Kinetic energy object vs opposing KE object?

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53 replies · 5K views
haruspex said:
If I understand your arguments correctly, if a mass m1, KE E1, has a head-on collision with a mass m2, KE E2<E1, each does E2 of work, leaving E1-E2 for their subsequent combined KE. Is that your thinking?
If so, consider m2 initially stationary, so E2=0. It would follow that no work is done in the collision, so it must be completely elastic (!)

Not combining...but subtracting KE...KE is used up...not added in this scenario...KE goes into work...
 
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normal_force said:
Not combining...but subtracting KE...KE is used up...not added in this scenario...KE goes into work...
Not sure what you mean. Please express that in terms of equations. What I wrote seems to match the logic you applied in post #1.
 
haruspex said:
Not sure what you mean. Please express that in terms of equations. What I wrote seems to match the logic you applied in post #1.

Okay, a moving object posses Kinetic energy...thus mechanical energy, thus the ability to do work...even without a constant accelerating force. Thus if an object has Kinetic energy, it has the ability to do work because of its energy by a virtue of motion.

Here is my equation

(TMEi ball - TMEi ball energy lost) + (- TMEi cart - (-TME cart energy lost)) = TMEf System (TMEi ball - TMEi ball energy lost) + (- TMEi cart - (-TME cart energy lost)) = TMEf System
(607.5J ball - 202.5J ball energy lost) + (- 354.37J cart - (-153.37 J cart energy lost)) = Ball 204 Joules Cart 0 Joules ---> Ball 204 Joules KE-->work-->cart displaced -->KE--> Work.
 
normal_force said:
Okay, a moving object posses Kinetic energy...thus mechanical energy, thus the ability to do work...even without a constant accelerating force. Thus if an object has Kinetic energy, it has the ability to do work because of its energy by a virtue of motion.

Here is my equation

(TMEi ball - TMEi ball energy lost) + (- TMEi cart - (-TME cart energy lost)) = TMEf System(TMEi ball - TMEi ball energy lost) + (- TMEi cart - (-TME cart energy lost)) = TMEf System
(607.5J ball - 202.5J ball energy lost) + (- 354.37J cart - (-153.37 J cart energy lost)) = Ball 204 Joules Cart 0 Joules ---> Ball 204 Joules KE-->work-->cart displaced -->KE--> Work.
How are you calculating those lost energies?
 
haruspex said:
How are you calculating those lost energies?

For the sake of getting the question, I just made up a value...thats all
 
normal_force said:
For the sake of getting the question, I just made up a value...thats all
That's a strange thing to do. How will you ever find the final velocities with any certainty that way? If we are given that the two objects coalesce you can use conservation of momentum to find the final velocity and the energy lost.
 
normal_force said:
For the sake of getting the question, I just made up a value...thats all
... Moreover, consider the equal initial KEs case, with no internal losses of energy. This should be an elastic collision, but your formula says there is no KE left.
 
well, this isn't a conservation of momentum problem, its better to use Work-energy, as this is what it is...its an example of mechanical energy to do work, that a small object with superior KE, inferior mass and momentum can stop a cart and do work on it.
 
normal_force said:
well, this isn't a conservation of momentum problem, its better to use Work-energy, as this is what it is...its an example of mechanical energy to do work, that a small object with superior KE, inferior mass and momentum can stop a cart and do work on it.
That is your claim, but everyone else on the thread disagrees with you. Please try to answer my post #37.
 
haruspex said:
That is your claim, but everyone else on the thread disagrees with you. Please try to answer my post #37.
Oops, sorry, didn't see it...

Okay, well that is because the KE is taken up to do work...

Look, Imma try to explain this a lot better...but you are not making sense.
 
I showed in my equation the energy lost which was not 100% of the energy, a great deal of energy dissipated...but there is some energy left to do work...as it clearly shows.
 
normal_force said:
I showed in my equation the energy lost which was not 100% of the energy, a great deal of energy dissipated...but there is some energy left to do work...as it clearly shows.
In my post #37, I applied your formula from your post #33 to the case of two masses of equal initial KE colliding head on elastically. This is the case I am challenging your logic on. According to your formula there is no KE left.
 
haruspex said:
In my post #37, I applied your formula from your post #33 to the case of two masses of equal initial KE colliding head on elastically. This is the case I am challenging your logic on. According to your formula there is no KE left.
Well, in short

(TMEi ball - TMEi ball energy lost) + (- TMEi cart - (-TME cart energy lost)) = TMEf System

(607.5J ball - 202.5J ball energy lost) + (- 354.37J cart - (-153.37 J cart energy lost)) = Ball 204 Joules Cart 0 Joules ---> Ball 204 Joules KE-->work-->cart displaced -->KE--> Work.

(405 JOULES)+ (-201 Joules cart) = 204 Joules remaining of ball...so it stopped the cart...but as long as its in contact with the cart...and is not elastic like a spring and compresses too much...it will use up the remaining Kinetic energy in non-conservative work...so the ball doesn't rebound, all energy is used up...no elastic potential...so in the end...

The ball stopped the cart, and pushed back the cart.
 
normal_force said:
Well, in short

(TMEi ball - TMEi ball energy lost) + (- TMEi cart - (-TME cart energy lost)) = TMEf System

(607.5J ball - 202.5J ball energy lost) + (- 354.37J cart - (-153.37 J cart energy lost)) = Ball 204 Joules Cart 0 Joules ---> Ball 204 Joules KE-->work-->cart displaced -->KE--> Work.

(405 JOULES)+ (-201 Joules cart) = 204 Joules remaining of ball...so it stopped the cart...but as long as its in contact with the cart...and is not elastic like a spring and compresses too much...it will use up the remaining Kinetic energy in non-conservative work...so the ball doesn't rebound, all energy is used up...no elastic potential...so in the end...

The ball stopped the cart, and pushed back the cart.
Stop dodging the issue. As I already explained twice, I'm asking you to apply the same logic to a slightly different set-up: one in which the two masses have the same initial KE and collide elastically.
 
haruspex said:
Stop dodging the issue. As I already explained twice, I'm asking you to apply the same logic to a slightly different set-up: one in which the two masses have the same initial KE and collide elastically.

Okay...well, the final velocity: -80.2 m/s for ball and cart is 5.22 M/s...

I get that, what is the point?
 
normal_force said:
Okay...well, the final velocity: -80.2 m/s for ball and cart is 5.22 M/s...

I get that, what is the point?
No, not your ball and cart; the situation I posed in post #37: two arbitrary masses with equal KE. According to your reasoning in post #33, the KEs should completely cancel, leaving none.
 
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haruspex said:
No, not your ball and cart; the situation I posed in post #37: two arbitrary masses with equal KE. According to your reasoning in post #33, the KEs should completely cancel, leaving none.

Okay, but I think you are miss reading me...

thinking of this, compression occurs, maybe a little...like 0.000098 meters...KE lost, thus it is doing work, Wnc to be correct. So, KE turns into work, that what stops it...
 
normal_force said:
Okay, but I think you are miss reading me...

thinking of this, compression occurs, maybe a little...like 0.000098 meters...KE lost, thus it is doing work, Wnc to be correct. So, KE turns into work, that what stops it...
We are only going to resolve this if you make clear what your reasoning is in a form that can be applied to all circumstances. In your post #33 (and in post #1 I think) your reasoning appears to be that if two masses collide head-on with KEs E1 and E2 < E1 then E2 of the one cancels E2 of the other, leaving E1-E2 as remaining total KE. If that is not your reasoning please state carefully what it is. (You have a habit of writing incomplete sentences, trailing off with ..., which leaves them rather vague. You also keep answering in terms of specific examples, which though useful leaves the general principle unclear.)
 
haruspex said:
We are only going to resolve this if you make clear what your reasoning is in a form that can be applied to all circumstances. In your post #33 (and in post #1 I think) your reasoning appears to be that if two masses collide head-on with KEs E1 and E2 < E1 then E2 of the one cancels E2 of the other, leaving E1-E2 as remaining total KE. If that is not your reasoning please state carefully what it is. (You have a habit of writing incomplete sentences, trailing off with ..., which leaves them rather vague. You also keep answering in terms of specific examples, which though useful leaves the general principle unclear.)
Okay, no, what I am saying is totally different.
 
Well, the Kinetic energy exerts force, it does work...so, my idea is that...they do work...so, its force vs force...work vs work, I guess...
 
normal_force said:
Well, the Kinetic energy exerts force, it does work...so, my idea is that...they do work...so, its force vs force...work vs work, I guess...
Doesn't help. To be usable, you have to be able to express that as a formula. You somehow used your concept to get an answer to your original ball and cart scenario. What actual process/formula did you apply to do that?
 
Closed pending moderation.

Edit: this thread will remain closed, but I wanted to thank haruspex for your extraordinary patience.
 
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