Yes, but for a school experiment the most reliable method is usually not to calculate distance from the image alone.

Best practice: add a measured reference in the scene, such as a board, backdrop, or grid with known spacing placed near the ball’s path. That lets you read position directly from the video frame. An even simpler check is to measure distances physically with a tape measure.

If you try to compute distance from the image only, you need accurate camera details such as sensor size and actual focal length, and the result may not be very precise on a phone camera.

Another workable method is calibration: place the tennis ball at several known distances from the fixed camera, record images, measure the ball’s apparent size in each image, and create a distance-vs-image-size chart. Then compare your experiment frame to that chart. This only works if camera position, zoom, and viewing setup stay unchanged.

Yes, camera height matters. The camera-to-ball distance is the straight-line distance, so if the camera is above the ground, the geometry changes and the line of sight is longer than the horizontal ground distance.