Final answer:
When walking downstairs, the energy source used is gravitational potential energy. This potential energy is converted into kinetic energy as you descend each step.
Step-by-step explanation:
When walking downstairs, the energy source used is gravitational potential energy. As you descend each step, your potential energy decreases, and this energy is converted into kinetic energy.
Gravitational potential energy is the energy stored in an object due to its position above the ground. When you walk downstairs, your potential energy decreases because you are moving closer to the ground. This energy is converted into kinetic energy, which is the energy of motion.
So, the energy source used to walk downstairs is the conversion of gravitational potential energy into kinetic energy.