Constant mass in a vacuum

In summary, the conversation discusses programming a simple physics engine for rockets in space, where there is a constant mass, thrust, and no gravity or drag. The question arises on how to calculate the rocket's top velocity, which leads to a discussion on the limitations of reaching the speed of light and the importance of considering fuel consumption and mass in a real rocket. It is noted that in a simple physics engine for a video game, these restrictions may not apply.
  • #1
JacksonMarks
4
0
I'm programming a simple physics engine that works with rockets in space. I haven't done any physics since high school and I'm embarrassingly lost on how to proceed.

What I have is a constant mass (say 50,000kg for example), and a constant thrust (75,000N for example) on the mass. The mass is in a vacuum. For simplicity, there is no gravity working on the mass either.

How do I calculate the mass' top velocity? I seem to be missing something or not taking something into account because as far as I can see, the mass will accelerate infinitely which I know doesn't make sense.

Any and all help would be appreciated.
 
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  • #2
Unless some other force acts on the rocket (and ignoring relativity), it will keep going faster and faster! In an atmosphere, drag force would limit the rocket to some maximum speed, because as you go faster the air pushes back on you harder and harder, until it cancels out your thrust. In a vacuum there's no such drag.
 
  • #3
The_Duck said:
Unless some other force acts on the rocket (and ignoring relativity), it will keep going faster and faster! In an atmosphere, drag force would limit the rocket to some maximum speed, because as you go faster the air pushes back on you harder and harder, until it cancels out your thrust. In a vacuum there's no such drag.

If that is true, then why is it considered impossible to travel at the speed of light? Or is that due to the fact that space is in fact not a perfect vacuum?
 
  • #4
This is why I added the caveat about relativity. In special relativity there is an absolute maximum speed which is the speed of light, even in a perfect vacuum. But I doubt you want to include relativistic effects (which would in any case be very small unless your ships will be traveling monstrously fast) in a simple physics engine.
 
  • #5
JacksonMarks said:
I seem to be missing something or not taking something into account because as far as I can see, the mass will accelerate infinitely which I know doesn't make sense.
You're not taking into account the fact that you don't actually have an infinite supply of fuel so you can't sustain that thrust forever.

You should also probably consider that burning fuel decreases the mass of the rocket
 
  • #6
The_Duck said:
This is why I added the caveat about relativity. In special relativity there is an absolute maximum speed which is the speed of light, even in a perfect vacuum. But I doubt you want to include relativistic effects (which would in any case be very small unless your ships will be traveling monstrously fast) in a simple physics engine.

Ah alright. Thank you very much for your quick responses.

dav2008 said:
You're not taking into account the fact that you don't actually have an infinite supply of fuel so you can't sustain that thrust forever.

You should also probably consider that burning fuel decreases the mass of the rocket

I'm aware of both those facts but it's not something that is important for my case.
 
  • #7
But that IS part of why a real rocket can't reach velocities anywhere close to speed of light. If we could build rockets that can sustain constant thrust without loosing mass, we'd be colonizing other stars by now.

Real rocket creates thrust by ejecting mass at some fairly constant speed Vp. If the fueled rocket had mass mi, and once it ejected all its propellant it has mass mf, then the final velocity is given by the rocket formula.

[tex]V = V_p ln\left(\frac{m_i}{m_f}\right)[/tex]

If you ignore this in your simulation, you are effectively using infinite Vp, and then you shouldn't be surprised that your top velocity is also infinite.
 
  • #8
Also, for an object with mass to reach light speed, you need an infinite amount of energy, feasible with your setup but not in reality.
 
  • #9
K^2 said:
But that IS part of why a real rocket can't reach velocities anywhere close to speed of light. If we could build rockets that can sustain constant thrust without loosing mass, we'd be colonizing other stars by now.

Real rocket creates thrust by ejecting mass at some fairly constant speed Vp. If the fueled rocket had mass mi, and once it ejected all its propellant it has mass mf, then the final velocity is given by the rocket formula.

[tex]V = V_p ln\left(\frac{m_i}{m_f}\right)[/tex]

If you ignore this in your simulation, you are effectively using infinite Vp, and then you shouldn't be surprised that your top velocity is also infinite.

I appreciate that rockets in real life are fuel based and that they must expel mass in order to actually achieve thrust, but this physics engine is just for a simple arcade video game and as a result players are not going to be restricted on fuel.
 

1. What does it mean for something to have constant mass in a vacuum?

Constant mass in a vacuum means that the mass of an object remains the same when it is placed in a vacuum, or an environment with no air or other gases. This is because in a vacuum, there is no air resistance or other external forces acting on the object to change its mass.

2. How is constant mass in a vacuum relevant to scientific experiments?

Constant mass in a vacuum is important in scientific experiments because it allows for accurate measurements and observations. In a vacuum, the mass of an object remains constant, so any changes in the object's behavior or properties can be attributed to other factors, rather than changes in its mass.

3. Can an object's mass ever change in a vacuum?

While an object's mass can remain constant in a vacuum, it is possible for its weight to change due to the effects of gravity. However, in terms of its actual mass, it will remain constant unless acted upon by an external force.

4. How does constant mass in a vacuum relate to the laws of physics?

Constant mass in a vacuum is a fundamental principle in physics, known as the Law of Conservation of Mass. This law states that in a closed system, mass remains constant, meaning it cannot be created or destroyed, only transferred or converted into different forms.

5. Can the concept of constant mass in a vacuum be applied to all objects?

Yes, the concept of constant mass in a vacuum applies to all objects, regardless of their size, shape, or composition. As long as an object is in a vacuum, its mass will remain constant unless acted upon by an external force.

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