Investigate conservation of energy by analysing a video of a water droplet falling.
- Surface area of a sphere of radius r = 4πr 2
- Volume of a sphere of radius r = (4/3)πr 3
- Surface area of a cylinder of radius r and height h = 2πr 2 + 2πrh
- Surface energy density of water/air = 0.073J/m 2
- Density of water = 1000kg/m 3
- What is the surface area of a spherical water drop with a width of 20mm?
- As a drop falls, it deforms, and its shape can be modelled as a cylindrical disk. If the drop from question 1 spreads to a maximum width of 24mm and corresponding thickness of 9.26mm, what is the new surface area?
- What is the change in surface area?
- Molecules at the surface of a material have more energy than those in the material’s body; surface energy measures this “excess” energy. Surface energy density describes the amount of energy required to create a surface of a certain area.
- What is the change in surface energy for the drop described above?
- Does the drop’s surface energy increase or decrease?
- Which shape (sphere or cylinder) is more energy-efficient? What do you think the most energy-efficient shape is? Why?