The law of conservation of momentum is stated for closed and isolated system. What are the two conditions for a system to be closed and isolated?

Step 1: Analyze the question The question asks for the two conditions that make a system "closed and isolated" for the law of conservation of momentum to apply.

Step 2: The concept

• Isolated system: No net external force acts on it. Without an external force, nothing from outside can change the system's total momentum.
• Closed system: No mass enters or leaves, so the total mass stays constant. If mass were gained or lost, it would carry momentum into or out of the system.

The law of conservation of momentum states: the total momentum of a closed, isolated system remains constant.

Step 3: Check each option

• A. No external force and no change in mass. This matches both conditions, so it is correct.
• B. A gain in mass means the system is not closed, so it is wrong.
• C. A loss in mass means the system is not closed, so it is wrong. (A rocket expelling fuel is an example.)
• D. An external force means the system is not isolated, so it is wrong.

Step 4: Answer The correct option is A: no external force acts on the system and there is no change in mass of the system.

Tip: Remember "Isolated = no Impulse from outside forces" and "Closed = no Change in mass." Both must hold for momentum to be conserved.