Regarding potential and kinetic energy, when a pendulum swings we do not count the force from the string because it doesn't affect the energy, but why? if we split the force from the string to it's components it does affect the force of gravity which in turn affects the acceleration of the pendulum which in turn affects the kinetic energy? Or something like skating down a ramp, the normal force can be split into it's components and therefore affecting the gravitational force.