He received the letter challenging him to a duel and began preparing for the duel.

Gender roles are an important element in defining individual behavior in society.

His role is to fulfill his responsibilities as a leader.

We should save electricity for energy conservation.

We should increase the use of alternative energy to protect the earth.

The potential energy of this ball is large because it is in a high position.

The potential energy of this ball is proportional to its height.

Ionization energy is the minimum energy required for an atom or molecule to lose an electron.

This park is an energy spot for the local people.

This park is known as a local energy spot.