Confusing Mechanics Question: Velocity of Rocket on Touchdown

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Discussion Overview

The discussion revolves around the confusion regarding the use of gravitational acceleration values (g = -9.8 m/s² versus g = 9.8 m/s²) in mechanics problems, specifically in the context of calculating the velocity of a rocket upon touchdown. Participants explore the implications of sign conventions in physics problems and how they affect calculations.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants note that the choice of using g = -9.8 or g = 9.8 depends on the direction defined as "up" and emphasize the importance of consistency in applying these conventions.
  • Others express confusion over a specific problem where different parts used different values for g, suggesting that the textbook author may have had varying conventions or possibly made an error.
  • One participant suggests that recognizing and adapting to different sign conventions is a valuable skill in physics.
  • Another participant points out that the complexity of the approach taken in the problem may not be necessary and proposes a simpler method for calculating impact velocity.
  • There is a request for the exact question to clarify the discussion further.
  • Concerns are raised about the lack of units in the problem, indicating that this could lead to misunderstandings.
  • One participant mentions a potential typo in the equation related to the rocket's motion, indicating that the solutions derived may not align with the expected outcomes.

Areas of Agreement / Disagreement

Participants generally agree that the choice of gravitational acceleration is a matter of convention, but there is disagreement regarding the specific application in the problem discussed, with some expressing confusion over the inconsistency in the textbook's approach.

Contextual Notes

There are unresolved questions about the exact problem being referenced, and participants highlight the importance of units in calculations, which may not have been provided in the original question.

Who May Find This Useful

This discussion may be useful for students learning mechanics, particularly those grappling with sign conventions and their implications in physics problems.

Falley
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So,while learning mechanics,I found that most questions use g=-9.8 ,but when calculating the velocity the instant a rocket touches the ground,the solution instead used g=9.8,which is very confusing.Other questions of similar workings used g=-9.8,anybody care to help?
 
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It depends on which direction you define as "up". As long as you are consistent, it doesn't matter which you use.
 
But for the same question,part a),b)c) a n b used -9.8 while c used 9.8
 
In every problem we get to choose whether we consider forces, velocities, and accelerations to be positive when they're in the upwards direction and negative when they're in the downward direction, or the other way around. Gravity pulls downward, so it will be negative if you use the "positive up" convention, positive if you use the "positive down" convention.

Either way works just fine - you just have to be consistent throughout the problem.
 
Falley said:
But for the same question,part a),b)c) a n b used -9.8 while c used 9.8

Most likely your textbook author was most comfortable thinking about parts a and b one way and part c the other way. Once you get used to it, you'll find yourself picking whichever one feels most natural for a given problem or subproblem without even thinking about it.

Or maybe he did it in purpose, just to keep you on your toes... Recognizing which sign convention is being used and being able to handle either one is an important skill and exercising it might have been the point of this problem.

Or maybe he was just careless. It happens :smile:
 
Post the exact question otherwise we are guessing.
 
Example 3,the rocket
 

Attachments

Both are meaningless without units... :)
 
Falley said:
Example 3,the rocket
That is just a typo. The equation, as written, doesn't have the solution 25.9 s. The solutions for +9.8 m/s^2 are both negative. Only -9.8 m/s^2 gives a positive solution and that soluiton is 25.9 s.
 
  • #10
In case it's not obvious... Their approach is far more complicated than necessary. They are working out the impact velocity starting from the point where the rocket motor shuts off on the way up. It's possible to do that but it's much easier to assume it just falls from the max height which you worked out in part (b).
 
  • #11
Thanks a lot,cleared up a lot of things for me
 

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