An object of mass five kilograms is dropped freely from a height of twenty meters above the ground. Neglecting air resistance and taking acceleration due to gravity as ten meters per second squared, what is the kinetic energy of the object exactly halfway down its fall?

💡 Solution & Explanation

According to the principle of conservation of mechanical energy, the total mechanical energy remains constant throughout the free fall if non-conservative forces are absent. At the starting height of twenty meters, the initial potential energy is calculated as mass multiplied by gravity and height, giving five multiplied by ten multiplied by twenty, which equals one thousand joules, while initial kinetic energy is zero. At the halfway point of ten meters, the potential energy is halved to five hundred joules, meaning the lost potential energy has converted entirely into kinetic energy, resulting in five hundred joules of kinetic energy. A useful related principle is that work done by gravity is equal to the negative change in gravitational potential energy.