# Walter Lewin demonstrates free momentum in an MIT lecture

Discussion in 'The Guru's Pub' started by UnrealGaming, Jun 13, 2021.

1. ### AsiJuAncient Guru

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All of which require work = energy.

Differentiating momentum, energy and work (and power) can be confusing as the concepts are quite abstract, not something you can "see".

While it's not a 1:1 accurate analogy, think of momentum as an object's ability to uphold its motion or resist a change in motion (related to Newton's First Law).
The theoretical infinite mass, infinitely low speed car with infinite momentum would just keep going until something stopped it.
Do note in all practical real world conditions at least some disruptive forces always exist so you'll never see something moving ad infinitum, even a lubricated piece of smooth steel on smooth ice will eventually stop.

Also remember that to get the first car moving, energy must be consumed and work on the object done (=exert a force on it to give it acceleration).
This is the maximum amount of energy or work any of the cars in the sequence are able to do.

No matter how many x2 times heavier cars you have on your experiment, the last one cannot compress any bigger a spring than you could by the force you used to set the first car in motion.

PS: I'm fairly certain the experiment you described hasn't been made as it doesn't really "make sense".
Feel free to try it out though!

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2. ### NoisivAncient Guru

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This is Newtonian dynamics. 3 Newton laws are all there is to it.
Conservation of energy follows directly from 2nd Newton law. You are free to ignore it, but you do so at your own peril.

Knowing momentum does not tell you anything about the work that the system can do. Energy is what gets work done, not momentum.

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3. ### UnrealGamingAncient Guru

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Yeah i know. I'm just indulging this scenario out of curiosity. If anyone has a link to a well done experiment, where a 3x more momentum can't move a bigger object in space, compress a spring more ( or compress a bigger spring ) and so on and so on. Please link it. And again thank you both Noisiv and AsiJu for the replies.

And I would totally try it myself, If had all the elements to properly set it up and so on.

Last edited: Jun 14, 2021
4. ### NoisivAncient Guru

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in elastic collision you can't get 3 times more momentum for 2nd body.

it can be shown that 2 times bigger momentum is maximum. and this case happening when 2nd body mass goes to infinity.

1 = -1 +2

with all the kinetic energy being concentrated in smaller body that rebounds at equal speed it went in, despite having only 1/2 of momentum of infinite mass body.

5. ### UnrealGamingAncient Guru

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Based on Lewin's video there is no evidence that the chain reaction would stop before reaching 2x or more momentum than what the initial car had.

Example:

It really comes down to: can an object with 2x or more momentum compress a spring more or not. Push a leaver with which you can restart the reaction on the other end etc. Seems like a super basic experiment that someone should've done at some point.

It's really gonna be you either need the "kinetic energy", which grows and drops exponentially with changes in velocity, or not.

I'd be really interested to see what happens.

6. ### AirbudMaster Guru

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I think you're trying real hard to find perpetual motion....PM me first if you find it!...we get rich.

7. ### UnrealGamingAncient Guru

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When the men in black come knocking at my door, I'm pinning everything on you

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8. ### AirbudMaster Guru

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9. ### NoisivAncient Guru

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we've been over this chain car scenario of yours. car energy converges to zero, momentum to infinity. did we leave something uncovered?

yeah. in the real world you can't have
1) perfectly elastic collision
2) infinite composition of cars

for the nth time (you gonna say you know this already) : momentum does not do work.
and momentum is not quite "object's ability to uphold its motion or resist a change in motion" as AsiJu said - that is (inertial) mass. think of momentum as total amount of motion (relative to your reference system).

Furthermore you don't need any kind of equipment to do kinetic experiments - everything can be done with pen and paper and 3 Newton laws.

And don't put kinetic energy in quotation marks It's a perfectly well defined quantity; nothing vague about it
In our experiment I called it simply energy because no other type of energy was worth considering. Yes, there is spring's potential energy, but only for a split second before it transfers entirely to kinetic. So simply energy, or kinetic energy if you insist

10. ### UnrealGamingAncient Guru

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String theory, 11 dimensions, a new universe being created with every interaction of a subatomic particle etc all work with pen and paper... but we all know that's woo. XD
I want non contrived experimental evidence.
Yes, more velocity can mean spending less time fighting the opposing force, so you end up doing more work. So, maybe you would lose exponentially more ability to do "useful" work, by losing velocity.
But, you are gaining exponentially more mass, which increases your ability to resist the opposing force for longer. Is the bias always for velocity? Isn't there a circumstance where having a giant mass moving slower is able to do more work. Especially in a case where you're doubling the mass, but not losing half the velocity.
If you never do the experiment, how do you know the equations mean anything in reality? Don't trust, verify.

11. ### AsiJuAncient Guru

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Then verify.

Newton's Laws of Motion and their derivatives including conservation of energy and momentum have been experimentally proven.

You can find info from hundreds of sources surely.
The video you linked is an experimental proof of conservation of momentum and energy.

The burden of proof is on the opposite side, if you want to show that an object with increasing momentum but decreasing kinetic energy can do an equal or increasing amount of work, show it.

Also yeah my momentum analogy wasn't good, momentum as measure of motion is better as Noisiv said.

I too easily confuse momentum with inertia.

12. ### UnrealGamingAncient Guru

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As I've said, I would, i just don't have any of the parts required for the experiment. If anyone runs into the experiment i'm looking for, feel free to link it.

The video i linked shows a car with a certain amount of momentum hitting another car, which leaves with more momentum than the first car had. If you do this a couple more times and end up with 2x or 3x more momentum, I'm interested in seeing whether that final car will do more work or not. I couldn't find an experiment demonstrating one way or another. That's it.

If you're not interested, okay. But if anyone is and has seen or done the experiment, link it, or weigh in please. Thx.

Last edited: Jun 15, 2021
13. ### vestibuleMaster Guru

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Obviously its a good idea to obey the laws of the universe when it comes to utilising energy/ Cost. That reminds me I need to lube the bearing on my bicycle wheels.
One of the down sides as some one who is using driver assist technology is how in its current state it pisses energy up the wall. Also its not nice having to second guess your vehicle. But more importantly: Screw the idea of the Chinese will destroy the earth if they all get a refrigerator. This new driver assist bollocks tech WILL: Whoops there goes another gallon and where have my brake pads gone.

Last edited: Jun 15, 2021
14. ### vestibuleMaster Guru

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I made an observation the other day whilst walking along the banks of the river Avon in Stratford uk.
Which was: there was a multi crewed rowing boat with a cox and it behaved largely like a piston engine. As in, it had a stop in its mechanism.

15. ### AsiJuAncient Guru

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The final car cannot do more work, it can do less than the first car by the factor of (2(2/3)^2)^n = (8/9)^n where n is the number of cars in your sequence.
As shown by Noisiv earlier.

Besides, as also stated by Noisiv, in real world conditions disruptive forces would bring the sequence to a stop rather quickly even in optimal conditions.
Some momentum is always "wasted" when objects collide - as completely elastic collisions do not exist - and work must be done by the former object on the latter to get it moving - overcome static friction or rolling resistance +air resistance etc. - so kinetic energy is also consumed on collision.

So any practical applications for a sequence of cars colliding on a level surface can't really be found I think.

PS: before you say how about an incline, that would be a completely different scenario.
Component of gravity would give the cars constant acceleration and they would start gaining kinetic energy.

Even so a practical application where it's better to start with a distant lighter object before striking something with a heavier object at the end of an incline is likely less viable than just exerting the necessary force via means X from the get go.

(Or just use a single object with enough mass)

Last edited: Jun 16, 2021

16. ### UnrealGamingAncient Guru

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You're not saying anything i don't know. Which is why i'm saying, the entire time, let's forget the math for a moment, the theory, and lets see an experiment. I'm talking about Lewin's video and his setup. Nice clean experiment, barely any losses.
The video shows the 2nd car gaining 33% more momentum, than what the initial car had. It does have only 66% the velocity tho, so there is a loss in kinetic energy.
The first car is, lets say, a 10kg car going 10m/s. That's 500joules. The 2nd car is a 20kg car going 6.66 m/s. that's 440joules. But the momentum is p100 for the the first one and p133 for the 2nd one.

I just want to see a well done, non contrived experiment, where the first car ( with its ke500 p100 ) does some work, then you do what Lewin did and have the 2nd car ( with its ke440 p133 ) do some work. That's it. Nothing else needs to be said.

( obviously just example numbers. As long as it's the same setup as in lewin's video, its good. And I'd rather see a 4th car down the line, really. That's now 80kg, going 2.9 m/s, and we're up to 2x+ more momentum. Again, obviously example numbers. Doesn't have to start at 10kg and go up to 80 on the 4th car... it can be exactly lewin's setup, or whatever )

I really cannot be any clearer. And I'm just repeating myself.

So again, if anyone has a link to an experiment I'm describing, link it here please. that's all i'm interested in.

Last edited: Jun 16, 2021

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