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Paul, spot the difference:

A) Momentum is not force.

B) An object with momentum cannot use its momentum to apply a force to something else.



Quote:

the centripetal force is in a constant direction.
and that direction is towards the center.
the direction of the force never changes it is always towards the center.


Don't you remember I used the word "obviously"? I explained that you can see it from common sense. You didn't see it, that means you misunderstood the whole idea of what I was saying.

Let me help. Drive a car around a round-about. Which direction is the center? Sometimes it's north, sometimes it's south, sometimes it's east, etc. It keeps changing as you go round.

The fact that you didn't try to see that shows you're not interested in working out anything or showing anything about nature. You just want to be contrary. But every trick you try fails because you ignore words you don't understand, so your comments don't relate to what I said.

You might wonder why we can't say its "constantly to the left" or "constantly to the right" of the driver. How about you work that one out? There's a good reason why we don't measure the direction from the point of view of the driver. Google "inertial frame". No, you'll never work it out, so just ignore this whole direction of the center thing.


This fundamental misunderstanding about the meaning of velocity seems to be the root of many of your problems. Why don't you just try to understand it? There are umpteen high school physics tutorials on the internet.

Maybe part of the confusion comes from the same operator symbols being used:

v2 - v1 The difference between two vectors
m2 - m1 The difference between two scalars

The "-" symbol in these two expressions has two different meanings! We reuse the same symbol and even the same word for two different operations. We call them both subtraction but they aren't the same.

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Originally Posted By: kallog

A) Momentum is not force.

correct

Originally Posted By: kallog

B) An object with momentum cannot use its momentum to apply a force to something else.

correct , because is has no force it only has momentum , because its not accelerating.
if the object were accelerating then it could apply a force to another object.

Originally Posted By: kallog

What about the masses in the 1st turn? They apply a force pushing the pipe backwards.

I've already demonstrated to your satisfaction that a mass changing direction will transfer double it's initial momentum to the pipe.


that was before you convinced me that an object that only has momentum cannot apply a force to another object.

there is no force available for a push , because the mass
only has momentum.

they only have momentum , there is no force that is causing them to accelerate through the turn so they dont have increasing angular acceleration so theres no force due to there mass x their acceleration that they can apply to the pipe.

if there was a force causing them to accelerate through the turn then their velocity would increase through the turn.

and when they exited the turn their velocity would be higher than 40 m/s

they only have mass and velocity.







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Paul, you still think velocity is the same as speed.

Why?

Haven't I explained the difference many times?

Haven't you ever bothered to look it up?

Most of what I've said in this thread will be meaningless to a person who doesn't know what velocity and acceleration are. Why don't you just tell me when you don't understand me?

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Originally Posted By: paul
that was before you convinced me that an object that only has momentum cannot apply a force to another object.


You so badly want to say F=mv that when I told you it's wrong you threw out the baby with the bathwater.

F=ma can predict the force the mass applies to the pipe.

But you can't use F=ma unless you know what acceleration means. You don't, so that formula is useless to you.

Wait! Maybe there is hope! Here you show you understand that the object is accelerating through the turn. I don't know why you changed your mind in the last message. This acceleration is how you can find the force. Which I think you also calculated!
Quote:
the mass that is passing through the turn is undergoing centripetal acceleration , this acceleration is toward the center of the turn , not away from the center of the turn.

Last edited by kallog; 09/24/11 11:34 AM.
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Originally Posted By: kallog
You so badly want to say F=mv that when I told you it's wrong you threw out the baby with the bathwater.


I must admit thats pretty much it.
LOL

but now Im really convinced that you were right.
you must have a force that applies to a object to change a objects momentum.
Originally Posted By: kallog

F=ma can predict the force the mass applies to the pipe.

no it cant , because the mass has no force it can apply to the pipe.
f=ma means the mass must be accelerating.
the mass is not accelerating.
just beforce the mass strikes the turn it is not accelerating
therefore it cannot apply a force to the turn.
it only has momentum.
and momentum is not a force.
Quote:

But you can't use F=ma unless you know what acceleration means. You don't, so that formula is useless to you.

from the looks of the above and the remainder of this post its you who doesnt fully understand acceleration.

Originally Posted By: kallog
Paul, you still think velocity is the same as speed.

Why?


dont you know that you can have an increase in velocity without having a change in direction?

when calculating angular acceleration you need to use the torque that is applied to push the object around in the circle.

to do that you must first find the torque.

t = r x f

t = torque
r = radius
f = force

DO YOU SEE VELOCITY IN THE ABOVE EQUATION?
DO YOU SEE FORCE IN THE ABOVE EQUATION?
notice the torque equation uses force , but theres no force applied to the mass that is pushing it around the in the circular path.

heres what I said.

Quote:
and when they exited the turn their velocity would be higher than 40 m/s

they only have mass and velocity.


is 60 m/s higher than 40 m/s?


Im sure your only picking more straws because I was explaining that the velocity of the mass would not increase.

and heres what Im going to make with all of them.



and that means that the velocity of the mass would not increase in the direction of its circular path.

and a velocity increase would be required for its angular acceleration to increase.

the torque equation is above.

angular acceleration = T/I
a = angular acceleration
T = torque
I = moment of Inertia

DO YOU SEE VELOCITY IN THE ABOVE EQUATION?
DO YOU UNDERSTAND THAT A FORCE IS REQUIRED TO HAVE A TORQUE?
DO YOU UNDERSTAND THAT A TORQUE IS REQUIRED TO HAVE ANGULAR ACCELERATION?

there is no force being applied to the mass as it follows the circular path so there is no torque that you can use in the above equation.

in other words its not being accelerated by a force in the direction of its circular path so it does not have angular acceleration , if you would notice I was talking about the angular acceleration of the mass which requires a increase in torque.

Originally Posted By: paul
they only have momentum , there is no force that is causing them to accelerate through the turn so they dont have increasing angular acceleration so theres no force due to there mass x their acceleration that they can apply to the pipe.


there is no increase in torque so there cannot be a increase in angular acceleration.

Quote:
Wait! Maybe there is hope! Here you show you understand that the object is accelerating through the turn. I don't know why you changed your mind in the last message. This acceleration is how you can find the force. Which I think you also calculated!


Quote:
Quote:
the mass that is passing through the turn is undergoing centripetal acceleration , this acceleration is toward the center of the turn , not away from the center of the turn.


kallog , a toward the center of the turn acceleration
does not mean that it is accelerating along the circular path.

angular acceleration = along the circular path

not towards the center of the circle.

it is accelerating towards the center of the circular path.
so I was right when I said that it has centripetal acceleration.

and I was right when I said that it has no angular acceleration.
it has angular velocity but its angular velocity is not increasing , its constant angular velocity is 40 m/s throughout the entire turn because we are using frictionless turns.

note: the only way that I can use the correct symbolism to prevent you from satisfying your straw picking habit would be to use 180 different angles in a 180 degree turn.

so I only used 40 m/s as the constant angular velocity through the turn.

I supposed I could have used the following
as the mass traverses the 180 degree turn from
40 m/s 0 degrees to 40 m/s 180 degrees
I was just thinking that you could follow that and I would not have write so much descriptive text to enable you to
understand.

heres a visual tool you can use to understand the concept of centripetal acceleration.

the blue dotted semi circle is the circular path that the mass follows , its not the direction of acceleration.

the direction of acceleration is the straight blue dotted line.

which may be where you are getting confused.



heres one that illustrates the direction of the centripetal force that causes the centripetal acceleration one that you might find less challenging to understand.



Quote:
This acceleration is how you can find the force. Which I think you also calculated!


you can calculate the force that the pipe applies to the mass
that accelerates the mass towards the center.
but the mass has no force it can apply to the pipe that can be used in a calculation.
the mass only has momentum.
the pipe has both momentum and force.

but maybe you cannot comprehend such complicated things as you have demonstrated in the past.



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from what I can tell.

the pipe has force of 8000N
and a momentum of +4000Ns

the mass has no force
and a momentum of -4000Ns

so what will happen when the two collide is their momentums
cancel each other out.

and the pipe continues with a force of 8000N

but your welcome to pick some more straws , and I know you will.






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Originally Posted By: paul

you must have a force that applies to a object to change a objects momentum.

Yipee.

Quote:

f=ma means the mass must be accelerating.
the mass is not accelerating.

The mass is accelerating. You said it yourself. "centripetal acceleration". That's acceleration towards the center of the turn. That's the acceleration that requires a force. Forget the accelerator, that's already stopped acting, as you said.

Quote:

dont you know that you can have an increase in velocity without having a change in direction?

Indeed you can.
You can also have a change in velocity without a change in speed. That's the point you keep missing.

Quote:

when calculating angular acceleration you need to use the

There is no angular acceleration in the turn! That's nothing to do with this machine. "angular acceleration" is a totally different concept from "centripetal acceleration". The different words indicate there is a different meaning.



Quote:
and when they exited the turn their velocity would be higher than 40 m/s

You're overthinking something trivial, but it's leading you to a completely wrong conclusion.


Before the turn, v=-40m/s
After the turn v=40m/s
Obviously (just look at the numbers), those two velocities are not the same. When velocity changes the thing accelerates.

I agree the speed is the same, 40m/s all through the turn and out the other side. But acceleration can occur without any change in speed. And in this case it does, with a force too according to F=ma.

Again, google "acceleration" and "velocity" if you're still unsure.


Quote:

you can calculate the force that the pipe applies to the mass
that accelerates the mass towards the center.
but the mass has no force it can apply to the pipe that can be used in a calculation.
the mass only has momentum.
the pipe has both momentum and force.

I thought you were being sarcastic with "it has only momentum so it can't apply any force". Now you seem to be almost serious. This is where you can simply throw a book at the wall and see that it deforms, and releases energy, and stops flying. All things that show a force was applied.

While the book is flying it's not applying any force, but as soon as it hits the wall it starts accelerating. The same F=ma again.

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Summary:

There is no angular acceleration in the turn.

There is no change in speed in the turn.

There is a change in velocity in the turn.

There is centripetal acceleration in the turn.

The pipe applies a force to the mass to cause that centripetal acceleration. (F=ma)

The mass applies a matching reaction force to the pipe (Newton's 1st law).


If you disagree, look up "acceleration" and "velocity".

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Originally Posted By: paul

the pipe has force of 8000N
and a momentum of +4000Ns

the mass has no force
and a momentum of -4000Ns


I should have picked up this misconception earlier. Below is where you made the mistake leading to the meaningless claims above. The explanations you quoted give a more correct picture.

Quote:

Force is between objects
Force is like giving some of this momentum to another object every second they are in contact.

Momentum is within an object
carried with the object
Momentum is like what the object has in its bank account

Paul: I would have to say that what is has in its bank account is force.


"between objects" means you need two objects for a force to exist between them. It's not something carried by objects. It just appears when they interact. It's not conserved - a small slow moving object can apply a huge force, or a big fast object can be stopped by a tiny force. It all depends on the details of the collision.

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Quote:
There is no angular acceleration in the turn.

Quote:
There is no change in speed in the turn.


finaly.

I didnt write the below.

Quote:
Force is between objects
Force is like giving some of this momentum to another object every second they are in contact.

Momentum is within an object
carried with the object
Momentum is like what the object has in its bank account


at least now you understand that there is no angular acceleration in the turn.

which is what I've been trying to communicate to you.


Quote:
the pipe has force of 8000N
and a momentum of +4000Ns

the mass has no force
and a momentum of -4000Ns


I should have picked up this misconception earlier.


theres no misconception at all.

the pipe has a mass of 1000 kg and there are 100 100kg masses being accelerated by the pipe ...

each of the 100 kg masses is being accelerated by a force of 80N

50 accelerating
50 decelerating

80N x 100 = 8000N

the pipe can apply a force of 8000N to another object.

the pipe has a mass of 1000 kg and a velocity of +4 m/s

p=mv = 1000 kg x +4 m/s = +4000Ns


the mass has no force being applied to it.
so the mass has no force it can apply.

the mass has a velocity of -40 m/s and a mass of 100 kg

p=mv = 100 kg x -40 m/s = -4000Ns

if you cant understand the above then we might as well just quit trying.

if all your going to do is take things out of context then the same applies.

I say the pipe has more force being applied to it in the (+) direction than any and all other influences that are in the (-) direction.

and the above causes the pipe to accelerate.

I have shown my reasons why I think this , if you disagree then show your reasons.




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Originally Posted By: paul
finaly.

Yes, finally I wrote it in enough different ways that you understood one of them. But we're not going to make any progress until you actually know what "acceleration" and "velocity" mean. Without knowing what acceleration is you can't use F=ma. Without knowing what velocity is you can't use p=mv or find a from the rate of change of v. This is why you're getting wrong results.

Quote:

the pipe can apply a force of 8000N to another object.


No. This is a 3rd misunderstanding (after v and a) that you need to solve before you can understand the system.

The force used to accelerate something doesn't determine the force it can apply to something else.

Eg: Accelerate a truck to 60mph in 20s then either:

A) Shift into neutral and let it coast to a stop.
B) Crash it into a cliff.

Do both cases apply the same force to the environment?


Extra question:

Change the 20s to 2 hours. Does that change the force in A or B?

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Originally Posted By: paul
the mass has no force being applied to it.
so the mass has no force it can apply.

Wrong. The mass can apply a force. The mass cannot apply a force while no force is applied to it. However it can always apply a force sometime in the future, for example when it touches something. This is common sense Paul. A moving object will apply a force when it hits something.

I've already completely solve the system for 1 mass. You haven't even done that. Your reasoning contains many mistakes, which I keep pointing out, such as the one above.

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Quote:
The force used to accelerate something doesn't determine the force it can apply to something else.


yes it does.

the pipe has a constant force being applied to it.
even during the collision!!!
and when it comes in contact with something else it will apply that force to it.

if the 1000 kg pipe comes in contact with another 1000 kg
object the force of +8000N that is accelerating the 1000 kg pipe will apply itself to the other 1000 kg object.

so that both 1000 kg masses will be being accelerated by the +8000N force.

I believe that you are thinking about the momentum of the pipe , momentum isnt force.

notice that I wasnt talking about momentum , I was talking about force only.

here it is again...

Quote:
theres no misconception at all.

the pipe has a mass of 1000 kg and there are 100 100kg masses being accelerated by the pipe ...

each of the 100 kg masses is being accelerated by a force of 80N

50 accelerating
50 decelerating

80N x 100 = 8000N

the pipe can apply a force of 8000N to another object.


now below you are again confussing force with momentum.

Quote:
However it can always apply a force sometime in the future, for example when it touches something. This is common sense Paul. A moving object will apply a force when it hits something.


no it cant , in order for the mass to apply a force to another object there must be a force being applied to the mass.

f=ma
its
force = mass x acceleration
not
force = mass x velocity

your talking about momentum and momentum is not force.

the mass has momentum but it has no force being applied to it.

and once again common sence cant be used as an element of an equation.

its like using the below elements in a equation.

common sence force = m*v

there no such force as common sense force.

if you dont believe me , perhaps you will believe yourself.
here is a similar situation from post # 40236 that you posted.

Originally Posted By: kallog
mass x velocity does not have units of force (N)


thats because mass x velocity is not force.
its momentum.

you said that sometimes you do a little research before you make post on the web.

maybe its a good time to do a little now before you get too confused.

correcting all of your mistakes is really getting to be a real pain.

so take a day off and do a little research.



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we are discussing the events after 101 seconds.

this will include the 10,100 kg of the 101 100kg masses.

at the start the pipe and the 101 masses have a combined mass of 11,100 kg.

after the first second the pipes mass decreases to 11,000 kg
because 1 of the 100 kg masses is accelerated toward the turn

50 masses are launched before the 1st 100 kg mass enters the turn.

and as the 1st 100 kg mass enters the turn another 100 kg mass is being launched.

the force of 80N applied to the first mass will accelerate the 11,000 kg pipe to an acceleration of .007 m/s/s

at the end of the 1st second the pipe will have a velocity of
+.007 m/s

after each 100 kg mass is launched the total mass of the pipe and masses reduces by 100 kg.

at the end of 50 seconds the pipe has a mass of 6100 kg and it has a velocity of +12.84 m/s !!!

its momentum is 6100 kg x 12.84 m/s = 78,324 Ns

so there are more things to consider than just your inability to understand the difference between force and momentum.

however you should really try to understand that you would really have a hard time stopping or even considerably slowing down a object that has a +78,324Ns momentum
with an object that has a mere -4000Ns momentum.

and that is only after the first 50 seconds.

so before there is any (-) influences that would impede the motion of the pipe.

it already has 78,324 Ns momentum.

and it already has a force being applied to it of
+4000N

looks to me like I just knocked all the legs off of the table that you were using.

and your whole side of the discussion just dissapeared.



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Originally Posted By: paul
if the 1000 kg pipe comes in contact with another 1000 kg
object the force of +8000N that is accelerating the 1000 kg pipe will apply itself to the other 1000 kg object.


OK, yes. If the objects stick together. If they bounce off (like ours) then the force depends on details of the collision and is not determined only by the 8000N applied force - it can be much higher or lower.

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Im glad you said that.

what would happen if a 6100 kg mass that has a momentum
of (+)78,000Ns comes in contact with a 100 kg mass that has a momentum of (-)4000Ns?

would they bounce off of each other?

(+)78,000Ns vs (-)4000Ns = ?

or wouldnt the object with (+)78,000Ns keep going in the same direction and the object with (-)4000Ns bounce off of the object with (+)78,000Ns







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Originally Posted By: paul
at the end of 50 seconds the pipe has a mass of 6100 kg and it has a velocity of +12.84 m/s !!!

its momentum is 6100 kg x 12.84 m/s = 78,324 Ns

OK. I didn't check the number, but it the reasoning sounds good.

Quote:

however you should really try to understand that you would really have a hard time stopping or even considerably slowing down a object that has a +78,324Ns momentum
with an object that has a mere -4000Ns momentum.


A hard time? We have to transfer -78324Ns of momentum to the pipe to stop it. How much momentum will all those masses give it as they pass through the turn?

Quote:

looks to me like I just knocked all the legs off of the table that you were using.

and your whole side of the discussion just dissapeared.

Have you noticed that you regularly make claims like this? Have you also noticed that in every case you have still been wrong?

You've just shown what happens when you fire 50 masses inside a pipe and they don't hit anything. That part's noncontroversial. The next two stages determine if the pipe keeps going or returns to its starting point. You haven't done them yet.

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Originally Posted By: paul
what would happen if a 6100 kg mass that has a momentum
of (+)78,000Ns comes in contact with a 100 kg mass that has a momentum of (-)4000Ns?

would they bounce off of each other?


I don't know! It's your concept. Would they? Do they have elastic ends? Do they have energy absorbing bumpers? Do they somehow fuse together?

If it's an elastic collision like our pipe, then you can work out the resulting velocities (or momentums) with one of the online calculators, or the formula on Wikipedia. It's a bit time consuming to derive.


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Quote:
A hard time? We have to transfer -78324Ns of momentum to the pipe to stop it. How much momentum will all those masses give it as they pass through the turn?


I would say each 100 kg mass would transfer only (-)4000Ns
each second.

Quote:
We have to transfer -78324Ns of momentum to the pipe to stop it.


yes you do !!!
and you cant do that with only (-)4000Ns


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Quote:
then you can work out the resulting velocities (or momentums) with one of the online calculators

https://www.msu.edu/~brechtjo/physics/airTrack/airTrack.html

okay

red cart 6100 kg mass v +12.84 m/s
blue cart 100 kg mass v -40 m/s

results show that the blue cart bounces off of the red cart
the red cart continues in the + direction.

red cart velocity = +11.13 m/s

blue cart velocity = +63.97 m/s

neither cart is traveling in the negative direction.




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