

Impulse = force x time, but work done = force x distance.

Are you saying that for a very small change, distance is proportional to time, and so impulse is proportional to work done? If so, that ignores the fact that, if there is acceleration, distance is never proportional to time (except in circular motion, in which case the work done and the change in KE are zero).

 

[aloshi]

i can't understand what is the difference between Newton's second law and the momentum?

 

[tiny-tim]

i can't understand what is the difference between Newton's second law and the momentum?

Newton's second law : impulse = force x time = change in momentum

(or force = rate of change of momentum).

What is worrying you about that?

 

[aloshi]

I can not understand the properties of momentum has. what is momentum?

 

[tiny-tim]

I can not understand the properties of momentum has. what is momentum?

Momentum is mass times velocity.

It is the derivative of kinetic energy … mv = d/dt (1/2 mv²)

 

[aloshi]

what is the difference physically, I understand the difference mathematical

 

[tiny-tim]

momentum equals available "oomph"

Momentum is always conserved in collisions (while kinetic energy is not),

so momentum measures the "oomph" available when something hits something else.

 

[aloshi]

I could not be pushed to the last:

"So the momentum measures the" oomph "available when something hits something else"
can you explain thanks

 

[tiny-tim]

Momentum is a quantity which an object has when it moves.

It can transfer that quantity to another object.

That quantity is never lost, it only moves from one object to another.

It measures the ability to move another object …

the more momentum you have, the more you can move something else …

the less you have, the less you can move something else …

if you do move something else, you must give up some of your own momentum.

 

[aloshi]

Requires no energy to give over part of their own speed?

 

[tiny-tim]

Requires no energy to give over part of their own speed?

Change of energy is not required …

if you bounce a ball off a wall, there is no change of energy, but the ball exerts an impulse on the wall …

if the wall is on wheels, it will move, with little or no change in the energy of the ball.

What moves the wall is the change in momentum: the change in energy is irrelevant.

 

[aloshi]

if we are dragging a ball from a height H. After the nozzles have the floor, it will not reach to the height H, but it will not be as high. it mean that we have lost energy?

if we are dragging a ball from a height H. After the nozzles have the floor, it will not reach to the height H, but it will not be as high. it does not mean that we have lost energy?
it can not mean that the floor gave a speed, and therefore its momentum. floor can not get a speed

 

[tiny-tim]

if we are dragging a ball from a height H. After the nozzles have the floor, it will not reach to the height H, but it will not be as high. it does not mean that we have lost energy?

Yes, if we drop a ball from a height H (sorry, i have no idea what you mean by "nozzles" ), it will not quite return to height H because of the small amount of thermal energy (heat) generated.

But that energy creates heat (and/or vibration and noise), not movement.

it can not mean that the floor gave a speed, and therefore its momentum. floor can not get a speed

No, the floor does get a speed …

the floor is fixed to the Earth, and when the ball bounces up, the whole Earth moves very very very slightly down!

 

[aloshi]

there are other ways of thinking that momentum? I want to understand it to 100%

 

[tiny-tim]

there are other ways of thinking that momentum? I want to understand it to 100%

No, I think I've covered everything, but if you still have any questions, just ask.

 

[aloshi]

No, I think I've covered everything, but if you still have any questions, just ask.

I do not know how to thank you, you have taught me much that my teachers could not teach me. I have that project and have chosen to investigate Compton scattering / Compton effect. Then I read about it, I saw that the momentum is the key element. so I've probably other issues that I need to ask. but it will be later, please respond during the Christmas holidays?

I will read more about momentum, therefore, I wondered if there was anything more that you can mention. thanks

 

[tiny-tim]

I assume you're starting with http://en.wikipedia.org/wiki/Compton_scattering" ?

Remember that both momentum and energy are conserved in Compton scattering.

(In a collision, to solve the equations, you need both conservation of momentum and either conservation of energy or some physical constraint such as a fixed speed.)

On momentum itself, you might also like to read the PF Library article at https://www.physicsforums.com/library.php?do=view_item&itemid=183", and the Australian website referred to in it, about how conservation of momentum explains how a sound wave can be reflected from the open end of a pipe.

happy holidays!​

 

[aloshi]

Can there is more fuller on Compton scattered? Then other links

 

[tiny-tim]

google book search

Can there is more fuller on Compton scattered? Then other links

Try a google book search …

on the google search page, type "Compton scattering" (including the quotation marks), then click on the drop-down menu marked "more" at the top of the page, and click on the first item, which is "Books" …

that will give you a lot of books on Compton scattering (for example, http://books.google.com/books?id=u7...r&dq="Compton+scattering"&client=safari&cd=1"), and any that are marked "Limited preview" can be read and downloaded free.

 

[aloshi]

Momentum is a quantity which an object has when it moves.

It can transfer that quantity to another object.

That quantity is never lost, it only moves from one object to another.

It measures the ability to move another object …

the more momentum you have, the more you can move something else …

the less you have, the less you can move something else …

if you do move something else, you must give up some of your own momentum.

you say that each okjekt has one quantity, then is my question: What comes this from the start? if we say a car, in order to it will drive so needs it no quantity from another item without it needs fuel in order to begin to drive. therefore, that one energy needs in order to can to drive with the car. if we take another example and it is then one runs. before I began to run had I no momentum, without in order to I will can opening so needs I to allocate a certain energy. this energy comes from the food that I eat. about now I run and since collides with another item that is placid comes a part of my momentum to be transferred to the item, but was come my momentum from? it cannot well come how as entire pcs, without it must come from something. I do not know about my English is understand

 

[tiny-tim]

Hi aloshi! Happy new year!

ok, you're asking where does the momentum come from, for example when a car accelerates, or when a person runs?

For a car, the extra momentum comes from the momentum of the piston, which in turn comes from the momentum of the gas molecules hitting the piston, which in turn comes from a chemical reaction …

the chemical reaction is basically the breaking of a bond … two objects are bound together with potential energy of a force field, and when that potential energy is released, the two objects fly apart.

Consider dropping something from a height … it will hit the ground with great momentum, but where did that momentum come from?

It also came from the potential energy of a force field (in this case, a gravitational field, but in the case of a chemical bond, it would be an electromagnetic field).

Basically, when a car accelerates, or when a person runs, ultimately the momentum has come, not from a collision, but from the release of energy of a force field

hmm … the Moon's momentum now is completely opposite to what it was two weeks ago … where did that change of momentum come from (since the potential energy is roughly the same)?

dunno! … i'll have to let someone else try to explain that ​

 

[aloshi]

thanks for the reply, but can you explain dett here once more:
the chemical reaction ice basically the breaking of a bond… two objects are bound together with potential energy of a force field, wild duck when that potential energy ice released, the two objects escape apart.

hag is not good on English and I translate the text in google translates. but it is value loose

 

[tiny-tim]

I always think of breaking a chemical bond as being like cutting an elastic band … the stored energy is released, and the two sides fly apart.

I really don't know enough chemistry to explain it any better.

 

[aloshi]

have you a good homepages where it stand about business amount? would you kuna send little lhemsidor (web) to me thanks

 

[tiny-tim]

hag is not good on English and I translate the text in google translates. but it is value loose

have you a good homepages where it stand about business amount? would you kuna send little lhemsidor (web) to me thanks

your google translator is really bad!

i can usually guess what you mean, but this time i have no idea.

 

[aloshi]

I wondered about there be websites that explain momentum

 

[tiny-tim]

Try this free online book … http://books.google.com/books?id=bY...ontcover&dq=momentum&lr=&client=safari&cd=31" by David P. Kessler, Robert Albert Greenkorn

Also The forces of nature by P. C. W. Davies.

(btw, your English is much better than the google translator … i recommend that you do your own translation! )

 

[aloshi]

Try this free online book … http://books.google.com/books?id=bY...ontcover&dq=momentum&lr=&client=safari&cd=31" by David P. Kessler, Robert Albert Greenkorn

Also The forces of nature by P. C. W. Davies.

(btw, your English is much better than the google translator … i recommend that you do your own translation! )

have you easier level, I mean that my level is not so here advanced

 

[tiny-tim]

You could try Chapter 11 of http://books.google.com/books?id=Wk...ient=safari&cd=62#v=onepage&q=impact&f=false" by Sander Bais (2005)

 

[aloshi]

I have read "An elementary treatise on mechanics" and i have some difficulty in understanding.


1)
“When two bodies in motion impinge, if their centers of inertia move in the same straight line perpendicular to a plane tangent to the bodies at their point of contact , the impact is said to be direct and central.”
I can not really understand:
“the same straight line perpendicular to a plane tangent to the bodies at their point of contact”
if I'm drawing a picture of how the impinge look;
[PLAIN]http://www.pluggakuten.se/wiki/images/2/22/F%C3%B6rsta.jpg

[PLAIN]http://www.pluggakuten.se/wiki/images/2/22/F%C3%B6rsta.jpg

Have I understand’t correct?

2)
I can not really understand:
“if the straight line described by the center of inertia of one of the bodies is not perpendicular to the tangent plane , the impact is said to be oblique.”

if I'm drawing a picture of how the impinge look:

[PLAIN]http://www.pluggakuten.se/wiki/images/c/c7/Andra.jpg

[PLAIN]http://www.pluggakuten.se/wiki/images/c/c7/Andra.jpg
Have I understand’t correct?



it is from 240. DEF. in "An elementary treatise on mechanics"

 

[tiny-tim]

1)
“When two bodies in motion impinge, if their centers of inertia move in the same straight line perpendicular to a plane tangent to the bodies at their point of contact , the impact is said to be direct and central.”
I can not really understand:
“the same straight line perpendicular to a plane tangent to the bodies at their point of contact”
…
2)
I can not really understand:
“if the straight line described by the center of inertia of one of the bodies is not perpendicular to the tangent plane , the impact is said to be oblique.”

Hi aloshi!

The "tangent plane" is the plane of contact between the two spheres …

imagine that, when they are touching, you put a flat piece of paper between them …

it will be tangent to both spheres at the point of contact.

Now, it's possible for one (or both) of the spheres to come towards that paper at an angle (though still hitting the same contact point) …

in that case, the centre of inertia is not moving perpendicular to the tangent plane.

 

[aloshi]

Hi aloshi!

The "tangent plane" is the plane of contact between the two spheres …

imagine that, when they are touching, you put a flat piece of paper between them …

it will be tangent to both spheres at the point of contact.

http://www.pluggakuten.se/wiki/images/8/83/Papper.jpg

Now, it's possible for one (or both) of the spheres to come towards that paper at an angle (though still hitting the same contact point) …

in that case, the centre of inertia is not moving perpendicular to the tangent plane.

http://www.pluggakuten.se/wiki/images/0/0e/Snett.jpg

like that??

 

[tiny-tim]

like that??

That's it!

Sphere B is approaching obliquely, and the impact is oblique.

 

[aloshi]

can you help me with this too, thanks;

"When the bodies impinge, they exert a mutual but varying pressure during the interval between contact and separation, an interval of time which is generally very short, and we suppose them to suffer a degree of compression, by wich, during a portion of this interval, their centers will approach each other, and during the remaining portion will recede by the action of an internal force rending to restore them to their original form. The force urging the approach if their centers is called the force of compression; the opposing force causing them to separate again is called the force of restitution or elasticity. The ratio of the force of restitution to that of compression is called the modulus of elastisk"

1)what is/does meant by compression?
the force that the balls come into each other, collide(the force at collide)??

2)what is/does meant by restitution? the force that removes them?
3)what is/does meant by “The ratio of the force of restitution to that of compression is called the modulus of elastisk” ?

 

[tiny-tim]

Hi aloshi!

Compression is the opposite of tension (negative tension) … it means the sphere is squashed very slightly, and there is an internal force, just like the internal force of tension in a rope, but acting inward instead of outward.

Tension and compression are stress when there are no shear forces (sideways forces).

Restitution is the restoring force when the sphere starts to expand again (back to its original size).

If the material is perfectly elastic, then the collision is also perfectly elastic, and no energy is lost. The force of restitution is then the same as the force of compression.

If the material is not perfectly elastic, then energy is lost, and the force of restitution is less than the force of compression.

The ratio is called the modulus of elasticity (or the coefficient of restitution).

See also http://en.wikipedia.org/wiki/Coefficient_of_restitution"

 

[aloshi]

Hi aloshi!

Compression is the opposite of tension (negative tension) … it means the sphere is squashed very slightly, and there is an internal force, just like the internal force of tension in a rope, but acting inward instead of outward.

Tension and compression are stress when there are no shear forces (sideways forces).

Restitution is the restoring force when the sphere starts to expand again (back to its original size).

I still can not understand what the difference by compression and Restitution.
So I Understand/ grasp;
1)
http://www.pluggakuten.se/wiki/images/6/63/1.JPG
2)
http://www.pluggakuten.se/wiki/images/7/75/2.JPG
3)
http://www.pluggakuten.se/wiki/images/6/6b/3.JPG
4)
http://www.pluggakuten.se/wiki/images/8/80/6.JPG

 

[tiny-tim]

compression and restitution

I still can not understand what the difference by compression and Restitution.

Hi aloshi!

I don't actually understand your first two diagrams .

Let's start again.

Tension: is irrelevant … these spheres are never under tension.

(a sphere could be under tension if, for example, it was hollow, and the pressure inside was more than the pressure outside)

Compression: when the spheres meet, they squash, so they experience compressive forces.

This happens both during the squashing and expanding periods.

Your book said …

we suppose them to suffer a degree of compression, by which, during a portion of this interval, their centers will approach each other, and during the remaining portion will recede by the action of an internal force rending to restore them to their original form.
The force urging the approach if their centers is called the force of compression; the opposing force causing them to separate again is called the force of restitution or elasticity.

What I've called squashing and expanding, your book calls approaching and receding.

In both these portions of the collision, the force is compression, but for some reason your books is distinguishing between them, and calling the first force the force of compression, and the second force the force of restitution.

The second force will always be less than the first force (unless the collision is elastic), and the ratio of the energies involved is called the coefficient of restitution.

 

[aloshi]

Hi aloshi!

I don't actually understand your first two diagrams .

Let's start again.

Tension: is irrelevant … these spheres are never under tension.

(a sphere could be under tension if, for example, it was hollow, and the pressure inside was more than the pressure outside)

Compression: when the spheres meet, they squash, so they experience compressive forces.

This happens both during the squashing and expanding periods.

have i understand it right?

Compression :
the force comes from the speed at which balls are? because the velocity is a vector. a vector has both direction and size, it is right ?

restroring force is the internal force that enables/make them to starts to expand again (back to its original size)
I'm sorry, but I am bad in English, sorry!

 

[tiny-tim]

Hi aloshi!

let's see … you got the original quotation from http://books.google.com/books?id=Wk...ient=safari&cd=62#v=onepage&q=impact&f=false" …

there's a worked example of the principle of that paragraph at para. 245, including …

Let e be the modulus of elasticity, or the ratio of the force of restitution to that of compression. Since these forces are proportional to the velocities they generate or destroy in the same mass, the velocity destroyed in m₁ by the force of restitution will be e(v₁ - v).​

(That's rather old-fashioned (1855) language … I think nowadays we'd talk of "change in velocity (or momentum)", rather than generating or destroying velocity.)

Perhaps the PF Library on coefficient of restitution (same as modulus of elasticity) puts it more clearly …

For a collision between two objects, the coefficient of restitution is the ratio of the relative speed after to the relative speed before the collision.

The coefficient of restitution is a number between 0 (perfectly inelastic collision) and 1 (elastic collision) inclusive.​

have i understand it right?

Compression :
the force comes from the speed at which balls are? because the velocity is a vector. a vector has both direction and size, it is right ?

restroring force is the internal force that enables/make them to starts to expand again (back to its original size)

Yes, force is (rate of) change of momentum, so it is proportional to change of velocity …

in that sense, the force comes from the velocity, and the velocity comes from the force.

And yes, as vectors, the force and the velocity will be in the same direction.

On the "way in", in that book, the force is called compression, and on the "way out", it is called the restoring (or restitutive) force (but I don't think a modern book would bother to make that distinction).

I don't think you really need to know how force comes into it when you have a problem like this …

In examination questions, just ignore these internal forces, and deal only with momentum before, momentum after, and coefficient of restitution. ​

 

[aloshi]

hi!
thanks for all the answers.

you have write;

If the material is perfectly elastic, then the collision is also perfectly elastic, and no energy is lost. The force of restitution is then the same as the force of compression.

If the material is not perfectly elastic, then energy is lost, and the force of restitution is less than the force of compression.

in that sense, the force comes from the velocity, and the velocity comes from the force.

And yes, as vectors, the force and the velocity will be in the same direction.
. [/INDENT]

So if the vector is equal before (compression vector) and after (restitution vector) collision , This means the material is perfectly elastic.
And if the vector is not equal before (compression vector) and after (restitution vector) collision , This means If the material is not perfectly elastic.

that must be right!

 

[tiny-tim]

Hi aloshi!

(btw, it's "you have written" … it's one of those verbs that come from old German, and whose past participles end in "en" instead of "ed", like forgotten, driven, stricken, given … )

So if the vector is equal before (compression vector) and after (restitution vector) collision , This means the material is perfectly elastic.
And if the vector is not equal before (compression vector) and after (restitution vector) collision , This means If the material is not perfectly elastic.

that must be right!

It is right!

 

[aloshi]

what is it that makes the vector is not equal before (compression vector) and after (restitution vector) collision?

 

[tiny-tim]

Mechanical energy is lost, and converted into heat, vibration, and sound.

 

[aloshi]

why is the force F=m_1*v_1??
is There a formula that says the force is F=m_1*v_1??
I have not seen that, but in the book you find it.