Mass at light speed

[BenSon]

Ok this sort of came from YT's thread so i'll ask my question here:)

In the eqn f = ma Lets say the object accellerates to 99.999c (lets just say light speed to keep it simple) Then wouldn't its mass change? That means that the force would be different when the acceleration is equal and the mass is determined by how long the object has been accellerating?

I'll give an example.

In a normal situation two identical objects traveling at the same constant acceleration will have the same force even if one object has traveled for 2second and the other for 4seconds

But if you lets another object, identical to the other two mentioned but let it accellerate for a realy long time if will approach light speed where its mass will change and it will get a different value for force then the other two which got equal values...Why is this?

 

~Scott

[Spyman]

As I said in YT's thread it's Newtons second law, it does'n include relativity.

 

According to relativity the force must change to be able to hold a constant acceleration.

 

The relativistic mass is determined by the objects rest mass and velocity.

Thus its mass will not change when it approaches light speed, it will change as soon as the velocity changes and then grow larger to reach infinite at c.

 

With relativity, "in a normal situation", two identical objects under the same constant acceleration will have to be under two different changing forces, if one object has accelerated for 2 seconds and the other accelerated for 4 seconds.

 

But "in a normal situation" the difference between relativity and Newton is so small that it is more practically to use the Newtonian way.

 

So when the velocity gets high, somewhere at half the speed of light, I think, the difference is large enough to make relativistic calculations worth the effort.

 

Why the world is created in such a manner nobody can tell You, it's just the way it is.

[swansont]

It depends on which definition of mass you use. If you use relativistic mass, then it changes. If you use rest, or invariant, mass it unsurprisingly doesn't change with reference frame.

[BenSon]

Thanks for clearing that up

 

I think we put too much weight on it i mean f dosen't realy equal mass times accelleration but everyone is taught that a school and everyone uses that equation. I think that they should teach it with less authority we are always told newtons laws are 'absolutely correct' then they teach us special relativity and thats 'absolutely correct'. I understand that theres no point calculating the diff using reativity in our everyday lives but i just think that they should also say but it is incorrect...

 

~Scott

Edit: I just saw swansnots post what is invarient mass?

[Spyman]

I don't know what "invariant mass" is either, hopefully that swansont will explain...

[Bob182]

I don't know what "invariant mass" is either, hopefully that swansont will explain...

 

it's just the rest mass, i.e. what you would measure if the object was not moving with respect to you.

[swansont]

It's called invariant because it doesn't depend on the frame of reference, i.e. it's invariant under a Lorentz tranformation.

[Bob182]

It's called invariant because it doesn't depend on the frame of reference, i.e. it's invariant under a Lorentz tranformation.

 

 

yes, it is obtained from the energy/momentum invariant equation and has the value:

 

m0 = sqrt(E^2/c^4 - p^2/c^2)

 

it is frame independant and has the same value as the rest mass.

[BenSon]

Got it thanks guys