Which of the following best describes the principle that the momentum of an isolated system is conserved?

• A. The total energy of an isolated system always increases over time.
• B. The total $p$ (momentum) of an isolated system remains constant if no external forces act on it.
• C. The total mass of an isolated system is converted into $E$ (energy).
• D. The total $p$ (momentum) of an isolated system is always $0$.

Answer: (B)

Verified step by step guidance

1. Understand that the principle of conservation of momentum states that in an isolated system (one with no external forces), the total momentum remains constant over time.
2. Recall that momentum \(p\) is defined as the product of mass \(m\) and velocity \(v\), so \(p = m \times v\).
3. Recognize that when no external forces act on the system, the vector sum of the momenta of all objects in the system does not change, meaning \(\sum p_{initial} = \sum p_{final}\).
4. Note that this principle applies regardless of the interactions between objects inside the system, as long as external forces are absent.
5. Therefore, the correct description is that the total momentum of an isolated system remains constant if no external forces act on it.