What is the SI unit of time?

Second (s)

The second is the SI base unit of time. It is defined by the vibrations of caesium-133 atoms.

Understanding Science questions

Science questions check whether you can connect a definition or law to a specific situation. Naming the underlying principle before answering keeps you from guessing on close options.

How to approach it

1. Identify the branch — physics, chemistry or biology — and the concept being tested.
2. State the definition or law in your own words.
3. Apply it to the specific case in the question.
4. Check the answer against everyday observation to see whether it is plausible.

Common mistakes to avoid

• Confusing similar terms such as mass and weight, or element and compound.
• Recalling a fact without checking that it fits the situation described.
• Ignoring units in numerical science questions.

Where it appears: Aligned to CBSE Class 3–12 science, NCERT, IGCSE and Common Core science standards.