Examining Density's Impact on Projectile Motion

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Discussion Overview

The discussion revolves around the impact of density on the motion of projectiles, particularly in the context of bottle rockets. Participants explore how the density of the medium and the object itself may influence the trajectory and behavior of projectiles, especially during the later stages of their flight.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions whether the density of the medium or the density of the object affects projectile motion, noting that drag plays a significant role in this context.
  • Another participant observes that bottle rockets tend to "fizzle out" and float down rather than following a parabolic path, prompting a discussion on the influence of the rocket's density on its motion.
  • It is suggested that the shape of the trajectory can deviate from a perfect parabola based on the ratio of air density to projectile density, as well as the drag coefficient.
  • A participant explains that during the burn phase, the bottle rocket accelerates, but after the burn, it slows down due to gravity and experiences significant drag, which affects its path.
  • There is a claim that on the moon, where there is no air resistance, the bottle rocket would follow a parabolic path after the burn phase.
  • One participant asserts that drag is proportional to velocity and does not depend on mass, suggesting that more massive objects are less affected by drag compared to lighter ones.
  • A later reply challenges the classification of a bottle rocket as a projectile, indicating a potential misunderstanding of the terms involved.

Areas of Agreement / Disagreement

Participants express differing views on the role of density and drag in projectile motion, with no consensus reached on the specific effects of the rocket's density or the classification of bottle rockets as projectiles.

Contextual Notes

Some discussions involve assumptions about the definitions of projectiles and the conditions under which drag affects motion, which remain unresolved.

apiche
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I'm still new to the field of physics and have undertaken a project in furthering my understanding of projectile motion. Can density affect the parabolic shape of a projectile especially concerning the tail end of its motion?
 
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Density of the medium it's traveling in or density of the object? Density of the medium would definitely affect it. This is called drag and it's why skydivers reach a terminal velocity. The center of gravity of an object follows normal projectile motion no matter what spin the object has. I have no idea what you mean by the tail end of it's motion.
 
Let me clarify. When launching bottle rockets, I've noticed that they typically start off strong and then have the tendency to "fizzle out" and almost float down to the ground instead of following a more parabolic motion. Does the density of the actual rocket affect that motion?
 
You are quite right. A projectile follows a curve that is sort of like the shape of a female breast. (I forget the technical name.) The amount of departure from parabolic depends upon the ratio of air density to projectile density. It also depends upon drag coefficient.
 
As the bottle rocket is in it's burn phase it's accelerating. As soon as the burn is over it slows down due to gravity. The reason why the bottle rocket doesn't follow a parabolic path on the way down is because it's pretty much an empty carboard tube and has major drag. If you were on the moon it would follow a parabolic path after the burn phase. I'm willing to bet if you fix some weights on the rocket it would follow that path as well. It was probably desiged like that anyway so rockets don't come back as deadly projectiles.


edit: forgot to say that the drag is directly proportional to the velocity and has mothing to do with the mass. so objects with more mass are unaffected by the miniscule drag where as less massive objects are greatly affected by it.
 
Last edited:
apiche said:
Let me clarify. When launching bottle rockets, I've noticed that they typically start off strong and then have the tendency to "fizzle out" and almost float down to the ground instead of following a more parabolic motion. Does the density of the actual rocket affect that motion?

A bottle rocket is not a projectile.
 

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