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Now our task is to find whether Newton’s laws are true in the mechanics of high velocities. If not, it would be necessary to look for other laws.
To draw conclusion about the 1st Law — Law of Inertia, analyse the motion of an object. If the object moves at a constant velocity 
relative to frame 
, and its velocity 
relative to the other inertial frame of reference, 
, is also constant, then the Law of Inertia must be valid.
It is easy to deduce that is constant provided 
and 
. Therefore, the 1st Law of dynamics is applicable in relativistic mechanics.
The 2nd Law of classical dynamics states that the applied force is a measure of the change in momentum of an object, 
, here the momentum is determined as a product of the mass and the velocity of the body.
Fig. 2.7
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It is obvious that the applied force causes the change of momentum (fig. 2.7) of a body in relativistic mechanics too. The difference is that the change of momentum consists of the change in mass and the change of velocity of the object and the momentum may be written as
.
Taking into consideration this statement we can apply the 2nd Law of dynamics to relativistic cases too.
The 3rd Law of dynamics is also valid in relativistic mechanics.
We can conclude that classical dynamics is therefore a particular case (under the conditions 
) of the general theory mentioned above.
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| AFTER STUDYING THE TOPIC A STUDENT IS TO | | | The total energy of a body equals the sum of its rest energy and its kinetic energy |