**The **branch of science concerned with the ” **nature and interaction** ” of **matter** and **energy.**

**Classical Physics**

The physics that existed before the formulation of **the ****theory of relativity** and **of quantum mechanics**.

It explains **matter **and **energy **only on a **scale familiar **to human experience.

2. **Quantum mechanics **

The branch of physics relating to the **very small**.

It explains the behavior of:

– matter and

– its interactions with energy

on the scale of atoms and subatomic particles

It describes how **light **and **matter **behaves at atomic stages.

**In quantum mechanics,** objects have **probability** of **certain chance** of **being at point A**, **another chance** of **being at point B** and so on.

**3. Relativistic Mechanics**

Science concerned with the motion of bodies whose relative velocities approach the speed of light *c*.