/ Newton’s Law:
Action & Reaction
/ 12 Feb 1998 David Tenenbaum /
A fisher spider in a real hurry "gallops" across the water, lifting itself off the surface while sprinting up to 75 centimeters per second!
That’s _____ inches!
/ < Look at the Photo!
See the bug? This fisher spider's legs make 8 dimples on the water & give it a lot of drag so it walks on top of water!
Photos courtesy Robert Suter, Vassar College
Action and reaction is handy stuff. Action: a jet engine forces burned fuel backwards. Reaction: the plane zooms forward. Action: a bicycle tire pushes the road -- and the Earth -- back. Reaction: the bike moves forward.
Action and reaction is not just handy: it's about the only thing that makes anything move. It's as plain as Isaac Newton and his Laws of Motion:
For Every Action There is an Opposite and Equal & Reaction
This Law of Motion is also true in Chemistry! For example, you usually get a reaction when you add something to water. (Water is the compound of chemicals – 2 hydrogen atoms + 1 oxygen atom – called H2O). If you add salt (1 sodium atom + 1 chlorine atom, called NaCl) it will dissolve in the water and make a solution. That’s because you made an action happen (adding salt) and got a reaction (salt disappearing in water)!
But what's the deal with the simple water spider? This eight-legged marvel speeds across the water fast enough to escape predators, find prey, and chase off other spiders. But how does it move? It must be pushing the water backwards, but what is doing the pushing? What’s the action and reaction?
That kind of question interests biology professor Robert Suter. Suter has turned his attention to figure out how this spider that walks on water. The fisher spider (called Dolomedes triton) lives in ponds in North America – there’s 15 different kinds of it. Two other groups of insects, including water striders, can do it, too.
Like a Cat, It Hates Water
Suter knows that surface tension – which is a force between a liquid and solid – makes the solid resist being wetted by that liquid. This explains why water beads up on a waxy surface like your car -- because of surface tension.
Objects like wax and glass that resist being wetted by water are called "hydrophobic" (hydro = water, and phobic = fear). The special hydrophobic legs of fisher spiders and other water-walking arthropods create so much surface tension they barely touch the water. The legs makes dimples where they contact the water. So how does it move if it barely touches the water? We can’t walk if our feet don’t touch the ground!
It's a paradox: the same phenomenon that lets the spider stay on the surface seems to stop it from moving. Yet the spider does move. Suter found spiders "row" across the water by pushing on the dimples of water under their legs. The dimple creates drag on the water, allowing the action of the leg to push the water backward while barely touching it. In reaction, the spider moves forward. Drag is the same thing that moves a rowboat - an oar pushes water backward, and the boat reacts by moving forward.
Science in Hawai‘i: Nā Hana Ma Ka Ahupua‘a – A Culturally Responsive Curriculum Project
Retrieved & adapted in part from: