Falling on a Planet with No Atmosphere: Observing a Thrown Ball

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Discussion Overview

The discussion revolves around the behavior of a ball thrown upwards while in free-fall on a planet with no atmosphere. Participants explore the implications of gravity and the absence of air resistance on the perceived motion of the ball relative to the thrower.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant asserts that while in free-fall, throwing a ball upwards would make it appear to move away until they hit the ground.
  • Another participant questions the initial claim, suggesting that the weight of the ball and aerodynamic factors might influence the outcome, although they acknowledge the absence of atmosphere.
  • Several participants emphasize that gravity remains a constant force, affecting the motion of both the thrower and the ball.
  • A participant clarifies that in a simplified scenario, if both the thrower and the ball are co-accelerating, the distance between them would remain constant unless the ball is given an initial velocity.
  • There is a discussion about the independence of gravity and magnetism, with one participant questioning whether a metallic ball would experience different gravitational effects.

Areas of Agreement / Disagreement

Participants express varying views on the influence of weight and aerodynamic factors, but there is a general agreement that gravity plays a significant role in the motion of the ball. The discussion remains unresolved regarding the implications of these factors in the absence of an atmosphere.

Contextual Notes

Some statements rely on assumptions about the conditions of the hypothetical scenario, such as the absence of air and the nature of gravitational effects on different materials.

cragar
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If I am in free-fall towards Earth or in this case let's use a planet with no atmosphere to get rid of air drag. So I am in free-fall and then I throw a ball up above me. From my point of view the ball would appear to keep going away from me until i hit the ground. Is this correct?
 
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cragar said:
If I am in free-fall towards Earth or in this case let's use a planet with no atmosphere to get rid of air drag. So I am in free-fall and then I throw a ball up above me. From my point of view the ball would appear to keep going away from me until i hit the ground. Is this correct?

At that moment, of course it would.
What's your point?
 
Im just making sure that from my point of view , it would always look as if it were going away from me .
 
I think it depends if the ball weigh more than you, and then you have to take aerodynamicability into account...

But saying basketball, tennis ball, etc... light ball objects, then yes...I'd reckon this would be the case.
 
Dory said:
I think it depends if the ball weigh more than you, and then you have to take aerodynamicability into account...

That would be good thinking under normal circumstances, but Cragar did specify that there is no atmosphere.
 
Danger said:
That would be good thinking under normal circumstances, but Cragar did specify that there is no atmosphere.

Yea, but there's still gravity.
 
Dory said:
Yea, but there's still gravity.

Correct. Aerodynamics, however, doesn't apply when there is no aer for the thing to be dynamic with.

edit: I originally, for reasons based upon excessive Scotch, treated "Aerodynamics" as a plural and thus followed it with "don't apply". How wretchedly embarrassing. :redface:
Anyhow... my friends will forgive me, 99.7% of you don't give a **** because you don't know me, and the remainder hiding in the trees can consider themselves invited to emerge and bite me.

Okay, now... I meant the first 2 sentences of my edit, up to the ":redface:". As I mentioned to another member earlier this evening, I used to be a pro writer. Since going onto the meds for my ADD about 10 years ago, I haven't been able to write a damned thing. For some reason, a touch of creativity crept in while I was doing my edit, and it was weird. I'm leaving it up, with this explanation, rather than delete it. I want to assure everyone that there is no animosity intended toward anyone... it's just something that my fingers did when my brain wasn't paying attention.
Cheers to all;
Dan
 
Last edited:
cragar said:
If I am in free-fall towards Earth or in this case let's use a planet with no atmosphere to get rid of air drag. So I am in free-fall and then I throw a ball up above me. From my point of view the ball would appear to keep going away from me until i hit the ground. Is this correct?

You write 'would appear to keep going away from me'.

An occupational hazard of being a physicist is that the word 'apparent' is used with more gravity than in everyday language. When a physicist says 'apparent' he means that looks are deceptive.

You write 'the ball would appear to keep going away from me', but to a physicist the word 'appear' makes the statement fuzzy. A sharper formulation is: 'the ball would keep going away from me'.

Indeed the ball would keep going away from you.

For the most simplified setting I take you falling straight down, no orbiting motion involved, and the height so small that tidal effects are negligable. Then acceleration and velocity are independent. If the two of you are co-accelerating then the distance between you and the ball remains the same, add a velocity of the ball relative to you and the distance between the ball and you keeps increasing.
 
Dory said:
Yea, but there's still gravity.

...and it's a constant negative acceleration so, regardless of the initial velocity conditions (as long as v_ball != v_person) they will always maintain the same velocity difference after the balls leaves hand, which will be a positive value, meaning that they're going away from each other, regardless of the planet's frame.
 
  • #10
Danger said:
Correct. Aerodynamics, however, don't apply when there is no aer for the thing to be dynamic with.

I imagine there has to be some gases floating around in any planet. Or we're talking simply purely gravity from a hypothetical PoV?

Also, don't gravity take magnetism into account? So a ball made of a metallic substance would be drawn more towards the gravitational pull.
 
  • #11
Dory said:
I imagine there has to be some gases floating around in any planet. Or we're talking simply purely gravity from a hypothetical PoV?

Also, don't gravity take magnetism into account? So a ball made of a metallic substance would be drawn more towards the gravitational pull.

hypothetical, of course. And no, gravity and magnetism are independent phenomena.
 
  • #12
I see.
 

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