During impact, the force on the ball and on the strings is always equal and opposite. This force begins at zero pounds just before impact, and climbs continuously to a peak value just at the stringbed deflects to its maximum, stops the ball, and changes its direction.
Force is the second most important characteristic of a string, because it is what you feel most at impact. It relates most to what you call “feel” or “comfort” or “crispness.”
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Stiff strings have higher peak forces than softer strings. Peak force is highest on polyesters and aramids and lowest on gut. Nylon falls in between, with multifilaments being lowest in force.
Force is related to dwell time. For stiff strings the force is very high and the time on the strings is short. For soft strings, the force is low but the time on strings is long. Hence, the power exerted by the force of the strings on the ball over time is about equal in all situations.