# The Naked Scientists Forum

### Author Topic: Kinetic Energy...what's that all about ?  (Read 7931 times)

#### neilep

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##### Kinetic Energy...what's that all about ?
« on: 17/08/2007 18:03:33 »
Kinetic energy is weird.

It allows the ball at each end to transfer the energy it has through the balls to the other
!

Amazing !

But what is it ?..and how comes it's transferred so easily ?...is there a limit to the number of balls ?...could it be a hundred ?

The ball then is just a conduit for the energy yes ? and does not have sole ownership of the thing called kinetic energy !

Say something is stopped very quickly...does the kinetic energy then leave the thing and just continue on outside the thing it was in ?

#### JP

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##### Kinetic Energy...what's that all about ?
« Reply #1 on: 17/08/2007 19:16:15 »
Kinetic energy is simply the energy contained in a moving object by virtue of its motion and is given by the equation KE=1/2*m*v^2, where m is the object's mass, and v is its velocity.

The balls shown above are an (approximate) example of what's called an "elastic collision."  Elastic collisions are collisions between objects which conserve the total kinetic energy.  Obviously these are idealized situations, but the above ball trick is a good approximation to an elastic collision.

There's one more concept you need in order to understand the balls above: the conservation of momentum.  Momentum is a quantity contained by a moving object and is given by p=m*v (momentum also carries with it a direction, but we won't worry about that here).  In a closed system, momentum is always conserved, just like energy.

In this particular ball system (an elastic collision), the only way to conserve both energy and momentum is for the first ball to transfer all its energy and momentum to the last ball.  If one of the middle balls were to start moving, it would have to push a bunch of the other balls with it (since they're all touching), and that would violate one of the two conservation laws.

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is there a limit to the number of balls ?...could it be a hundred ?
There isn't a limit in theory if all the balls are perfectly elastic (no kinetic energy is lost).  Energy cannot be created or destroyed in a closed system, so the only way to lose kinetic energy is if its transferred into some other form of energy.  In practice, each ball you add probably causes energy to be lost due to friction, so there would be some limit.

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The ball then is just a conduit for the energy yes ? and does not have sole ownership of the thing called kinetic energy !
Kinetic energy is an accounting tool: it's like the energy "cash" the ball has by virtue of its speed and mass.  If it changes either speed or mass, it will lose or gain some energy.

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Say something is stopped very quickly...does the kinetic energy then leave the thing and just continue on outside the thing it was in ?
Kinetic energy only exists in moving objects with mass, so it can't just continue on if an object stops.  If something stops quickly, the kinetic energy it had has to go somewhere (since total energy can't be created or destroyed).  This could either go into kinetic energy of another object or objects, such as the balls above, or it could be into other forms of energy (such as a car's brakes, which transform kinetic energy into heat* energy through friction).

*Heat is in turn just a measure of the kinetic energy of the atoms and molecules in something as they jiggle about.

#### neilep

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##### Kinetic Energy...what's that all about ?
« Reply #2 on: 17/08/2007 19:52:16 »
OUTSTANDING !!

THANK YOU so much for the time you have taken to address my questions and answer each part individually in such a way that even I can get a grip on what you have said.

THANK YOU again.

#### syhprum

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##### Kinetic Energy...what's that all about ?
« Reply #3 on: 17/08/2007 20:00:38 »
The group of balls can be likened to an electrical delay line or coaxial cable, a pulse is injected at the input travels down the line to the end where there is no termination so it is reflected back to the input where again it travels down the line and so repeats until dissipated by the inherent losses.
This is very easy to demonstrate with the aid of an oscilloscope and a drum of coaxial cable.

#### neilep

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##### Kinetic Energy...what's that all about ?
« Reply #4 on: 17/08/2007 20:27:57 »
The group of balls can be likened to an electrical delay line or coaxial cable, a pulse is injected at the input travels down the line to the end where there is no termination so it is reflected back to the input where again it travels down the line and so repeats until dissipated by the inherent losses.
This is very easy to demonstrate with the aid of an oscilloscope and a drum of coaxial cable.

WONDERFUL!

Thanks syhprum for helping me understand the nature of kinetic energy.

It just seems such a strange phenomena in particular with the above group of balls as  the kinetic energy continues to flow through the 'inbetween'  balls which in themselves remain still only to be introduced as movement in the last ball. Presumably the speed of the kinetic energy (as I am learning) is dependent on the speed of the object in the first place !

#### syhprum

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##### Kinetic Energy...what's that all about ?
« Reply #5 on: 18/08/2007 07:02:20 »
The speed that the energy flows down the the line depends on the speed of sound in the steel balls and is not affected by how hard the first ball is struck (it can be likened to the propagation speed of the electrical energy in the coaxial cable which is about 0.9c)
There is actually some very small movement in the intermedate balls but it happens very fast, as each ball is struck a wave of compression moves thru it at the speed of sound until it reaches the next ball whence the energy is transfered to that one.
If a much slowed down film was taken no doubt this could be seen (the use of rubber cylinders instead of steel balls would make the effect more apparent).
The large movement of the last ball occurs because the energy is converted to a velocity which lifts it against gravity until it reaches a zenith then falls back sending its stored energy back down the line
« Last Edit: 18/08/2007 07:10:18 by syhprum »

#### neilep

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##### Kinetic Energy...what's that all about ?
« Reply #6 on: 18/08/2007 19:28:16 »
THANK YOU again syhprum.

The speed of sound within the steel balls themselves eh ?..

At the time that the ball reaches it's zenith , I take it at that precise moment there is no kinetic energy stored !..it is only as it proceeds to gather speed on the way back that it regains it's kinetic energy yes ?

Thank you for helping me understand.

#### lightarrow

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##### Kinetic Energy...what's that all about ?
« Reply #7 on: 18/08/2007 21:31:16 »
THANK YOU again syhprum.

The speed of sound within the steel balls themselves eh ?..

At the time that the ball reaches it's zenith , I take it at that precise moment there is no kinetic energy stored !..it is only as it proceeds to gather speed on the way back that it regains it's kinetic energy yes ?

Thank you for helping me understand.
Ok, Neil. This time you take the part of the expert. I have a question for you:
if sound's speed in steel is 5,000 m/s and earth's circumference is 40,000 km, after how much time you will see moving the last of a series of steel balls all along earth's circumference?

(Clearly this is an ideal situation, since there is a tiny loss of energy in each collision, which would prevent the movement of a ball after a great number of them, as jpetruccelli explained).
« Last Edit: 18/08/2007 21:41:19 by lightarrow »

#### neilep

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##### Kinetic Energy...what's that all about ?
« Reply #8 on: 18/08/2007 22:01:27 »
This isn't fair....it's my brother who is the mathematical genius !!...but I reckon it'll take 8000 seconds...that being 2.22 hours ?.....

....Do I win a prize ?...I'll take the car !!

#### lightarrow

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##### Kinetic Energy...what's that all about ?
« Reply #9 on: 19/08/2007 15:25:17 »
This isn't fair....it's my brother who is the mathematical genius !!...but I reckon it'll take 8000 seconds...that being 2.22 hours ?.....

....Do I win a prize ?...I'll take the car !!
Ok, and how many hours, minutes and seconds are 2.22 hours?

"it's my brother who is the mathematical genius !!"

Now I understand why you ask so many questions on this Forum! You don't want to deface at his comparison!
« Last Edit: 19/08/2007 15:27:09 by lightarrow »

#### neilep

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##### Kinetic Energy...what's that all about ?
« Reply #10 on: 19/08/2007 15:41:45 »
Is it 133 minutes and 12 seconds !! ?..I'l take the round the world cruise !!...does your city have a port Alberto ?

Fortunately, my brother has no knowledge of my existence here !

#### lightarrow

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##### Kinetic Energy...what's that all about ?
« Reply #11 on: 21/08/2007 14:10:30 »
Is it 133 minutes and 12 seconds !! ?..I'l take the round the world cruise !!...does your city have a port Alberto ?

Fortunately, my brother has no knowledge of my existence here !
Right answer! You have won a medal! When you have the possibility to come here, I'll be honored to give you!

#### The Naked Scientists Forum

##### Kinetic Energy...what's that all about ?
« Reply #11 on: 21/08/2007 14:10:30 »