What is a second's pendulum?

Explanation

### Step-by-Step Solution:

1. **Definition of a Pendulum**: A pendulum is a mass (called the bob) attached to a string or rod that swings back and forth under the influence of gravity.

2. **Understanding Time Period**: The time period (T) of a pendulum is the time taken for one complete cycle of motion, which includes moving from one extreme to the other and back again.

3. **Second's Pendulum**: A second's pendulum is specifically defined as a pendulum that has a time period of exactly 2 seconds. This means it takes 2 seconds to complete one full swing back and forth.

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