an apple falling from a tree branch doorbell ringing when a switch is pushed a wagon rolling across a yard when pulled two sticks getting warm when rubbed together
I am hesitating between 4 options to answer the following question, which option should I choose?
Question: What is an example of a force producing heat?
Possibilities:
- an apple falling from a tree branch
- doorbell ringing when a switch is pushed
- a wagon rolling across a yard when pulled
- two sticks getting warm when rubbed together

two sticks getting warm when rubbed together