Deacceleration causes object to move backwards?

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Discussion Overview

The discussion centers around the concept of deceleration and its effects on perceived weight in various scenarios, particularly in vehicles and elevators. Participants explore the implications of deceleration when moving vertically, questioning how it relates to gravitational forces and weight perception.

Discussion Character

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

Main Points Raised

  • One participant describes an imaginary vehicle decelerating upwards at 5 m/s² and questions how weight decreases despite moving away from the Earth's center.
  • Another participant compares the situation to being in a car or train that suddenly slows down, asking whether one feels pressed into the seat or thrown forward.
  • A participant clarifies that deceleration means a decrease in velocity but questions the terminology used, noting that 'deacceleration' is not a vector quantity.
  • There are corrections regarding the units of acceleration, with suggestions for more accurate representations.
  • One participant uses the example of an elevator to illustrate how weight perception changes with acceleration, noting that weight feels greater when accelerating upwards and less when accelerating downwards.
  • Another participant comments on modern elevators and their acceleration profiles, suggesting they minimize perceptible changes in weight.
  • Participants discuss the analogy between deceleration in a car and an elevator, noting that both scenarios involve changes in perceived weight due to acceleration in opposite directions.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the relationship between deceleration and perceived weight. Some agree on the analogy between different scenarios, while others remain uncertain about the implications of deceleration in the context of weight perception. No consensus is reached on the terminology or the underlying physics.

Contextual Notes

There are unresolved questions about the definitions of acceleration and deceleration, as well as the implications of these concepts on weight perception. The discussion includes varying interpretations of how these forces interact in different scenarios.

inuka00123
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a imaginary vehicle that travels vertically is deaccelerating at the rate of 5ms/s in upward direction. I understand that when the vehicle is accelerating upwards, at the rate of 5ms/s, the weight of the people inside the vehicle increases, respect to the newtons third law, peoples have a opposite force to the acceleration of the vehicle plus the gravity. but when a vehicle is deaccelerating to the same direction, (vehicle travels vertically) according to the text books the weight decreases, I can't understand if the vehicle is travelling to the same upward direction away from the center of the earth, how the weight decreases, if it does deacceleration must mean it travels back to earth.
 
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If you are traveling in a car or a train that suddenly begins to slow down, are you pressed into your seat, or thrown forward?
 
No it just means the velocity is decreasing as it moves upward.

Imagine a rocket traveling upward thats runs out of fuel it continues upward and depending on its speed and the force of gravity slows down as it moves forward and then begins to fall back to earth.
 
inuka00123 said:
a imaginary vehicle that travels vertically ...
Downwards or upwards?

inuka00123 said:
... is deaccelerating at the rate of 5ms/s in upward direction.
What is "deaccelerating in upward direction" supposed to mean? There is no vector quantity called 'deacceleration', just acceleration. "Deaccelerating" means that the magnitude of velocity decreses, but that is a scalar quantity and has no direction.
 
inuka00123 said:
deaccelerating at the rate of 5ms/s
FYI, you have written the units of acceleration (or deceleration) incorrectly.

More correct: 5m/s^2

Better: 5m/(s^2)

Best (using LaTeX): ##5\frac{m}{s^2}##

I will send you a DM with tips for posting math at PF using LaTeX (see the "LaTeX Guide" link below the Edit window). :smile:
 
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inuka00123 said:
... I can't understand if the vehicle is travelling to the same upward direction away from the center of the earth, how the weight decreases, if it does deacceleration must mean it travels back to earth.
You can feel the variation of your weight in a regular elevator:
Static or moving at constant velocity (up or down) = Your feet feel your normal weight.
Accelerating upwards = Your feet feel a greater than normal weight.
Breaking just before reaching the upper floor = Your feet feel a lesser than normal weight until the cabin stops.
 
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Lnewqban said:
You can feel the variation of your weight in a regular elevator:
In modern elevators you barely feel anything, despite getting much quicker to a given floor than in the past. They seem to optimize the acceleration profile by distributing it throughout the entire travel distance, to keep acceleration and jerk below perceptable levels.
 
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Imagine standing on spring scales in a lift. You will be subjected to Earth's gravity of about, g ≈ 10 m/s². While the lift is not accelerating relative to the building, the scales will show your weight to be 100% of normal.

While the lift accelerates upwards at 5 m/s², you will be subjected to an acceleration of, g + 5 m/s² = 15 m/s², so your weigh on the scales will be 150%, = 50% more than normal.

While the lift accelerates downwards at 5 m/s², you will be subjected to an acceleration of, g - 5 m/s² = 5 m/s², so your weight on the scales will be just 50% of normal.
 
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gmax137 said:
If you are traveling in a car or a train that suddenly begins to slow down, are you pressed into your seat, or thrown forward?
I will be thrown forward, the minus acceleration can be thought as slowing down, but I can not understand how slowing down affect my weight, like in a elevator, slowing down causes weight to be less.
 
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inuka00123 said:
I will be thrown forward, the minus acceleration can be thought as slowing down, but I can not understand how slowing down affect my weight, like in a elevator, slowing down causes weight to be less.
It's the same thing:
Car moving forward & slowing down -> acceleration backward -> you are thrown forward
Elevator moving up & slowing down -> acceleration downward -> you are thrown upward
 
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  • #11
A.T. said:
It's the same thing:
Car moving forward & slowing down -> acceleration backward -> you are thrown forward
Elevator moving up & slowing down -> acceleration downward -> you are thrown upward
did not thought about that, thank you.
 
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