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Mass (and Momentum) in Special Relativity
We tend to think of the mass of a body as a constant property of the body itself.  
In some sense this is justified: if you go to the moon, you might weigh about six times less than you weigh on earth but your mass doesn't change (assuming you keep the same diet!).
However, we will be concerned here with the mass of a body as measured by an observer in motion relative to that body.
Any measurements made on distant, rapidly moving bodies must involve the use of light or some other electro-magnetic radiation, as was discussed here.  
What follows is a possible method by which observer A can make measurements from which the mass of space-ship B can be calculated.  
We will start with A and B close together and at rest relative to each other.  
 
B now starts the rocket motor and accelerates away from A.  
 
B leaves the motor running for a given time (according to his/her clock, of course).  
Now, by an amazing coincidence, just at the very instant the B stops the motor, looking out the window, he/she sees C.  
If that's not enough, it is observed that B and C are stationary relative to each other (now, there's a real coincidence!)  
 
A now makes a measurement of the velocity of B (and C) relative to A and let's say this turns out to be half the velocity of light, as shown in the diagram above.  
As soon as this measurement has been made, B starts the motor again and leaves it running at the same power, for exactly the same time (on his/her clock) as during the previous period of acceleration.  
 
We must now have the situation shown in this diagram (seen from C's point of view).  
Having been informed by A that the measured velocity was 0.5c, C must conclude that the velocity of B relative to C is also 0.5c (starting from rest, same rocket motor power, same time, therefore, same change in velocity).  
We can now use the equation for combining relative velocities, to transform C's observations to A's frame of reference:  
 
This means that during the first period of acceleration, the velocity of B relative to A changed by 0.5c.  
During the second period of acceleration (starting from rest, same rocket motor power, same time), it only changed by 0.3c.  
Since the force causing the acceleration is exactly the same in the two cases, A must conclude that, according to his/her measurements, the mass (inertia) of the space-ship has increased.  
From A's point of view, it is getting harder and harder to increase the speed of B.   
   
We conclude that the (inertial) mass of a body is a relative concept.   
Observers in relative motion will obtain different values for the mass of a body.  
   
The mass of a body measured by an observer at rest relative to the body is called (not surprisingly) the rest mass of the body. Other observers will find values greater than the rest mass.  
   
As the variation of (measured) mass is basically due to the time dilation effect, you should not be surprised to find that, if the rest mass of a body is mo, then its mass, m, as measured by an observer moving with speed v relative to the body is given by  
 
abbreviated to  
 
Looking at this equation we see that as v approaches c, the mass, m approaches infinity.  
As infinite mass seems unreasonable, we can consider this as an explanation of why we can never measure a relative velocity between two bodies (possessing rest mass) equal to (or greater than) the velocity of light.    
   
Momentum  
As in Newtonian mechanics, the concept of momentum (or "quantity of motion" as Newton called it), is also useful in special relativity.  
In special relativity momentum is defined in exactly the same way (the product of mass and velocity) but we must be careful to stress that the velocity, v, is the velocity of the body relative to the observer  
 
This reminds us that momentum too must be a relative concept.  
 The momentum possessed by a body depends on the state of motion of the body relative to the observer measuring the momentum  
 
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