Understanding Momentum
* Momentum: A measure of an object's mass in motion. It's calculated as momentum (p) = mass (m) x velocity (v).
* Net Momentum: The total momentum of a system. In a closed system (one where no external forces act), the net momentum remains constant.
The Stone-Earth System
1. Before the Drop: The stone is at rest, and the Earth is also considered to be at rest (its velocity is very small compared to the stone's). This means the initial momentum of the system is zero.
2. During the Fall: When you drop the stone, gravity acts on it, causing it to accelerate downwards. This means the stone gains momentum. But, at the same time, the Earth is also pulled upwards (very slightly) due to the gravitational attraction between the stone and Earth. This tiny upward movement of the Earth gives it a very small momentum in the opposite direction.
3. Key Point: Because of Newton's Third Law (for every action, there is an equal and opposite reaction), the stone's momentum and the Earth's momentum will always be equal and opposite. This ensures the net momentum of the system remains zero throughout the fall.
The Answer
The net momentum of the stone-Earth system is zero at all times, even as the stone falls. The key is recognizing that the Earth also gains a tiny but equal and opposite momentum.
Additional Notes
* External Forces: If air resistance is significant, it would introduce an external force, and the system wouldn't be perfectly closed. The net momentum would no longer be zero.
* Momentum Transfer: We can think of momentum as being transferred from the Earth to the stone, and back again. This transfer is what causes the stone to accelerate downwards.
Let me know if you have any more questions!
