# The Naked Scientists Forum

### Author Topic: Why does an objects mass/weight increase as it gets closer to the speed of ligh?  (Read 2114 times)

#### Dragon Ice

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##### Why does an objects mass/weight increase as it gets closer to the speed of ligh?
« on: 17/04/2011 12:01:29 »
I was looking at some equations and as far as i can see an objects mass/weight cannot change as it speeds up.
Yet we know that it does this.

#### JP

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##### Why does an objects mass/weight increase as it gets closer to the speed of ligh?
« Reply #1 on: 17/04/2011 14:43:48 »
The confusion here is that there are actually two commonly used definitions of mass in relativity.

Relativistic mass follows E=mc2.  This means as the energy of something goes up as it increases in speed, this mass goes up.  This mass is basically a measurement of energy.

Invariant mass follows E2-(pc)2=m2c4.  This mass doesn't change a something speeds up, as it's equal to the mass you would measure if the object were at rest.

It sounds like you're looking at equations of invariant mass, but thinking about relativistic mass.

#### Dragon Ice

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##### Why does an objects mass/weight increase as it gets closer to the speed of ligh?
« Reply #2 on: 17/04/2011 15:05:47 »
The confusion here is that there are actually two commonly used definitions of mass in relativity.

Relativistic mass follows E=mc2.  This means as the energy of something goes up as it increases in speed, this mass goes up.  This mass is basically a measurement of energy.

Invariant mass follows E2-(pc)2=m2c4.  This mass doesn't change a something speeds up, as it's equal to the mass you would measure if the object were at rest.

It sounds like you're looking at equations of invariant mass, but thinking about relativistic mass.

i was always taught that the E in E=MC^2 was the energy in the object itself
(i.e. in 1 gram of antimatter when combined with normal matter)
so that would explain why i was confused

thanks JP

#### yor_on

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##### Why does an objects mass/weight increase as it gets closer to the speed of ligh?
« Reply #3 on: 17/04/2011 20:06:29 »
And I think you were taught right :)

Take a object speeding towards us near light speed. When it hit Earth we will feel it real bad as compared to the same object just getting dragged down by Earths gravity.

Imagine now that you decide to measure this 'relativistic energy'. You take your spacecraft and accelerate out and then  set your speed to the object coming at earth. Where is its extra 'energy'? That 'relative energy/mass' is a relation between earth/the object but also a relation between you landing/the object. In one case the 'energy' or momentum or relative mass is enormous, in the other negligible, simultaneously.

And I expect this is the reason why physics mostly use 'mass' as in the 'invariant kind' nowadays. The invariant mass is, at least is expected to be, the exact same in all 'frames of reference' so counting on that you then have to look at the momentum relative whatever you measure it against to see the interaction. But, in a way they seem interchangeable to me.

#### lightarrow

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##### Why does an objects mass/weight increase as it gets closer to the speed of ligh?
« Reply #4 on: 17/04/2011 20:33:52 »
i was always taught that the E in E=MC^2 was the energy in the object itself
E it's the energy if the object is not moving with respect to your frame of reference. If it's moving, E is not the energy.

#### The Naked Scientists Forum

##### Why does an objects mass/weight increase as it gets closer to the speed of ligh?
« Reply #4 on: 17/04/2011 20:33:52 »