Newton's 1st Law: Proving Equal Forces Acting on a Box

  • Context: High School 
  • Thread starter Thread starter goforit
  • Start date Start date
  • Tags Tags
    Doubt Law
Click For Summary

Discussion Overview

The discussion centers around the application and understanding of Newton's first and third laws of motion, particularly in the context of a box resting on the floor and the forces acting upon it. Participants explore the implications of these laws, the nature of forces, and the concept of equal and opposite reactions.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions how to prove that the forces acting on the box (gravitational force and upward thrust) are equal, suggesting that if gravitational force were greater, the box would move downwards.
  • Another participant introduces the idea of drag and terminal velocity in relation to objects in water, implying that forces can vary based on conditions.
  • A participant argues that the upward reaction force is not necessarily equal to the gravitational force, stating that the gravitational force exerted by the box on the Earth is the actual reaction.
  • One contribution explains that Newton's third law involves equal and opposite forces between two interacting objects, using the example of an object in water to illustrate how forces can lead to acceleration.
  • Another participant humorously suggests a method to measure forces using a scale, indicating that equal readings would imply equal forces.
  • A later reply discusses the nature of Newton's laws as postulates rather than proven facts, suggesting that they are convenient assumptions that simplify understanding of physical interactions.
  • One participant emphasizes that Newton's third law is supported by extensive experimental data and is tied to the conservation of momentum.
  • Another participant argues that the upward force is presumed to exist to explain the box's lack of movement, suggesting that alternative laws could be proposed but would complicate the understanding of mechanics.

Areas of Agreement / Disagreement

Participants express differing views on the nature of forces acting on the box and the validity of Newton's laws. There is no consensus on whether the upward force is equal to the gravitational force or whether Newton's laws require proof, leading to an unresolved discussion.

Contextual Notes

Participants highlight the limitations of Newton's laws as postulates and the dependence on definitions of force. The discussion reflects varying interpretations of the laws and their implications in different scenarios.

goforit
Messages
2
Reaction score
0
Hello friends! I'm new to this site and can see its good.
Well I have a doubt which I would like to get clarified.

According to Newton's 1st law ,
Any body continues to be in its state of rest unless and until an external force acts on it.

Imagine a box kept on the floor.
There are 2 forces acting on it.
1)gravitational force ie downwards.
2) upward thrust by the floor.

So according to Newton's 3rd law,
For every action there is an equal and opposite reaction.

How do we know it is equal?What is the proof?

Assume that the magnitude of gravitational force is greater,then the box should start moving in the direction of the gravitational force.ie downwards.

Am I right?
Please help.
 
Physics news on Phys.org


Yes. Like some object in water, denser than the water. But there is drag also so the object will have some terminal velocity.
 


Well then how do you prove Newton's 3rd law?
Assuming the gravitational force is greater than the upward thrust so the box starts moving downwards.

Like can the magnitude of gravitational force be higher than upward thrust?
If no why?
If yes why?

Please help.
 


I don't think I know about any proof about Newtons laws(but they are accepted by expts) but I would like to say something..The upward reaction IS NOT the reaction to the gravitational force .. hence it may or may not be equal! The actual reaction is the gravitational force that the box exerts on the earth.

Hence magnitude of gravtational force may be greater the upward reaction or maybe not. (eg just like vlado said)
 


The third Newton law states that when two objects interact they exert equal and opposite forces on one another. This means that whenever a first object exerts a force F on a second object, the second object exerts a force −F on the first object.
The water can oppose only with some maximal force on the object (that depends on the its density ) let's say 100 N. So the object can only apply 100 N on the water. If the weight of the object is let's say is 150 N, it will have extra 50 N to accelerate with, to its terminal velocity.
But the action-reaction forces between the object and the water will be the equal, and the object will accelerate downwards.
 


goforit said:
How do we know it is equal?What is the proof?

Because a bizillion perpetual motion machines have failed to work.
 


Get a room with a glass floor. Place the box on a scale and read what it says. Then flip the scale upside down and read what it says again. If the readings are equal, the forces are equal. :biggrin:
 


russ_watters said:
Get a room with a glass floor. Place the box on a scale and read what it says. Then flip the scale upside down and read what it says again. If the readings are equal, the forces are equal. :biggrin:

That's a very flip-ant answer! :redface:

To the OP - Newton's 'laws' are postulates. That is, things taken to be obviously true and therefore requiring no proof. They are just mathematical re-statements of the conservation of energy and momentum.

In fact, it's mathematically possible to create an entirely new set of 'laws' based on a new definition of force that says 'the action is equal to half of the reaction' or it's square or anything you like.
But if you do that, the price you pay is that all the other laws become extremely complicated.

It's much the same situation as with the old view of the Earth as the centre of the universe. All the astronomical 'laws' became so complicated that it was almost impossible to calculate simple things like the orbit of a planet. When we moved to a sun-centred view, things instantly became easier.

So, the answer is - because it's easier that way.
 


In mechanics Newton's third law embodies the conservation of momentum. So any experiment which violates the conservation of mechanical momentum will falsify Newton's third law.

It is not just accepted for convenience, there are centuries worth of experimental data including freshman lab experiments.
 
  • #10


DaleSpam said:
It is not just accepted for convenience

If you have an object, sitting on the floor, it doesn't move up or down because although there is a downward force acting on it (gravity), there is also a presumed force acting upward.
We are forced to presume the upward force because without it the object would be required to move (by the first and second laws).We are then safe to say the two forces are equal and opposite and that's why it doesn't move.

However, you could do something else - for example, you could invent a new law that 'forces acting on a body in contact with another pass through that body without effect and act on the other side'. That works too.

Newton's third law is not experimentally verifiable because it is simply a convenient postulate.
 

Similar threads

High School Are frictional forces action forces or reaction forces?
  • · Replies 14 ·
Replies
14
Views
2K
High School Newton's 3rd Law interactions with 2nd and 1st Law Partners
  • · Replies 8 ·
Replies
8
Views
2K
High School Newton's Third Law in space
  • · Replies 20 ·
Replies
20
Views
3K
High School Why Is an Object's Weight Equal to the Force of Gravity It Experiences?
  • · Replies 51 ·
2
Replies
51
Views
5K
High School How's it possible to push on systems with 2 parts? (Newton's third law)
  • · Replies 14 ·
Replies
14
Views
1K
High School Confused about Newton's 3rd law
  • · Replies 22 ·
Replies
22
Views
3K
High School Newton's third law (equal and opposite force)
  • · Replies 40 ·
2
Replies
40
Views
4K
Undergrad Extending Newton's laws -- Is the concept of force still defined?
  • · Replies 35 ·
2
Replies
35
Views
5K
Undergrad Why a book on a table is not an example of Newton's 3rd law?
  • · Replies 10 ·
Replies
10
Views
4K
High School What forces act on a bar when you do pull-ups?
  • · Replies 5 ·
Replies
5
Views
2K