Calculating Speed of a Rocket Using Mass and Thrust

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Discussion Overview

The discussion revolves around calculating the speed of a rocket based on its mass and thrust. Participants explore the implications of thrust measurements, acceleration calculations, and the effects of changing mass during flight. The context includes theoretical and practical aspects of rocket motion.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant inquires about calculating speed using a mass of approximately 150 grams and an engine thrust of 1700 grams, expressing uncertainty about the calculations.
  • Another participant points out that thrust should not be measured in grams, suggesting a conversion to Newtons for proper calculations.
  • A different participant provides a method to find net force by converting thrust to Newtons, subtracting the weight of the rocket, and then calculating acceleration and final velocity using Newton's laws.
  • One participant clarifies that the thrust measurement was taken by weighing the engine, indicating a misunderstanding of thrust versus power.
  • Another participant emphasizes the importance of zeroing scales before measuring thrust and provides calculations for net force and acceleration, while cautioning that the resulting velocity may be unrealistically high without considering air resistance.
  • A later reply highlights the complexity of the problem due to the changing mass of the rocket as propellant is burned, suggesting the use of the Tsiolkovsky rocket equation for further understanding.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct method for calculating speed, with multiple viewpoints on thrust measurement, the effects of air resistance, and the changing mass of the rocket during flight remaining unresolved.

Contextual Notes

There are limitations regarding the assumptions made about thrust measurement and the neglect of air resistance in calculations. The discussion also reflects a lack of clarity on the application of the Tsiolkovsky rocket equation in this context.

qalz
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Hi guys, I would like to count sth., but to do that i need a speed of thing. The promblem is that I've got mass ~150 gram (0,33 pound), enginee thrust 1700 gram (3,75 pound) which work 4 seconds (it moves up) and I don't know how to count this. Do you know, it is possible to count speed based on that data?
Sorry for the low level of English, I'm from Poland (there is no forum where somebody helped me). :/
 
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qalz said:
engine thrust 1700 gram
Engine thrust cannot be in grams.
 
Find the engine thrust in Newtons, subtract from it ( 0.15 * 9.81 ), leaving the net force, calculate the acceleration rate, accelerate for 4 seconds, use Newtons rules of motion for the solution ( final velocity).
 
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Sorry, this is power of engine. I measured it putting engine upside-down on weight :)
 
Did you zero the scales before firing the rocket? I assume you did so..

1.7kg equates to 1.7*9.81 = 16.7N of thrust.

The total force on your rocket is

16.7N - (0.15 * 9.81) = 14.4N

The acceleration is given by F=ma or a=F/m

a = 14.4/0.15 = 96m/s/s

That's very fast. I suspect if you use that figure to calculate the velocity after 4 seconds you will get a figure that is too fast. eg You can't ignore air resistance.
 
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If this is a rocket, the problem is further complicated by the fact that the mass changes during the four-second period as the propellant is burned. Try googling for "Tsiolkovsky rocket equation"; you'll find what you need there.
 
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Yes, that's a rocket. I'll google it, thanks for help :)
 

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