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A query on force and system of bodies


IndianScientist
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I don't know if this is a foolish question or a decent one, I don't know if there is any official subjective answer to this, this query is popping in my mind and lately I can't stop myself from thinking what the scientific or 'physical' cause might be....

Please don't mind if it is a question of  a darn fool cause I am only a high schooler from India.     :P         XD

Suppose a strong  man of mass M is pushing a wagon with a force F ,which contains load of same mass M. His force overcomes the static friction and acceleration is caused in the system of the load and a wagon and it moves. 

But consider a case, where the man of mass M is standing in the wagon(in place of load of mass M) and he applies the same force F. The wagon does not move. Whatever mass of the man is and how brute the force he applies, there is no acceleration in the system. Thinking as  a Layman, this is simple. But what is the theoritical physics behind this?

 First I thought that the  weight of the man acting downward  is restricting any acceleration, but it is not the case because the man can push the wagon which is containing a loaf of his same mass exerting the same weight , if he has the strength.

If anyone guides about the correct physics behind this, I will be grateful to that person. Because of this query , I sometimes lose my atten on other subjects.

My drawing is not very good but attached is a rough diagram of the two scenarios

Best wishes

A science aspirant and lover from India 

Quote

 

16119102081881861557280.thumb.jpg.1f88b34fef0dda0c54e738790645a3b1.jpg

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58 minutes ago, IndianScientist said:

I don't know if this is a foolish question or a decent one,

This far from a foolish question and highlights a very important principle in mechanics.

"A single body cannot move itself without interaction with an external force or other body"

 

So to look at why you man in the waggon cannot push himself and the waggon, but the man ouside the waggon can.

Simply put the man in the waggon has nothing to push against but the man outside pushes against the ground.

 

There is a further twist to this you might like to think about.

The man pushing the waggon can push a heavier weight (mass) than his own.
Why is this ?

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4 hours ago, IndianScientist said:

If anyone guides about the correct physics behind this, I will be grateful to that person. Because of this query , I sometimes lose my atten on other subjects.

I agree with @studiot and I'll add some comments about principles and physical laws. First one you may use is Newton's third law: If an object A exerts a force on object B, then object B must exert a force of equal magnitude and opposite direction back on object A. 

In your case this means that when the man is in the wagon he pushes the wagon with force F and the wagon pushes back with force F. There is also a force along the floor of the wagon affecting the man and and equal and opposite force affecting the wagon. This means that when you analyze and sum the forces on the wagon and on the man they add to zero for the combination man + wagon.

Given that the force is zero you may look at Newtons second law (or use the first, if you prefer); Newton's second law of motion is F = ma, or force is equal to mass times acceleration. 

In your case that means that since the force on man + wagon is zero they will remain at rest.

 

4 hours ago, IndianScientist said:

But what is the theoritical physics behind this?

For further details one can discuss underlying principles; for instance how Newtons laws are derived from more fundamental concepts and what those fundamental concepts are. There are plenty of experts here that can provide explanations, feel free to post followup questions.

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On 1/29/2021 at 3:27 PM, studiot said:

Simply put the man in the waggon has nothing to push against but the man outside pushes against the ground.

Im extremely sorry for late reply @studiot and @Ghideon as I was preparing for social science class test. When the man is struggling to push the wagon, he is pushing on a wall  of the wagon. But two ideas then immediately come to mind:

1. When the man is pushing from outside the wagon, he pushes the ground and the horizontal component of the reaction force by the ground{if I am correct(because application of vectors in walking of a man is taught one grade ahead of me and I studied it beforehand😁)} propels the man forward. Let's say the net force applied by the man is F. There will be accelaration in the system if the unbalanced forced  is sufficient(pls correct me of any mistakes)

 

2. @Ghideon I can in a way get a sense of what you say but can't fully understand the 'physico-mathematical cause'(😁). The man is inside the wagon and the man is diagonally pushing on the floor of the wagon. The horizontal component of reaction by the floor should propel him forward and he must be applying a very same net force F on the wall. 

AS I have told, I can get a  sense of what is happening in the second case , but if I am not acting immature, the question reverses, why is motion possible in the 1st case. 

A little more detailed explanation, you'll all be geniuses because a darn fool like me is making simple things complicated and you'll be the ones who can be making the complex thing simple.... That's what the definition of genius is....😁😂.

Jokes apart, please elaborate a little more. I thank you all for your help.

Best Wishes

A science aspirant and lover from India

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20 minutes ago, IndianScientist said:

Im extremely sorry for late reply @studiot and @Ghideon as I was preparing for social science class test. When the man is struggling to push the wagon, he is pushing on a wall  of the wagon. But two ideas then immediately come to mind:

1. When the man is pushing from outside the wagon, he pushes the ground and the horizontal component of the reaction force by the ground{if I am correct(because application of vectors in walking of a man is taught one grade ahead of me and I studied it beforehand😁)} propels the man forward. Let's say the net force applied by the man is F. There will be accelaration in the system if the unbalanced forced  is sufficient(pls correct me of any mistakes)

 

2. @Ghideon I can in a way get a sense of what you say but can't fully understand the 'physico-mathematical cause'(😁). The man is inside the wagon and the man is diagonally pushing on the floor of the wagon. The horizontal component of reaction by the floor should propel him forward and he must be applying a very same net force F on the wall. 

AS I have told, I can get a  sense of what is happening in the second case , but if I am not acting immature, the question reverses, why is motion possible in the 1st case. 

A little more detailed explanation, you'll all be geniuses because a darn fool like me is making simple things complicated and you'll be the ones who can be making the complex thing simple.... That's what the definition of genius is....😁😂.

Jokes apart, please elaborate a little more. I thank you all for your help.

Best Wishes

A science aspirant and lover from India

I see you are still on line so I will try to make this quick, but it may be a bit jumbled as a result.
So carry on asking questions.

You do not really need vectors or Newton's Laws to understand the Physics of this.

The man in the trick is a single bodythat mey be considered as a single entity.
He has no direct contact with the ground (or anything other than the truck).
So pushing against the truck floor, as well as one side,  is the same as pushing against the opposite wall of the truck or any other part of it.
The is no overall or net effect on the ouside world.

The fundamental physical principle is that internal forces always cancel or balance out.

This is illustrated by the following.

If you stand a book on a table the book exerts a force on the table due to its weight.
If you stand another book on top of the first one then this book exerts a force on the first book which then transfers the extra weight to the table.
Internal forces within the book cancel out.

Can you draw a diagram to illustrate this ?
It would help you to try first, then I will offer one.
If you

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1 hour ago, studiot said:

see you are still on line so I will try to

@studiot I actually was not online at the time you posted this.  I just saw this Now. Usually I keep these type of tabs open in the device and so that's why probably it was showing.

I am now gradually getting a feel of it.

I have tried to draw the diagram of your scenario as elaborately as possible . Though I did not understand this analogy clearly. It would be helpful if you elaborate a little more. But I have climbed up to the nodes of the branches of the tree , I just need to climb to the apex of the shoot.😁

If you elaborate the diagram of my case (with your explanation) a little more, I will feel like a book.

I can't describe anything in words but thanks for the help

 

16121016450461805785413.jpg

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Archimedes is supposed to have said "Give me  long enough lever and a place to stand and I can move the earth".  The "place to stand" is crucial!  When your strong man is pushing against the wagon what are his feet pushing against?  When he is standing on the ground, outside the wagon, His arms are pushing the wagon one way while his feet are pushing the ground in the opposite direction.  Of course the earth is so large it isn't going to move so it is just the wagon that moves.  When he is standing in the wagon his arms are pushing the wagon one way and his feet are pushing the wagon the opposite way.  it is a stalemate!  Nothing moves.

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