1 Which length of the ruler shook or vibrated the most? (1) 2 Did you feel any shaking (vibrations) in the hand that held the ruler down on the desk? (1) 3 What did you learn about vibrations from doing this task? (1)

here are the answers to the questions based on the experiment described:

1.1. The length of the ruler that shook or vibrated the most was the $\boxed{\text{20 cm length}}$.

1.2. Yes, you would feel $\boxed{\text{shaking (vibrations)}}$ in the hand that held the ruler down on the desk.

1.3. From doing this task, you learn that the $\boxed{\text{length of a vibrating object affects how it vibrates and the sound it produces}}$.