Force/magnitude/acceleration scenario

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Homework Help Overview

The problem involves an object moving vertically while being influenced by a rope and significant air resistance. The question focuses on comparing the object's acceleration while rising versus descending, given that the magnitudes of all forces remain constant. Participants are asked to create free body diagrams (FBDs) and derive expressions for acceleration using Newton's second law.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the implications of force vectors and their magnitudes, noting that while the magnitudes may be the same, the forces themselves are not necessarily identical. There are suggestions to derive expressions for acceleration in both scenarios and to explore the differences between them.

Discussion Status

Some participants have pointed out potential duplications of previous threads and have noted the need for clarity in understanding the relationship between forces and acceleration. There is an ongoing exploration of how to approach the problem algebraically without substituting numerical values, with guidance offered on considering different cases for acceleration.

Contextual Notes

Participants are reminded of the constraints regarding the starting equations allowed for the assignment and the requirement to derive symbolic expressions for acceleration. There is also mention of the need to clarify the directionality of the forces involved.

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Homework Statement


1. Homework Statement
An object is moving vertically while being pulled from above by a rope (or cable, wire, or string). The object is also subject to a significant air resistance force that can't be ignored. All forces acting on it are strictly vertical (pointing up or down only).

Question: If the magnitudes of all forces remain the same, does the object have a greater acceleration if it's rising or if it's descending?

To answer this question, create an appropriate scenario, then draw two FBDs for the object, one for the case in which it's rising, and one for the case in which it's descending. Use N2L and your FBDs to derive expressions that will allow you to calculate the magnitude of the object's acceleration in each case.

Review the summary sheet on FBDs before starting your solution

The only starting equations permitted for this assignment are ΣF = ma and FG =mg

Derive symbolic expressions for the acceleration in each case, then substitute and calculate
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This appears to be a duplicate of https://www.physicsforums.com/threads/magnitude-force-acceleration.856945/, which has been marked solved. I assume you did this because the original images were not right.
Forces are vectors. Their magnitudes may be the same but it does not follow that the forces are the same.
You correctly obtained two different expressions for the two accelerations. Try subtracting one from the other.
 
haruspex said:
This appears to be a duplicate of https://www.physicsforums.com/threads/magnitude-force-acceleration.856945/, which has been marked solved. I assume you did this because the original images were not right.
Forces are vectors. Their magnitudes may be the same but it does not follow that the forces are the same.
You correctly obtained two different expressions for the two accelerations. Try subtracting one from the other.
I'm having troubling understanding how to put numbers into the equation.
 
paperboy221 said:
I'm having troubling understanding how to put numbers into the equation.
You don't need to.
You have two algebraic expressions, one for the acceleration if moving up, one for the acceleration if moving down. You want to find out which acceleration has the greater magnitude.
There are two cases to consider: the accelerations may be in the same direction or in opposite directions.
Pick one of those cases, and write an expression for the difference in the magnitudes.
 

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