1. The problem is to understand and use the concept of the speed of light, denoted as $C$, which is a fundamental constant in physics. 2. The speed of light in vacuum is approximately $C = 3 \times 10^{8}$ meters per second. 3. This constant is used in many physics formulas, such as Einstein's famous equation for energy-mass equivalence: $$E = mC^{2}$$ where $E$ is energy, $m$ is mass, and $C$ is the speed of light. 4. Important rules: - $C$ is always positive and constant in vacuum. - It is used to relate energy and mass. 5. Example: If you have a mass $m = 2$ kg, the energy equivalent is: $$E = 2 \times (3 \times 10^{8})^{2} = 2 \times 9 \times 10^{16} = 1.8 \times 10^{17}$$ 6. This means 2 kg of mass is equivalent to $1.8 \times 10^{17}$ joules of energy. This is the basic use of the concept of $C$ in physics.