How are obligations mutual in feudalism

Feudal lords are obligated to offer up soldiers and taxes to the king in exchange for essentially leasing the land from him , since in the Feudal system , the king literally owns all the lands of his country . Unless the feudal lords get too powerful , then they can start dictating termstomthe king instead , like what happened in Englad , when the magma carta was forced down the Kings throat .

I believe there is something wrong with the experiments.

First of all, there is no control group  which make the researcher able to distinguish which variables are the one that would change the nature of independent variable. The second reason is because there is biased or non representative sample in the study ( only from colleagues in the same field)