Calculating Kinetic Force for Sci-Fi Story: Help Needed

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Discussion Overview

The discussion revolves around calculating the kinetic energy produced by a railgun for a science fiction story. Participants explore the implications of high-speed projectiles, the relationship between speed and energy, and the potential need for fictional elements to balance realism with narrative needs.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant calculates the kinetic energy of a 300kg shell accelerated to 290,000 m/s to be about 12.6 million megajoules, questioning the validity of this figure compared to military claims.
  • Another participant suggests that the focus should be on the fictional aspect of the story, proposing the creation of a new power source rather than adhering strictly to scientific accuracy.
  • A participant expresses a desire to maintain some realism in the calculations, despite acknowledging the fictional nature of the story, and mentions previously ignoring physics for narrative purposes.
  • One participant corrects the initial energy calculation, stating that the correct energy is about 13 terajoules and suggests aiming for 200 pentajoules, emphasizing the exponential increase in energy required.
  • A later reply reveals a correction in the speed of light used in calculations, leading to a revised energy output of 1.261x10^13 megajoules, or 3,015 megatons of TNT, which is deemed excessive.
  • The same participant proposes hurling a 40kg shell at 0.4c (120m/s) to achieve 68.8 megatons of TNT, which aligns better with their narrative needs.
  • There is a mention of the need to apply Einstein's laws of kinetic energy for speeds above 0.1c, indicating that relativity is a relevant factor for accurate calculations.
  • The participant concludes that understanding the energy requirements will help determine the capabilities of fictional ships in the story.

Areas of Agreement / Disagreement

Participants express differing views on the balance between scientific accuracy and fictional storytelling. While some focus on precise calculations, others advocate for creative liberties in the narrative. The discussion remains unresolved regarding the best approach to integrate science into the story.

Contextual Notes

Participants acknowledge limitations in their calculations, including the need for accurate definitions of speed and energy, and the relevance of relativistic effects at high velocities. There are also unresolved mathematical steps and assumptions regarding the nature of the fictional technology.

Who May Find This Useful

Writers of science fiction, particularly those interested in incorporating realistic physics into their narratives, as well as enthusiasts of theoretical physics and engineering concepts related to high-speed projectiles.

Richard Noel
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Can you help me work something out, please?

I'm writing a science fiction story, and want to work out how much impact energy would be produced by railgun. As a starting point, though I'm sure it should be way too high, if I accelerated a 300kg shell to 290,000 m/s, the kinetic energy should be about 12.6 million megajoules. Given that the US military said a shell fired at nearly 10k mph would give 1.5 megajoules, this seems pretty high. But when I converted this into megatons to get the power, it only gives me 0.003. I tried it a couple of times and checked it on an online calculator and came up with same answer. Which just doesn't work out.

Now I wanted to be able to produce about 40-50 megatons. I know just short of light speed is ludicrous, but I needed a starting point. How do I work this out? I'm sure I've heard that object moving at that speed would go right through the planet, so how do we work that out? Do I have to factor in the stationary object, or does relativity kick in with the mass increasing with speed, in which case I'm lost.

Can anyone help with this?
 
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Richard Noel said:
I'm writing a science fiction story,
The most important part of that is fiction. While you may want some science in it, invent some new crazy power thing that can do whatever you want it to do. You need to balance out science and fiction. I'm not quite sure how to work the math of it, but that's my advice.
 
Isaac0427 said:
The most important part of that is fiction. While you may want some science in it, invent some new crazy power thing that can do whatever you want it to do. You need to balance out science and fiction. I'm not quite sure how to work the math of it, but that's my advice.

Yes, I could do that. But all I really want is to work out fast I'd have to hurl a shell in order for it to have that level of impact force. I like to make the numbers real where possible because it gives you nice consistency and a little realism where you can. I've ignored the laws of physics enough when I've had to say that we have a drive system that can reach speeds of up 3-4 pentameters and hour.
 
Ok, I checked out the math, and you have energy of about 13 terajoules (1.3*1012). You want about 200 pentajoules (2*1017). We are talking about exponentially increasing your value. I am going to stand by my original claim. Try being able to accelerate things beyond the speed of light. I mean think about it, in Star Wars, people can travel across galaxies within hours.
 
I managed to figure it out. The problem is out of stupidity, I calculated the speed of light to be 300k m/s, not 300m m/s as it should be (rounded off I know, but it was only for approximation). This made a significant difference. We then ended up with 1.261x1013 megajoules, or 3,015 megatons of TNT. Way above what was necessary.

With a bit of trial and error, I then came to hurling a 40kg shell at 0.4c, or 120m m/s for east maths. This then have me 68.8 megatons of TNT, which is just right.

HOWEVER, if I wanted an accurate measurement I shouldn't have used Newton's laws of motion. Einstein's laws of kinetic energy would be better suited to working this out at anything over about 0.1cc, because relativity becomes an issue. But I did only need a eurgh idea so I can decide how big to make the guns.

This was useful to work out, because now I need to make sure how fact ships can move so as not to outrun the shells, and I know how much energy is required and therefore how many fusion reactors the battleship would need to keep 18 of these guns firing in sequence at a rate of 1 shell every second.
 

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