Trouble with FBD: Is There an Upward Force?

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Homework Help Overview

The discussion revolves around the concept of free body diagrams (FBD) in the context of forces acting on masses in an accelerating elevator. Participants are exploring the implications of upward acceleration on the forces involved, particularly focusing on the normal force and its relationship to gravitational force.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are questioning the necessity of an additional upward force due to acceleration and discussing how this affects the normal force in relation to gravitational force. There are inquiries about the sensation of weight in an elevator and whether effective gravity changes during acceleration.

Discussion Status

The discussion is active, with participants raising questions about the relationship between forces in an accelerating system. Some guidance has been offered regarding the analysis of vertical forces and the application of Newton's second law, but there is no explicit consensus on the interpretation of the forces involved.

Contextual Notes

Participants are considering the effects of acceleration on perceived weight and the forces acting on the blocks, indicating a need for clarity on how to approach the FBD in this context.

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Homework Statement
Two blocks (m1 = 15 kg and m2 = 25 kg) are placed side by side on an elevator
accelerating at 2.2 m/s2
[up]. The coefficients of friction between the blocks and the
floor are µs = 0.8 and µk = 0.6. If a horizontal force of 400 N is applied to the first block, find the acceleration of the system.
Relevant Equations
F = ma
Fg = mg
I am having trouble coming up with a FBD. I know there is a Fg, Fn, friction, and applied force on the masses, but would there be another force going upwards because of the acceleration?
 
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What is the effect of the upwards acceleration? In a nutshell?
 
PeroK said:
What is the effect of the upwards acceleration? In a nutshell?
won't the upwards force have to be greater than the force of gravity so that the elevator can accelerate upwards
 
Physicsstudent2 said:
won't the upwards force have to be greater than the force of gravity so that the elevator can accelerate upwards
Okay, but that doesn't help you much. Put it another way: what does it feel like in the elevator? Does it still feel like planet Earth?
 
no it doesnt. depending on which direction you are moving you will feel you're a different weight
 
Physicsstudent2 said:
no it doesnt. depending on which direction you are moving you will feel you're a different weight
So, can we just take the "effective" gravity to be greater than the usual ##g##, but otherwise proceed as normal?
 
Physicsstudent2 said:
but would there be another force going upwards because of the acceleration?
The upward force acting on the blocks is the normal force Fn. To find the normal force on each block, analyze the vertical forces (apply Newton's 2nd law).
 
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