To understand why raindrops fall at a constant speed by the time they reach the ground, we need to consider the forces acting on them as they descend through the atmosphere. The correct answer to your question is option b: "The force due to air resistance increases with the speed of the raindrop until it balances the gravitational force." Let's break this down further.
The Forces at Play
When a raindrop begins to fall, it is initially accelerated by gravity, which pulls it downward. However, as the raindrop gains speed, it encounters air resistance, also known as drag. This drag force opposes the motion of the raindrop and increases with its velocity.
Understanding Terminal Velocity
As the raindrop continues to fall, the drag force increases until it equals the gravitational force acting on the raindrop. At this point, the forces are balanced, and the raindrop stops accelerating. This state is known as terminal velocity. The raindrop then continues to fall at a constant speed, which is the terminal velocity.
Why Other Options Are Incorrect
- Option a: "Raindrops are too light to be accelerated by gravity." This is incorrect because gravity affects all objects, regardless of their mass. Raindrops do experience gravitational acceleration initially.
- Option c: "The force due to air resistance is constant and balances the gravitational force." This is misleading. The drag force is not constant; it increases with the speed of the raindrop until it reaches a balance with the gravitational force.
- Option d: "Raindrops originate in outer space, where the gravitational effects are negligible." This is not true; raindrops form in the atmosphere due to condensation of water vapor, and they are very much affected by Earth's gravity.
Real-World Example
Think of a skydiver jumping out of an airplane. Initially, they accelerate downward due to gravity. As they pick up speed, air resistance increases until it equals the force of gravity. At that point, they stop accelerating and fall at a constant speed—this is their terminal velocity, similar to what happens with raindrops.
Conclusion
In summary, raindrops fall with a constant speed when they reach the ground because the increasing air resistance balances the gravitational force acting on them. This balance leads to a state of terminal velocity, allowing them to descend steadily without further acceleration.
