Any object requires energy to increase its speed.
If m0 is the mass of an object when it is at rest, its internal energy ( potential energy) is given by m0 CC.
If m is its mass when it moves with a speed V, now its total energy including its energy when it was at rest is given by m CC
The difference in these two energies (m-mo) CC is the energy that we have given to the object to move with a speed V.
Thus the energy that we have given is used to increase its mass from m0 to m.
Note that MASS IS A FUNCTION OF SPEED.
When we say that the mass is increased from m0 to m, it means, we have made it to move with a speed V.
It is because the value m denotes the speed V. we cannot use both m and v simultaneously to calculate energy ( as in 1/2 m VV)
In terms of speed, the total energy when it moves with a speed V is m0 CC / Sq.root of {1- (V/C) ^2}
Note that in this equation we are not using m.
Thus the answer to your question is the energy that we gave is used to increase its mass from m0 to m.
Another way of saying this is the energy that we gave is used to make the object move with a speed of V or used to provide a kinetic energy of (m- mo) cc.
It is not an apparent increase of mass. It is actual increase of mass. More over it is not addtion of extra mass from outside.
Mass is a quantity which depends upon speed. Higher is the speed higer is its mass. The minimum mass is it when it is at rest.
Its energy equivalent is mass times the square of the speed of light.
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