I need to create an object that will sink 100 cm in 100 seconds. The closer, the better. This means that it needs to be sinking at the rate of 1cm/second, if it is constant (although it is not possible), and it needs to be just slightly more dense than water. I've tried a water bottle with rocks and water in it, but it increases in momentum too fast and too much (not sure if that was the correct term). It begins at the rate of .5cm per second, but then after about 10cm, it goes at the rate of 3cm/second. If I remove the slightest of water (just a drop), it floats. I also tried a glass container, and kept adding small rocks (very small). However, the same thing happened. The glass container started off sinking very slowly, but then picked up speed very fast. I know that is how gravity works, but how else am I supposed to accomplish this? Perhaps the shape? Carrying on, I attached a CD on the lid of the glass container, with the water and the rocks. I was able to get to about 60cm in 45 seconds, but when I tried again, it sank in about 20cm... My next idea is to use a balloon, and filling it up with water, and adding/subtracting air. Any ideas? This project is simply a HIGH SCHOOL experiment challenge, so it may not be very difficult. Thank you. PS, I am not sure if this is in the correct section (perhaps the Physics?). Would be appreciated if someone would come to aid.