Acceleration when an object hits the ground

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Discussion Overview

The discussion revolves around the acceleration experienced by an object when it impacts the ground, particularly focusing on the implications of velocity changes at the moment of landing. It explores concepts in kinematics, material deformation, and the nature of impacts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the acceleration of an object hitting the ground could be considered negative infinity, given that its velocity drops to zero at the moment of impact.
  • Another participant argues that the object does not stop instantaneously but rather slows down rapidly due to deformation of both the object and the ground upon impact.
  • A subsequent reply suggests that both the object and the ground may bend slightly at the moment of contact, raising questions about the nature of stopping.
  • Another participant emphasizes that even in a theoretical scenario with rigid bodies, electrostatic forces would prevent an instantaneous velocity change, and that real-world interactions involve deformation.
  • One participant notes that real objects are not perfectly rigid and will compress during impact, leading to very high accelerations, citing examples from shock testing that can reach significant multiples of gravitational acceleration.

Areas of Agreement / Disagreement

Participants express differing views on the nature of stopping upon impact and the implications for acceleration. There is no consensus on whether the acceleration can be defined as negative infinity or how to characterize the stopping process.

Contextual Notes

The discussion highlights assumptions about rigidity and the nature of forces at play during impacts, as well as the limitations of idealized models in explaining real-world behavior.

Uranium235
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Ok so this is a problem that's been bothering me ever since the first few days of learning kinematics.

We've been taught that when an object falls, the object has a positive velocity up until the moment that it hits the ground. At that moment, the velocity becomes zero. Wouldn't this imply that the acceleration that the object experiences when hitting the ground is negative infinity since it stops the very moment that it lands?
 
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The falling object will dent the ground slightly. also the object itself will be deformed a little by the force of the impact. Thus the object does not stop the very moment it lands, it just slows down very rapidly.
 
Does it really stop the moment that it lands? Might not both the object and the ground bend a slight amount the moment it touches the ground?
 
That's not realistically what happens when an object hits the ground. Even if the the ground and the object were rigid, electrostatic forces that prevent solid objects from passing through each other would not result in an such an instantaneous change of velocity. In reality, though, an object and the ground probably would not be perfectly rigid, and the ground would deform downward, a bit like the object was being caught in a net.
 
Ok got it! Thanks for the answers guys!
 
No real objects are perfectly rigid, so even if there is no permanent change in shape of either the object or the ground, both will be compressed a bit during the time of the impact and then spring back again.

The accelerations when a "rigid" object like a brick falls on the ground can be very large - "shock testing" to check that accidental bumps and falls won't cause damage to objecst is often done at accelerations up to 3000 times the acceleration due to gravity (i.e. about 30,000 m/s2)
 

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