We hear often that the new mechanics in tennis is the "modern forehand". What makes for the "modern"? Then there is invariably the mention of angular momentum. How does this apply to tennis?
There has been a change in how the forehand swing is shaped, and the top guys of today hit the forehand much differently from how the top forehand hitters did in the 1990s. They employ angular momentum.
Let's begin with the Wikipedia definition: angular momentum is a vector quantity that represents the product of a body's rotational inertia and rotational velocity about a particular axis. For a rigid body rotating around an axis of symmetry (e.g. the blades of a ceiling fan), the angular momentum can be expressed as the product of the body's moment of inertia, I, (i.e. a measure of an object's resistance to changes in its rotation rate) and its angular velocity.
So, if we think of the blades of a ceiling fan, we can transpose the idea to the tennis player's body and his limbs. How would someone move to conserve the momentum. The body is rotating: arms, shoulders and hips. The legs produce the vertical ground force which is transferred to the hips and arms. What way would be best for the arms to windup, to take advantage of angular momentum?
Federer, Djokovic and Nadal all have different windups. They all have different grips: extreme Eastern, conservative Western, and full Western. But they all have the same main idea: wind inwards using a unit turn, balance on the pivot leg, close the racquet face midway, and open outwards and finish across the body, with a lower followthrough.
Instead of starting the windup high and following a vertical C shaped windup, they prefer to windup inwards and compact. They open outwards with full knowledge their racquet face is closed to the ground and accelerate with ground force. The windups are always consistently done. They do not compromise themselves by doing half of a C shaped swing (doing only the bottom of the swing) when rushed. They are able to finish their entire compact windup cycle against the fastest serves. And they are able to go across the ball more than the 1990s players did, because of these mechanics.
I believe that because of this greater knowledge, the Top 4 have been able to dominate tennis as no other top 4 in the history of tennis. Sure, they may lose an occassional upset. But these basic physics concepts are a significant advantage the Top 4 have been able to utilize with great success.