A theoretical question about forces

  • Context: Undergrad 
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Discussion Overview

The discussion revolves around a theoretical scenario involving forces and motion, specifically examining the effects of an impact on a person lying in bed. Participants explore the implications of different types of impacts and the resulting forces, considering both direct and indirect impacts.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions whether the force would be the same if a knee hit the head directly compared to an indirect impact on the bed.
  • Another participant suggests that if there is a headboard, it would exert a force on the head opposite to the direction of impact.
  • One participant argues that if the bed were frictionless, the force on the head would be negligible or zero.
  • A different participant uses the analogy of pulling a tablecloth from under a plate to illustrate that the forces in the two scenarios could be very different.
  • Another participant introduces the equation F=ma, noting that the mass differs in the two cases, which would affect acceleration despite the force being the same.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the forces involved, with no consensus reached on whether the forces in the two scenarios are equivalent.

Contextual Notes

Assumptions about friction, the presence of a headboard, and the nature of the impact are not fully resolved, and the discussion relies on hypothetical scenarios that may not account for all variables.

freexd
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Hello,

Imagine the following scenario: a person is lying in bed, straight on the back. A second person passes the bed and accidently hits the edge with it's knee. The created force results in a fast head movement of the person on the bed, towards the direction where the impact was.

Is it correct that the force would have been the same high, when the knee hit the head directly?
 
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freexd said:
The created force results in a fast head movement of the person on the bed, towards the direction where the impact was.

I'm assuming there's a headboard, and in such a case it is the headboard that moves towards the person's head. This exerts a force on the head opposite of the direction of impact.
 
No, it' not correct. Supposing the bed was like a clean table with some kind of oil to eliminate the friction between the body on it and the bed the force on the head is going to be close to zero or zero (idealy).
 
freexd said:
Is it correct that the force would have been the same high, when the knee hit the head directly?

No. Consider the trick where you pull a table cloth from under a plate. The plate does not accelerate as quickly as the cloth. The forces could be very different.
 
F=ma, in the two cases the mass is different, the force is the same, so you would get a different acceleration.
 

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