Cut a small notch out of the back of the boat.
Float the boat in a tub of water. What happens? Not much!
Now, place a small sliver of soap in the notch and watch. What happens? The boat moves across the tub!
Why?
Without soap, the water pulls on the boat from all directions, resulting in little to no movement. When the soap is added, it reduces the pull of the water at the back of the boat. The pull at the front of the boat remains strong and you see movement.
Students can experiment with boat shape to find the fastest (and straightest) racer!
Because the soap reduces the water's surface tension, the water in the tub will need to be dumped out and replaced often.
