Physics

15 breaks

Forces, light, time and the odd behaviour of the very small.

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Why a small push at the right beat builds a huge swing: resonance and the narrow peak Every child learns to pump a swing without a lesson: small shoves, well timed, build a huge ride, while big shoves at the wrong moment achieve nothing. That… Physics4 min read Why a skater spins faster when she pulls her arms in: angular momentum and r squared A skater starts a slow spin with her arms wide, pulls them in, and blurs into a fast spin with no push from anything. The trick is a quantity that cannot… Physics4 min read Why a small ball on a big ball shoots up nine times higher: the two-step bounce Drop a tennis ball on top of a basketball and the small ball can fly far above the height it fell from. Nothing is cheating: it is a two-step bounce. The floor… Physics4 min read Why the sky is blue and the sunset red: Rayleigh scattering and the length of the air The sky is blue and the sunset is red, yet the sunlight is the same. Air scatters short blue waves about four times more strongly than long red ones, a… Physics4 min read Why a siren drops in pitch as it passes: the Doppler effect An ambulance siren sounds high as it comes at you and drops the moment it passes, yet the driver hears one steady note. The trick is in how crests per second… Physics3 min read Why light bends in water: the lifeguard's shortest-time path (Fermat's principle) A lifeguard runs faster than she swims, so the quickest route to a swimmer is not a straight line. It bends at the shoreline, and the bend follows one tidy… Physics4 min read Why a spinning ball curves in mid-air: the Magnus effect Clip a football with the side of your boot and it bends around a wall with nothing touching it in the air. A spinning ball drags air around with its surface… Physics5 min read Why a boat's wake is always a 39° V: Kelvin's wedge Behind every boat spreads a V of ripples, and it is always about 39° wide, whether the boat crawls or speeds. Lord Kelvin worked out why in 1887: only waves… Physics4 min read Why the bottom of a dropped Slinky hangs in mid-air: a wave has to bring the news Hold a Slinky by its top, let it stretch, and let go. The top rushes down while the bottom coil hovers, apparently ignoring gravity, until the collapsing coils… Physics5 min read Why a dried coffee drop leaves a ring: the edge is stuck and the rim dries first Spill coffee, let it dry, and you get a dark ring with a pale middle, not an even stain. The pigment started out spread through the whole drop. The drop's edge… Physics2 min read Why a tossed tennis racket flips: the intermediate axis theorem Spin a tennis racket about its handle or its face and it flies cleanly. Spin it about the third axis and it flips, even in orbit where nothing pushes on it… Physics3 min read Why water drops last longer on a hotter pan: the Leidenfrost effect Flick water onto a warm pan and it sizzles away in a second. Make the pan hotter, past about 193 °C, and something odd happens: the drop pulls into a bead… Physics5 min read