A windmill that lifts a weight is a simple project that shows how moving air can do useful work. When wind or a fan pushes on the blades, the blades spin and turn an axle. A string wrapped around the axle winds up and raises a small weight, such as a paper clip cup or washer.
This makes energy transfer easy to see because motion in the air becomes motion in the weight.
Key Facts
- Work is done when a force moves an object: W = Fd.
- The weight force of a lifted object is Fg = mg.
- The gravitational potential energy gained is GPE = mgh.
- A larger blade area can catch more wind, but blades that are too heavy may spin slowly.
- A smaller axle radius usually lifts the weight more slowly but can make lifting easier.
- Energy changes form: wind kinetic energy to rotational energy to gravitational potential energy.
Vocabulary
- Wind energy
- Wind energy is the kinetic energy carried by moving air.
- Axle
- An axle is the rod or tube that spins with the windmill blades and winds the string.
- Work
- Work is the energy transferred when a force moves an object through a distance.
- Torque
- Torque is a twisting effect that makes an object rotate around an axis.
- Gravitational potential energy
- Gravitational potential energy is stored energy an object has because it is raised above a lower position.
Common Mistakes to Avoid
- Making the weight too heavy, which is wrong because the windmill may not produce enough torque to lift it. Start with a light load and add mass slowly.
- Using floppy or unbalanced blades, which is wrong because uneven blades wobble and waste energy. Make the blades the same size and attach them evenly around the hub.
- Letting the axle rub tightly against the supports, which is wrong because friction steals energy from the spinning windmill. Leave a small gap and make sure the axle turns freely.
- Winding the string in the wrong direction, which is wrong because the spinning axle may unwind the string instead of lifting the weight. Test the spin direction before attaching the load.
Practice Questions
- 1 A 0.05 kg weight is lifted 0.40 m by the windmill. Using g = 9.8 m/s^2, how much gravitational potential energy does it gain?
- 2 A windmill lifts a 0.10 kg load. What is the weight force on the load in newtons if g = 9.8 m/s^2?
- 3 If two windmills use the same fan and the same load, explain why the one with lighter, balanced blades and a smoother axle might lift the weight higher.