Japan

Japan

This water strider makes its home at a zen garden in Hiroshima, Japan. They have the relatively unique ability of not only running across the water, but even standing still on the water's surface without ever falling through. Water striders can live this way due to surface tension and application of Conservation of Momentum.

Why doesn't the water strider sink?

The water strider's weight is pressing down on the water with a force of mass x gravity, or mg. In order for the water strider to avoid sinking, the water must be exerting a force up on the insect. This force is a combination of surface tension caused by hydrogen bonding between the water molecules and buoyency force that says the water exerts a force on the insect that is equal to the weight of the water displaced by the insect. The weight of the insect is also distributed over six long legs covered with thousands of hairs (Hu, 2003).

How does the water strider move forward?

The water strider's legs make a rowing motion that pushes the water in the opposite direction that the insect wants to go (Hu, 2003). The Law of Conservation of Momentum states that momentum in a system is conserved, which means that the insect gains forward momentum when it imparts backwards momentum to the water.

The Law of Conservation of Momentum can be described by pinitial = pfinal. In the case of the water strider moving forward from standing still, the initial momentum is 0, and the final momentum includes both the water strider's forward momentum and the water's backwards momentum, so 0 = pinsect - pwater. Therefore, the momentum from the insect and the water must be equal to cancel out. The small vortices in the photo are caused by the backwards movement of the water.