Explanation: Key Points

• Hydropower plants are used for generating electricity by constructing the dams across the stream, a turbine is located at the narrowing opening of the dam, waterfalls on the turbine due to gravity, and this is used to make the motors run to produce electricity.
• The geothermal power plant utilizes the temperature inside the earth's surface for generating electricity.
• Windfarms utilize the kinetic energy from the wind to rotate the blade of the turbine and shaft of the generator so that electrical energy produced.
• Nuclear power uses radioactive elements that undergo the controlled chain fission reaction to generate heat, and this heat used to produce high-pressure steam to rate the shaft of the generator.

So, the hydropower plants work on the basis of the gravity of the earth.