why does the kinetic energy and gravitational potential energy decrease the longer a pendulum swings?

Answer 1

Aye...there's the rub.

And that's the answer...the rub, the friction on the pivot point of the pendulum. The initial energy given a pendulum by raising it (increased PE) and then letting it go (initial KE), is dissipated through heat created by the friction force acting in opposition to the direction of the swing. Thus, for each subsequent swing, the potential energy is less because the pendulum fails to get as high as the swing before and the ensuing KE is less because of the decreased PE with each swing.

This is a prime example of why there is no such thing as a perpetual motion machine. There is always friction wherever two bodies rub together. Even if that friction is very very tiny, there is always some. Thus, eventually, the system (the pendulum in this case) will eventually come to a halt unless additional energy is input.

Good question.

Answer 2

Kinetic energy is diminished by dispersal non-directionally, while gravitational energy is realized from the potential through less resistance from the kinetics. Gravity is a constant, kinetics is not.

Answer 3

Friction at the pivot and air resistance rob the pendulum of energy, eventually slowing it to a stop.