Fallrate Calculation: Constant Upward Force

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Discussion Overview

The discussion revolves around the calculation of fall rates for objects subjected to a constant upward force. Participants explore the implications of applying such a force to objects of different weights, specifically comparing a 100 lb object with a 25 lb upward force to a 75 lb object. The conversation touches on concepts from physics, including Newton's laws and the distinction between force and mass.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether a 100 lb object with a 25 lb upward force would fall or accelerate at the same speed as a 75 lb object.
  • Another suggests using a free body diagram and Newton's 2nd law to analyze the forces acting on the objects.
  • A participant emphasizes the importance of distinguishing between force and mass, proposing a restatement of the problem in metric units for clarity.
  • One participant concludes that the fall rate would be the same in both scenarios but expresses confusion when comparing the behavior of a helicopter descending versus a free-falling object.
  • Another participant asserts that free-falling objects fall at the same rate regardless of mass, referencing Galileo's experiment, and states that applying an upward force would slow the object down.
  • A participant challenges the conclusion that the fall rates are the same, suggesting that a mistake may have been made in the calculations.

Areas of Agreement / Disagreement

Participants express differing views on the fall rates of the objects under discussion, with some asserting that they would fall at the same rate while others argue that the application of an upward force would result in a slower descent. The discussion remains unresolved with competing perspectives on the calculations and their implications.

Contextual Notes

There is a noted distinction between American units (pounds) and metric units (kilograms and Newtons), which may affect participants' understanding of the problem. Additionally, the role of air resistance is not fully addressed, which could influence the outcomes discussed.

mrdans777
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I'm trying to figure out how fast an object will fall/accelerate with a constant upward force applied to it. For instance, a 100 lb object with a 25 lb upward force applied to it. Would it fall/accelerate at the same speed as a 75 lb object?
 
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Draw a free body diagram of the mass and use Newton's 2nd law i.e. , the sum of forces on it is equal to ##ma##.
mrdans777 said:
Would it fall/accelerate at the same speed as a 75 lb object?
What do you think about it??
 
This is one of those problems where it helps to be careful about the distinction between force and mass. It's also easier to get it right if you use the metric system because the distinction between mass (in kilograms) and force (in Newtons, and the gravitational force on a 1 kilogram mass is ##9.8 \approx 10## Newtons) is explicit - American units blur the two concepts together with the word "pound".

So I'll restate it in metric units and I'll bet that you can work it out for yourself: does a 100 kilogram object subjected to an upward force of 250 Newtons (the gravitational force on a 25 kilogram object) accelerate downwards more or less quickly than a 75 kilogram object in free fall? Calculate the net force on each, plug into ##F=ma##, see what comes out.
 
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Thanks. What I get from these calculations is that fall rate would be the same in both instances. What trips me up is thinking about a helicopter or other VTOL aircraft descending straight down to land with a net mass of maybe 20 lbs or something, versus a 20 lb object. The helicopter comes down slowly but the object seems it would drop much faster, even initially, before acceleration becomes a factor.
 
mrdans777 said:
Thanks. What I get from these calculations is that fall rate would be the same in both instances.
Please do the calculations first and judge the results after.
 
Free falling objects all fall at the same rate, irrespective of mass (ignoring air resistance) - Galileo experiment. Applying an upward force would slow it down. Net result it would fall slower than the free falling object.
 
mrdans777 said:
Thanks. What I get from these calculations is that fall rate would be the same in both instances
If that's how the calculations came out, you made a mistake somewhere. Post your work and someone here will be able to spot the problem.
 

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