Why doesn't DEceleration add mass as well?

In summary, the concept of relativistic mass gain at higher speeds is due to the contribution of kinetic energy to the total mass of an object, along with its rest mass. However, this does not mean that decreasing speed will continue to add mass, as the decrease in kinetic energy will compensate for it. Additionally, the idea of gaining or losing kinetic energy is relative to a particular inertial frame and cannot be seen as an absolute concept. Observers in different frames may see different changes in mass and energy depending on their perspective.
  • #1
Meatbot
147
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If I accelerate to .9c I am gaining mass, but if I reverse thrust and decelerate why aren't I still adding mass since I'm still putting energy into the system? I thought it wasn't about speed but about changing speed. It takes energy to slow down as well. Or does deceleration lower mass? Or does the decrease in kinetic energy from the lower speed make up for it? I'm sure I just don't understand it properly.

I mean, one observer might think you're accelerating and another might think you're decelerating. Isn't it irrelevant whether you are slowing down or speeding up? How do you know which one you are really doing? What if you and a planet are both moving at the same speed in the same direction but you are in front? For me to get to the planet I'd need to reduce speed, but from my perspective I'd be increasing speed.

Thanks!
 
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  • #2
Meatbot said:
If I accelerate to .9c I am gaining mass, but if I reverse thrust and decelerate why aren't I still adding mass since I'm still putting energy into the system? I thought it wasn't about speed but about changing speed. It takes energy to slow down as well. Or does deceleration lower mass? Or does the decrease in kinetic energy from the lower speed make up for it? I'm sure I just don't understand it properly.

Thanks!

The fact that you need energy for slowing down is a consequence of using dissipative processes (burning fuel) for that. If you imagine you didn't have a rocket engine in your spaceship but a contracted spring connected to a large mass then releasing the spring accelerates you but as you pass the equilibrium length of the spring you start to decelerate without requiring energy. That's an example of a conservative propulsion.
 
  • #3
The gain in relativistic mass at higher speeds can be understood in terms of kinetic energy contributing to the relativistic mass along with the object's rest mass. So, the only way in which it makes sense to say that relativistic mass increases because you're "putting energy into the system" is if by "putting energy into the system" you just mean "increasing its kinetic energy". And under this definition, decreasing speed is clearly not putting energy into the system, since the kinetic energy is decreasing.
 
  • #4
JesseM said:
The gain in relativistic mass at higher speeds can be understood in terms of kinetic energy contributing to the relativistic mass along with the object's rest mass. So, the only way in which it makes sense to say that relativistic mass increases because you're "putting energy into the system" is if by "putting energy into the system" you just mean "increasing its kinetic energy". And under this definition, decreasing speed is clearly not putting energy into the system, since the kinetic energy is decreasing.

But how do you know whether kinetic energy is increasing or decreasing since you may not have been aware of any previous increases/decreases? What if you are already moving at .9c and have been since birth. You think you are stationary and accelerate opposite the direction of motion. Are you gaining or losing kinetic energy? How do you know?
 
  • #5
Meatbot said:
But how do you know whether kinetic energy is increasing or decreasing since you may not have been aware of any previous increases/decreases?
Kinetic energy is just a function of present speed in your frame, it doesn't depend on past history. In classical mechanics kinetic energy is (1/2)*mv^2, where m is mass and v is speed, while in relativity it's given by [tex](\gamma - 1)mc^2[/tex], where m is rest mass, c is the speed of light, and [tex]\gamma = \frac{1}{\sqrt{1 - v^2/c^2}}[/tex]
Meatbot said:
You think you are stationary and accelerate opposite the direction of motion. Are you gaining or losing kinetic energy? How do you know?
You can only talk about gaining or losing kinetic energy relative to a particular inertial frame, it's not an absolute concept. In any frame where the acceleration increased your speed, you've gained kinetic energy, while in any frame where the acceleration decreased your speed, you've lost kinetic energy.
 
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  • #6
JesseM said:
In any frame where the acceleration increased your speed, you've gained kinetic energy, while in any frame where the acceleration decreased your speed, you've lost kinetic energy.

So an observer in your frame would see you gain mass while an observer in an outside frame would see you lose mass, right? So anyone who would see you as decelerating would also see you as losing mass, even if in your frame you were gaining mass. I was mixing frames I guess.
 
  • #7
Meatbot said:
So an observer in your frame would see you gain mass while an observer in an outside frame would see you lose mass, right? So anyone who would see you as decelerating would also see you as losing mass, even if in your frame you were gaining mass. I was mixing frames I guess.
If you're actually in the process of accelerating, you don't have a single inertial rest frame--"inertial" just means non-accelerating. The usual equations of SR, such as the time dilation equation or the equation I gave for kinetic energy, can only be used in inertial frames.
 
  • #8
JesseM said:
If you're actually in the process of accelerating, you don't have a single inertial rest frame--"inertial" just means non-accelerating. The usual equations of SR, such as the time dilation equation or the equation I gave for kinetic energy, can only be used in inertial frames.

What about an observer that remains in the frame of your starting location before you accelerated. He would see you gain mass, but an observer who was able to see that you and the planet were moving fast relative to him would see you lose mass.
 
  • #9
Meatbot said:
What about an observer that remains in the frame of your starting location before you accelerated. He would see you gain mass, but an observer who was able to see that you and the planet were moving fast relative to him would see you lose mass.
Yes, that's right (assuming that you were losing speed relative to the second observer).
 
  • #10
Meatbot said:
You think you are stationary and accelerate opposite the direction of motion. Are you gaining or losing kinetic energy? How do you know?

In your own reference frame you are always stationary. You can't observe your own "relativistic mass."
 
  • #11
I think this is a problem of misunderstanding "what is an Inertial Reference Frame ( IFR )".

So, the only way in which it makes sense to say that relativistic mass increases because you're "putting energy into the system"
You don't define system.

If you're actually in the process of accelerating, you don't have a single inertial rest frame--"inertial" just means non-accelerating.
This is not true. How do you define acceleration ? Respect to a IFR.

Kinetic energy ( and relativistic mass, I suppose ) is relative to a IFR.
How much energy can you get stopping this body that is moving ? This is kinetic energy.
And how could you stop this body ? Exerting a force between this body and and your IFR.

And the only way you can exert a force from your IFR is that your IFR have a mass, but this belongs to another thread.
 
  • #12
Meatbot said:
What about an observer that remains in the frame of your starting location before you accelerated. He would see you gain mass, but an observer who was able to see that you and the planet were moving fast relative to him would see you lose mass.

When you say that someone gains or loses mass, you undoubtedly mean "relativistic mass".

Relativistic mass is not a property of an object alone, but just like energy in Newtonian mechanics, depends on the observer.

Thus, if we consider one object, it has one value for relativistic mass when viewed from a frame that's "moving along" (co-moving) with the object in question, but has a different value for relativistic mass when viewed from a frame that's moving at a different velocity.

There is a sort of mass that is a property of an object (at least if the object is isolated), but this sort of mass is not relativistic mass. It's called invariant mass.

I get the impression that you are viewing "relativistic mass" as a property only of the object, and that this is what is confusing you. Relativistic mass is not a property of the object alone, but depends both on the object and the frame from which it is viewed, i.e. relativistic mass is observer dependent.
 
  • #13
Relativistic mass is not a property of the object alone, but depends both on the object and the frame from which it is viewed, i.e. relativistic mass is observer dependent
.
And it doesn't depend on acceleration, just on relative velocity. True ?
 
  • #14
alvaros said:
.
And it doesn't depend on acceleration, just on relative velocity. True ?

Correct, the relativistic mass of a (small) body that is accelerating depends only on its rest mass (aka its invariant mass), and its velocity.

This is similar to the way that the energy of a body in Newtonian mechanics, .5 m v^2, is a function only of m and v, and is not a function of the acceleration.

Of course a body that is accelerating cannot be an isolated system.
 
  • #15
Meatbot said:
If I accelerate to .9c I am gaining mass, but if I reverse thrust and decelerate why aren't I still adding mass since I'm still putting energy into the system?
First, "relativistic mass" is not really considered a very useful concept, so I would not recommend using it.

Having said that, consider two inertial reference frames here. Frame A where you are initially at rest and then accelerate to .9c, and frame B where you are initially at -.9c and accelerate to rest. In A you gain relativistic mass and in B you lose it. If you then reverse thrust you will undo any change in "relativistic" mass.

-Regards
Dale
 
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1. Why is there a misconception that deceleration should add mass?

Many people misunderstand the concept of mass and assume that it is directly related to velocity. However, mass is a fundamental property of matter and does not change with velocity. Therefore, deceleration does not add mass.

2. How does the theory of relativity explain the relationship between deceleration and mass?

The theory of relativity explains that mass and energy are equivalent and can be converted into each other. When an object accelerates, it gains kinetic energy, but this does not affect its mass. Similarly, when an object decelerates, it loses kinetic energy, but its mass remains unchanged.

3. Can deceleration affect the weight of an object?

Weight is a measure of the force of gravity on an object, and it is directly proportional to mass. Since deceleration does not affect mass, it does not have any direct impact on weight. However, if an object is decelerating due to gravity, its weight may appear to change because the force of gravity is decreasing.

4. How is the concept of inertia related to the misconception about deceleration adding mass?

Inertia is the tendency of an object to resist changes in its velocity. Many people mistakenly believe that this resistance is due to an increase in mass when an object accelerates. However, inertia is an inherent property of matter and does not change with velocity, so deceleration does not add mass.

5. Are there any scenarios where deceleration could add mass?

No, deceleration cannot add mass. As discussed previously, mass is a fundamental property of matter and does not change with velocity. Therefore, deceleration does not have the ability to add mass under any circumstances.

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