Collision between moving and stationary object

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Discussion Overview

The discussion revolves around the physics of collisions, specifically analyzing the scenario of a moving object (a feral elephant) colliding with a stationary object (a fly). The focus is on understanding the resulting speed of the fly after the collision, considering the mass difference between the two objects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant asserts that the speed of the fly after the collision is 10 m/s, reasoning that the fly's mass is negligible compared to the elephant's mass.
  • Another participant explains that in the center of mass (c.m.) system, the fly collides with the elephant at 5 m/s and recoils at the same speed in the opposite direction, leading to a total speed of 10 m/s when considering the c.m. speed.
  • A different perspective is introduced by a participant who compares the scenario to a ball bearing colliding with a locomotive, suggesting that the principle of momentum transfer is more intuitively understood through this analogy.
  • This participant also mentions the application of similar principles in slingshot orbits around large planets to increase the speed of space vehicles, indicating a broader context for the discussion of momentum transfer.

Areas of Agreement / Disagreement

Participants present different models and analogies to explain the collision, but there is no explicit consensus on the best way to conceptualize the scenario. The discussion remains open with multiple viewpoints on the implications of momentum transfer.

Contextual Notes

There are assumptions regarding the negligible mass of the fly and the idealized nature of the collision that may not account for real-world complexities. The discussion does not resolve these assumptions.

Ezio3.1415
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Consider a feral elephant that charges a hovering stationary fly with
a speed of 5m/s at a hovering fly. What is the speed with which the fly will recoil?

Well I know the answer's 10m/s as the mass of the fly is negligible with respect to the mass of the elephant... But how?
 
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Since the fly mass is negligible, the center of mass is coincident with the elephant. In the c.m. system (stationary elephant) the fly hits the elephant at 5 m/s, bounces off and recoils at 5 m/s in the opposite direction. Add the c. m. speed and the fly will be going 10 m/s.
 
Thank you... :)
 
I prefer the model of a ball bearing being fired at an oncoming express locomotive with a thick steel plate on the front. Same principle but the resulting high speed ball bearing is somehow a more attractive idea than a squashed fly.

Also, a really useful version of this is the way you can use slingshot orbits around large planets to speed up space vehicles. It's all to do with transfer of momentum.
 

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