If I went tubing with my little sister, and I have a larger mass, why would we both reach, in terms of energy, the bottom of the hill at the same time;

Originally I thought becuase we would both be accerlerating at the same rate but that doesn't explain the energy part does it?

The reason you and your little sister would reach the bottom of the hill at the same time in terms of energy is due to the principle of conservation of mechanical energy. In this scenario, let's assume that both of you start at the top of the hill with the same initial gravitational potential energy.

When you go tubing down the hill, your larger mass would require more force to overcome inertia and friction compared to your sister's smaller mass. This means you would experience a greater acceleration. However, although you are gaining more kinetic energy as you accelerate, you are also converting some of your initial potential energy into kinetic energy.

Now, according to the conservation of mechanical energy, the sum of potential energy and kinetic energy remains constant as long as no external forces, such as friction, are acting on the system. This means that even though you are gaining more kinetic energy due to greater acceleration, you are simultaneously losing an equivalent amount of potential energy.

As a result, the rate at which your potential energy decreases and your kinetic energy increases is balanced in a way that you and your sister will reach the bottom of the hill with the same total mechanical energy. Therefore, in terms of energy, you both will reach the bottom of the hill at the same time.

In summary, while your acceleration might be different due to your larger mass, the conversion of potential energy into kinetic energy ensures that the total energy remains the same, allowing you and your sister to reach the bottom of the hill simultaneously.