Forces in inertial and non-inertial frame of reference

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Discussion Overview

The discussion revolves around the forces acting on a man standing in a bus that is braking with constant negative acceleration. Participants explore the implications of analyzing the situation from both inertial and non-inertial frames of reference, particularly focusing on the role of friction and inertia.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant proposes that the force of friction is the only force acting on the man relative to the bus when it begins to brake.
  • Another participant questions the scenario by suggesting what would happen if the floor were slippery, leading to zero friction, and whether the man would still move relative to the bus.
  • A participant argues that the man does not start moving due to a force but rather continues moving because he was already in motion, emphasizing that no negative acceleration is applied directly to him.
  • It is noted that in an inertial frame, the bus is the one accelerating, not the man, and the change in relative position is attributed to the bus's acceleration.

Areas of Agreement / Disagreement

Participants express differing views on the role of friction and inertia, with some agreeing that the man continues moving due to his initial motion rather than being acted upon by a force. The discussion remains unresolved regarding the implications of friction and the interpretation of motion in different frames of reference.

Contextual Notes

There are unresolved questions about the definitions of forces in inertial versus non-inertial frames and the assumptions regarding the effects of friction in this scenario.

ChessEnthusiast
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Imagine such situation:
A bus is moving at constant speed, a man is standing on the bus and is not holding to anything - he is simply standing.
Now, the bus starts breaking with constant negative acceleration and so the man will change his relative position due to the force of inertia.
Now, let's say we don't believe in the existence of the force of inertia and want to solve this problem in an inertial frame of reference.

The only force of contact between the man and the bus is the force of friction.
The man will move forward due to braking.
Therefore, is the force of friction the force causing his motion relative to the floor on the bus?
 
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ChessEnthusiast said:
Therefore, is the force of friction the force causing his motion relative to the floor on the bus?
What do you think would happen if we made the floor very slippery* so the friction force was 0? Would he still move relative to the bus or not?

*perhaps he should not remain standing for this portion of the experiment, it might be difficult to pass the ethics committee review!
 
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Right, the force of friction was not the right guess.
However, there are no more forces in the x direction.

What about this analysis:
There is no force acelerating him towards the windshield, there is simply a force opposing his motion - the force of friction. The fact that this man starts moving is wrong - the man simply KEEPS moving because he was in motion and no negative acceleration was applied directly to him.
 
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ChessEnthusiast said:
Right, the force of friction was not the right guess.
However, there are no more forces in the x direction.

What about this analysis:
There is no force acelerating him towards the windshield, there is simply a force opposing his motion - the force of friction. The fact that this man starts moving is wrong - the man simply KEEPS moving because he was in motion and no negative acceleration was applied directly to him.
Yes. Don't talk about "relative to the bus" if you decided to use the inertial frame.
 
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ChessEnthusiast said:
The fact that this man starts moving is wrong - the man simply KEEPS moving because he was in motion and no negative acceleration was applied directly to him.
Yes, exactly. In the inertial frame the bus is accelerating, not the man. The change in relative position is due to the bus’ acceleration
 
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Possibly useful:
Frames of Reference (1960) .. go to 13m27s
 

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