# The Naked Scientists Forum

### Author Topic: What is kE?  (Read 5283 times)

#### alancalverd

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##### Re: What is kE?
« Reply #25 on: 28/03/2016 00:42:43 »
The higher you lift an object, the heavier it becomes relative to the ground, terminal velocity is the weight maximum limit and at its relativistic heaviest.
Not true.

The higher you lift an object, the less it weighs because the gravitational field is divergent. F= GmM/r2, if you recall.

Terminal velocity depends on the shape of a falling object, and the density and viscosity of the medium through which it falls. It's about 150 mph for a human body, from any height above a couple of hundred feet. I don't think this is what you meant.

You can calculate ground impact velocity (ignoring air resistance) by putting mgh = ½mv2, i.e. by assuming that energy is conserved. But even though the calculation is simple and the answer is always correct, I'm sure you will not believe it.

#### Thebox

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##### Re: What is kE?
« Reply #26 on: 28/03/2016 17:13:28 »

Force is rate of change of inertia. You can't equate force with inertia.

No it is not, I totally disagree with valued reasoning.

''a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force''

In my terms a resistance to change of velocity, an object at rest on Earth has inertia, this inertia is created by the linear force of gravity, inertia how you define it is an exact equal to force and means the same thing.

F=ma in a Y axis and F=ma in a x-axis, change x to change y,  inertia is just another add-on that means force,  so yes I can equate inertia with force, because they are the same thing, we don't need mass, we don't need inertia, we don't need kE, Newtons of force explains all of them and is equal to them, they are all the same thing and have no differential.

A change of force is needed to change the force hold on the object, inertia is the force hold which is gravity.

In plain English, the resistance to force is a force but you call it inertia when it is Newton's.

« Last Edit: 28/03/2016 17:17:35 by Thebox »

#### Thebox

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##### Re: What is kE?
« Reply #27 on: 28/03/2016 17:23:06 »
The higher you lift an object, the heavier it becomes relative to the ground, terminal velocity is the weight maximum limit and at its relativistic heaviest.
Not true.

The higher you lift an object, the less it weighs because the gravitational field is divergent. F= GmM/r2, if you recall.

Terminal velocity depends on the shape of a falling object, and the density and viscosity of the medium through which it falls. It's about 150 mph for a human body, from any height above a couple of hundred feet. I don't think this is what you meant.

You can calculate ground impact velocity (ignoring air resistance) by putting mgh = ½mv2, i.e. by assuming that energy is conserved. But even though the calculation is simple and the answer is always correct, I'm sure you will not believe it.

Yes by terminal velocity I meant the maximum speed something falls at, why what else could I mean?

The higher you lift an object, following the inverse square law the weight of Newton's will lessen as the gravitational force lessens, however when the object falls it regains relativists weight and the scales the object lands on will record that it gained relativistic kg (mass) .

Newton's of force is what gives an object it's mass which is equal to Newton's. (the same thing)

(F=0)=(m=0)

« Last Edit: 28/03/2016 17:25:41 by Thebox »

#### chiralSPO

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##### Re: What is kE?
« Reply #28 on: 28/03/2016 21:56:14 »

No. You are on the wrong track here. I am not quite sure what track you are on, but it ain't right...

A bit contradictory mate, you say you are not sure what I am talking about yet you claim it is not right.

Imagine someone says they are trying to make chocolate mousse, and you walk into the kitchen to find them struggling with a tin of anchovies. You don't know why they are using anchovies, but you can be sure that it won't help them make the chocolate mousse...

Quote
F = m*a means force = mass times acceleration
force is in units of Newtons (N = kg*m*s–2)
mass is in units of kilograms (kg)
acceleration is in units of m*s–2

Huh?   why are you putting - 2

Putting the negative sign in the exponent is equvalent to taking the reciprocal. For instance:

32 = 9
3–2 = 1/9
5*2–3 = 5/(23) = 5/8
m/s2 = m*s–2

Force = mass times acceleration

force is measured weight , in Newtons,

a=9.81m/s2
ok
which means that for every  meter falling it increases its acceleration *2

no

You lost me , I know what F=ma means, or I thought I did.

Let me confirm,

for 1kg mass we put 0.1 in the calculation for mass?

F= 0.1 * a

acceleration is 9.81m/s2

for the first  meter

9.81m/s

2nd meter

9.81m/s*2

3rd meter

9.81m/s*3

?????????????

no. For 1 kg, F = 1*a (and for 23 kg, F = 23*a)
An acceleration of 9.81m/s2 means that the velocity of an object (vt) after t seconds of acceleration will be whatever the velocity was at zero seconds (v0, ie before the acceleration started) plus the acceleration times t:

vt = v0 + a*t

If in object is initially not moving (v0 = 0) and then begins to fall at a = 9.81 m/s2
it will be moving at 9.81 m/s at 1 second, 19.62 m/s at 2 seconds, 29.43 m/s at 3 seconds...

EDIT: equations corrected thanks to hamdani yusuf!
« Last Edit: 29/03/2016 03:37:46 by chiralSPO »

#### evan_au

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##### Re: What is kE?
« Reply #29 on: 28/03/2016 22:01:19 »
Quote from: TheBox
an object at rest on Earth has inertia, this inertia is created by the linear force of gravity
How do you explain that an object orbiting the Earth in free-fall (no weight) has the same inertia as the same object sitting on the Earth's surface?

Gravity is different in orbit, so how could the inertia be the same?

It is a bit of a mystery to physicists why inertial mass should be identical to gravitational mass; but it is believed that both characteristics are maintained far from an external gravitational field.

#### alancalverd

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##### Re: What is kE?
« Reply #30 on: 28/03/2016 23:15:56 »
Time for an apology. Force is rate of change of momentum. Must have left my brain in the sky.

#### hamdani yusuf

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##### Re: What is kE?
« Reply #31 on: 29/03/2016 02:35:35 »
vt = v0 + a*t2

If in object is initially not moving (v0 = 0) and then begins to fall at a = 9.81 m/s2
it will be moving at 9.81 m/s at 1 second, 39.24 m/s at 2 seconds, 88.29 m/s at 3 seconds, 156.96 m/s at 4 seconds...
The correct formula is vt = v0 + a*t
hence at 1 second, v=9.81 m/s, at 2 seconds v=19.62, and so on.
or if you want to calculate distance st = s0 + v0*t + ½*a*t²

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#### chiralSPO

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##### Re: What is kE?
« Reply #32 on: 29/03/2016 03:34:15 »
vt = v0 + a*t2

If in object is initially not moving (v0 = 0) and then begins to fall at a = 9.81 m/s2
it will be moving at 9.81 m/s at 1 second, 39.24 m/s at 2 seconds, 88.29 m/s at 3 seconds, 156.96 m/s at 4 seconds...
The correct formula is vt = v0 + a*t
hence at 1 second, v=9.81 m/s, at 2 seconds v=19.62, and so on.
or if you want to calculate distance st = s0 + v0*t + ½*a*t²

You are absolutely correct--my mistake!!!  [:I] I will remove this incorrect info from my original post...

I was thinking in terms of displacement, and writing velocity... as you state,
xt = x0 + v0*t + 0.5*a*t2
« Last Edit: 29/03/2016 03:42:08 by chiralSPO »

#### Thebox

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##### Re: What is kE?
« Reply #33 on: 29/03/2016 07:51:30 »

no. For 1 kg, F = 1*a (and for 23 kg, F = 23*a)

The horrible persons on other forum learnt me that for 1kg you put 0.1 thank you for the correction.

You also say no for *2,

2nd meter  9.81*2= 19.62

3rd meter 19.62*2= 39.24

Is that not correct?

added- sorry correction, I did it correct in the first place

acceleration is 9.81m/s2

for the first  meter

9.81m/s

2nd meter

9.81m/s*2

3rd meter

9.81m/s*3

*2 is only for the first one sorry.

« Last Edit: 29/03/2016 08:03:37 by Thebox »

#### alancalverd

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##### Re: What is kE?
« Reply #34 on: 29/03/2016 08:54:51 »
So, to answer the original question,

(1) k.e. is a scalar equal to ½mv2 and

(2) we find that if an object falls from rest through a height h in vacuo, gh =  ½v2.

Which is not at all surprising for those who understand the meaning of "acceleration". The interesting bit is the discovery (phenomenologically attributed to Galileo, maths by Newton) that g = GM/r2 near the surface of a large spherical planet.

Like it or lump it, Mr Box.

#### Thebox

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##### Re: What is kE?
« Reply #35 on: 29/03/2016 09:03:22 »
So, to answer the original question,

(1) k.e. is a scalar equal to ½mv2 and

(2) we find that if an object falls from rest through a height h in vacuo, gh =  ½v2.

Which is not at all surprising for those who understand the meaning of "acceleration". The interesting bit is the discovery (phenomenologically attributed to Galileo, maths by Newton) that g = GM/r2 near the surface of a large spherical planet.

Like it or lump it, Mr Box.

Your answer is either ambiguous or something else.   To me you have just said that kE is a piece of maths and does not exist as E in any sense.

I asked you what is kE?, the question is not what is the maths for kE.

A battery  has energy,   a mass has energy, from what I have understood, science says an objects increases in energy(kE)   ,  so it gains more light?  what is kE?

#### alancalverd

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##### Re: What is kE?
« Reply #36 on: 29/03/2016 14:18:32 »
For the hard of learning:

If you take a lump of stuff with mass m and velocity v, then you times together the mass and velocity and velocity again and halve it, you get a number that grownups call kinetic energy.

If you chop a lump of stuff so that the mass of all the bits added up comes to less than the mass you started with, and the difference is m, then m times the speed of light times the speed of light again is a number called mass-equivalent energy or rest-mass energy.

Energy only "exists" in the minds of the child-hating idiots who write the National Curriculum. For the rest of us, it is a number that tells us something about things that happen.

#### Thebox

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##### Re: What is kE?
« Reply #37 on: 29/03/2016 14:21:36 »

Energy only "exists" in the minds of the child-hating idiots who write the National Curriculum. For the rest of us, it is a number that tells us something about things that happen.

That's what I said, so kE is not really a physical power, it is just a number so pretty meaningless and abstract.

#### jeffreyH

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##### Re: What is kE?
« Reply #38 on: 29/03/2016 19:52:29 »
It may be abstract but it certainly isn't meaningless.

#### alancalverd

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##### Re: What is kE?
« Reply #39 on: 29/03/2016 23:05:43 »
Interestingly, it's what the police mostly talk about in road safety lectures. If Mr Plod understands it, and can somehow transfer that understanding to speeding teenage halfwits, it seems strange that anyone who contributes to this forum has a problem with it.

#### Thebox

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##### Re: What is kE?
« Reply #40 on: 30/03/2016 08:21:05 »
Interestingly, it's what the police mostly talk about in road safety lectures. If Mr Plod understands it, and can somehow transfer that understanding to speeding teenage halfwits, it seems strange that anyone who contributes to this forum has a problem with it.

Oh please, I clearly understand it but like to dig deep for a deeper understanding, I have my answer, it is abstract and not really a physical thing involving energy.

Do falling objects regardless of their mass have the same terminal velocity?
« Last Edit: 30/03/2016 08:33:58 by Thebox »

#### Thebox

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##### Re: What is kE?
« Reply #41 on: 30/03/2016 08:29:07 »
It may be abstract but it certainly isn't meaningless.

Well we could describe it mas, height, acceleration= d(x) it will travel

h*a=F

#### hamdani yusuf

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##### Re: What is kE?
« Reply #42 on: 30/03/2016 11:08:44 »
Do falling objects regardless of their mass have the same terminal velocity?
No. Small rain drops have lower terminal velocity than bigger rain drops, even though they consist of the same material.
Terminal velocity of a falling object is achieved when down force by gravity equals up force by air resistance, hence it is affected by many factors, e.g. air pressure, temperature, humidity, and the object's density, form, orientation, size, etc.

#### alancalverd

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##### Re: What is kE?
« Reply #43 on: 30/03/2016 17:13:53 »
I clearly understand it but like to dig deep for a deeper understanding

There's nothing deeper to understand. The kinetic energy of a mass m moving at velocity v is ½mv2. It's a useful concept because (by experiment) energy is conserved in classical physics, so we can use it to predict what happens when our moving object interacts with something else.

#### Thebox

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##### Re: What is kE?
« Reply #44 on: 31/03/2016 14:29:43 »
I clearly understand it but like to dig deep for a deeper understanding

There's nothing deeper to understand. The kinetic energy of a mass m moving at velocity v is ½mv2. It's a useful concept because (by experiment) energy is conserved in classical physics, so we can use it to predict what happens when our moving object interacts with something else.

I am sure there is another present piece of maths that gives the exact same answer, something involving the equivalance principle.   Is it F=ma?

#### jeffreyH

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##### Re: What is kE?
« Reply #45 on: 31/03/2016 18:17:03 »
Except that kinetic energy that is constant has no acceleration. Therefore there is zero force. As well as having stated F=ma, Newton also has a first law which precludes acceleration. You should look it up. It describes states with no forces acting on an object. So that an object at rest remains at rest and an object with constant speed carries on in a straight line with the same constant speed. Because NO FORCES ARE ACTING UPON THE OBJECT!

#### Thebox

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##### Re: What is kE?
« Reply #46 on: 01/04/2016 10:55:11 »
Except that kinetic energy that is constant has no acceleration.

A piece of maths has no acceleration or velocity or anythng else for that matter because it is abstract.   I think you mean the object in motion will remain in motion unless acted upon by external forces.  An object in spacial motion away from an inertial reference frame, remains without force until it impacts another body  . A falling body that the force is redirected of, will continue in a parallel motion until the potential force disperses an dis lost to the external force of gravity.

p=F

p0=Fn

var(p)=var(Fn)

Newtons third law ''the ground pushes back'',

It is positive energy that pushes back and is the resistance to compression.

« Last Edit: 01/04/2016 11:11:29 by Thebox »

#### evan_au

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##### Re: What is kE?
« Reply #47 on: 01/04/2016 12:53:31 »
Quote from: TheBox
I am sure there is another present piece of maths that gives the exact same answer, something involving the equivalance principle.

This thread is discussing conservation of Energy (of which Kinetic Energy and Potential Energy are two forms), under the influence of a gravitational field.

Emmy Noether came up with a very general concept which links conservation laws to symmetries in nature.

The Conservation of Energy (and its component Kinetic Energy) is proved in the following example:
http://en.wikipedia.org/wiki/Noether%27s_theorem#Example_1:_Conservation_of_energy

Occasionally, conservation of momentum comes up in this thread - this is another principle which can be proved by Noether's theorem.

Quote from: TheBox
Do falling objects regardless of their mass have the same terminal velocity?
"raindrops falling through air to reach terminal velocity" is a complex system in which it is extremely hard to add up all the tiny contributions of kinetic energy which are distributed amongst all the individual air molecules. So if you wish to understand kinetic energy, look at objects falling in a vacuum (or the proverbial cannonballs falling from the leaning tower of Pisa) - it is so much easier to analyze.

You can see something real, without being diverted into fractal flurries of turbulence which dissipate lots of energy and get you nowhere.

Quote
p0=Fn etc
The mathematical notation used in Noether's Theorem may look superficially similar to some equations previous posted in this thread. It is not.

#### Thebox

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##### Re: What is kE?
« Reply #48 on: 01/04/2016 15:04:53 »
Quote from: TheBox
I am sure there is another present piece of maths that gives the exact same answer, something involving the equivalance principle.

This thread is discussing conservation of Energy (of which Kinetic Energy and Potential Energy are two forms), under the influence of a gravitational field.

Emmy Noether came up with a very general concept which links conservation laws to symmetries in nature.

The Conservation of Energy (and its component Kinetic Energy) is proved in the following example:
http://en.wikipedia.org/wiki/Noether%27s_theorem#Example_1:_Conservation_of_energy

Occasionally, conservation of momentum comes up in this thread - this is another principle which can be proved by Noether's theorem.

Quote from: TheBox
Do falling objects regardless of their mass have the same terminal velocity?
"raindrops falling through air to reach terminal velocity" is a complex system in which it is extremely hard to add up all the tiny contributions of kinetic energy which are distributed amongst all the individual air molecules. So if you wish to understand kinetic energy, look at objects falling in a vacuum (or the proverbial cannonballs falling from the leaning tower of Pisa) - it is so much easier to analyze.

You can see something real, without being diverted into fractal flurries of turbulence which dissipate lots of energy and get you nowhere.

Quote
p0=Fn etc
The mathematical notation used in Noether's Theorem may look superficially similar to some equations previous posted in this thread. It is not.

That is new to me, I have no idea what it means yet.

Φ→← =  Φ←→  = 4/3 pi r³

>Φ→← = <4/3 pi r³

>Φ←→ = >4/3 pi r³

E=>Φ→←

#### alancalverd

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##### Re: What is kE?
« Reply #49 on: 01/04/2016 17:41:08 »
I am sure there is another present piece of maths that gives the exact same answer, something involving the equivalance principle.   Is it F=ma?
Kind of.

Energy = work = force x distance = (mass x acceleration) x distance =  mass x ½(velocity change)2, if you think about it.

Classical physics really is very simple. It's all about precise definitions of mathematical quantities, and finding the conservation laws that apply to them. And as I've said many times before, dimensional analysis will always resolve any mistakes or misunderstandings.

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##### Re: What is kE?
« Reply #49 on: 01/04/2016 17:41:08 »