This is fun...Aatish Bhatia maps out the forces and motions involved in doing an ollie on a skateboard.
It's a neat piece of science art, and it also tells us something interesting. The arrows show us that the force on the skateboard is constantly changing, both in magnitude as well as in direction. Now the force of gravity obviously isn't changing, so the reason that these force arrows are shrinking and growing and tumbling around is that the skater is changing how their feet pushes and pulls against the board. By applying a variable force that changes both in strength and direction, they're steering the board.
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Aatish Bhatia noticed a plant in his backyard whose leaves naturally repelled water. He took a sample to a friend who had access to a high-speed camera and an electron microscope to investigate what made the leaves so hydrophobic.
But how does a leaf become superhydrophobic? The trick to this, Janine explained, is that the water isn't really sitting on the surface. A superhydrophobic surface is a little like a bed of nails. The nails touch the water, but there are gaps in between them. So there's fewer points of contact, which means the surface can't tug on the water as much, and so the drop stays round.
The leaf is so water repellant that drops of water bounce right off of it:
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