A logical problem about a new system.

  • Context: Graduate 
  • Thread starter Thread starter Richlair
  • Start date Start date
  • Tags Tags
    System
Click For Summary

Discussion Overview

The discussion revolves around a hypothetical scenario involving a force termed "air resistance" that acts on a body in a unique manner, opposing its motion by a specific acceleration. Participants analyze the motion of a ball thrown upwards under different conditions of this force, exploring the implications and outcomes of each case.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant proposes a hypothetical force that acts against the motion of a body only when it is in motion, leading to different cases of ball motion based on the relationship between the force's acceleration (a) and gravitational acceleration (g).
  • In Case I, if a < g, the ball decelerates and returns to the ground with less acceleration due to the opposing force.
  • In Case II, if a = g, the ball comes to rest and is suggested to float due to equal opposing forces from gravity and air resistance.
  • In Case III, if a > g, the ball experiences complex interactions between gravity and air resistance, leading to a proposed scenario where it never moves at all.
  • Another participant questions the validity of the hypothetical force, suggesting it resembles inertia and challenges the feasibility of the scenarios presented.
  • A later reply clarifies that the proposed force does not oppose acceleration but acts against instantaneous motion, suggesting a different interpretation of the scenario.
  • Some participants express skepticism about the implications of the proposed force, citing potential violations of conservation laws and the absurdity of the scenarios.

Areas of Agreement / Disagreement

Participants generally do not reach consensus on the validity of the hypothetical force or the scenarios presented. There are competing views on the feasibility and implications of the analysis, with some participants expressing confusion and skepticism.

Contextual Notes

The discussion involves a hypothetical scenario that lacks real-world applicability, leading to unresolved questions about the nature of the proposed force and its effects on motion. Participants highlight limitations in the assumptions made and the implications for established physical principles.

Richlair
Messages
8
Reaction score
0
I was thinking of a problem on my own when I came upon an entirely new idea.

Assume that a force(call it air resistance) acts on a body such that it opposes the motion of the body by an acceleration a. If the body is at rest, it doesn't act. I know that it is not possible but just assume such a system. Now, suppose you throw a ball upwards with considerable speed. Let us consider cases and analyse the motion of the ball.

Case I: If a<g.
The ball moves up with deceleration(a+g) and comes to rest and comes down with less acceleration (g-a) on it.

Case II: a=g
The ball moves up with fair deceleration 2g, comes to rest. Now, at the instant when it is at rest, air resistance doesn't act on the ball. But since gravity does, the ball starts coming down but air resistance pulls it up with same force and so, the ball just floats in air neither going up nor coming down.

Case III: a>g
The ball moves up with powerful deceleration (a+g) and comes to rest. Now, consider the instant when it is at rest. Only gravity acts on it and so, it tends to come down. But if it does come down for an instant, air resistance acts upon it pulling it upwards with acceleration more than gravity. So, just as the ball starts moving down, it is pulled forcibly upwards and hence, for an instant, tends to move up. Now, again, air resistance changes its direction(remember that it just tends to oppose the motion like friction), and begins pulling it downwards and gravity also pulls and so, the ball again starts falling down for a fraction of time. Finally, we can see that the ball never moves at all.

My question is, am I right in this analysis of the motion of the ball? Because it seems vague and incorrect(for the third case).
 
Physics news on Phys.org
The problem here is that you are asking us to gauge whether your made up force is going to have X effect. I don't really even understand it. It opposes the acceleration of a body? We have a term for that already, called Inertia. Also, you seem to be neglecting to take into account the acceleration provided by your arm to throw the ball upwards to begin with. I also don't understand your use of A and G. Gravity is a constant acceleration of 9.8 m/s^2 here on earth, no matter the inertia of the object. In case 1 the ball would be hard to toss into the air, and would slowly come to a stop and slowly come back down. In case 2, you couldn't even move the ball since all acceleration would be opposed equally. Case 3 is just crazy, as it seems that it would be pushed opposite of the normal direction.

I think that is what would happen, but since it isn't real its hard to say. You should try to stick to real concepts, they are generally much easier to work through lol.
 
My force doesn't oppose the acceleration of the body. It just gives acceleration against the instantaneous motion of the body.
Since everything is hypothetical, you can just assume that somehow the ball was just tossed into the air. If you feel the tossing of the ball in the air is absurd, assume such an atmosphere of air(as given in the question) starts from above 2 metres in the ground and goes to a considerable height. So, you can throw the ball from less than two metres height normally and notice the motion of the ball as soon as it reached the special atmosphere.
I know this is completely impossible, but I wanted to analyse the motion in such a case by assuming it exists in another planet.
 
So your force is exactly like resistance moving through a fluid, but variable then. Alright, got it. If so, then I think your analysis is correct. Although case 3 might result in a paradox where my grandfather comes out from a singularity and gives birth to an alien bicycle. :)
 
Drakkith said:
So your force is exactly like resistance moving through a fluid, but variable then. Alright, got it.

Exactly. I am surprised it didn't cross my mind about viscosity. Anyway, you got the point.

Drakkith said:
If so, then I think your analysis is correct. Although case 3 might result in a paradox where my grandfather comes out from a singularity and gives birth to an alien bicycle. :)

Oh, thanks for your help.
 
Richlair said:
Oh, thanks for your help.

Just saying that it makes no sense and would violate conservation of energy and momentum and all kinds of other things.
 

Similar threads

Graduate Please Explain Elementary Physics Elevator Question
  • · Replies 22 ·
Replies
22
Views
2K
Undergrad Ball free fall 100m at equator, where is the landing spot?
  • · Replies 45 ·
2
Replies
45
Views
5K
Graduate Can a Spinning Astronaut Use a Tethered Bowling Ball to Gain Motion in Space?
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad Acceleration after applying a force
  • · Replies 4 ·
Replies
4
Views
3K
High School What happens to buoyancy when a non-uniform force is applied to the liquid?
  • · Replies 5 ·
Replies
5
Views
2K
High School Does weight affect the acceleration of an object?
  • · Replies 9 ·
Replies
9
Views
2K
High School What are the forces involved in the following case?
  • · Replies 2 ·
Replies
2
Views
1K
Graduate Anamolous Aerodynamic behaviour of small spheres?
  • · Replies 4 ·
Replies
4
Views
1K
Undergrad Why does the apparent weight decrease when a body accelerates down a rope?
  • · Replies 9 ·
Replies
9
Views
3K
High School Newton's 3rd Law interactions with 2nd and 1st Law Partners
  • · Replies 8 ·
Replies
8
Views
2K