# Simulating the Raindrop Problem

## The Simulation:

The diagram at left shows a simple simulation of the "Raindrop Problem". By adjusting the speeds of the "raindrop" and the "car window", as well as the angle of the car window, you can explore the relationship among these quantities.

Hopefully, the simulation can:

• give you some insight into the physical situation, and
• give you some "experimental data" with which to check your theoretical analysis.

## Building the Simulation:

1. Load the Interactive Physics program.
2. Use the circle tool to build a "raindrop", and the rectangle tool to build a "car window".
3. Use the rotation tool to adjust the angle of the window.
4. Give the raindrop a downward velocity by either:
1. grabbing the center point of the circle with the mouse and pulling the velocity vector down, or
2. opening the Properties Window (in the Window menu) and clicking on the circle, then enter a value for vy. (vx should be 0.)
5. Turn off gravity so the raindrop will move at constant velocity (and the window won't fall!).
1. In the World menu, select Gravity...
2. Select None.
6. Give the car window a horizontal velocity..

## Running the Simulation:

Adjust the velocities of the raindrop, window, or both until the distance between the drop and the window stays constant as the simulation progresses. If the drop hits the window, then the car is going too slowly (or the drop is falling too fast), and if the distance between the drop and the window increases, then the car is going too fast (or the drop is falling too slowly). It might help if you:

• turn on Tracking (in the World Menu)
• use the magnifier tool to zoom in
• slow down the action. You don't need "realistic" velocities here since you are looking for a relationship. Velocities in the range of 0.5 - 5 m/s make the simulation easier to follow.

You can use the rotation tool to change the angle of the car window and try again. The diagram at right shows how you can measure the angle of the window. Record your "data" carefully.