A sandscape builder filled with water and air bubbles is set by trial and error to build a pyramid with a stream of descending sand fed by collapse from the top of the upper reservoir. A fancy impractical egg-timer.

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Science activity: "Make an unattended vehicle to go three metres in less than 10 seconds." Daughter wanted a land yacht. Fan-driven" counts as "unattended". Piece of cake! Only problem is the three metres: nobody steering.

Design solution: spinnaker HOLDING SHAPE well forward. Ballast out back. Contraption self-corrects for the straight line with a torque about the wheels under the deck. Maiden voyage. (The roaring fan is out of sight under the camera.) You saw it first on flickr.

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The uppermost rings of the spring contract in order. The collapsing top section descends at an almost constant velocity, reaching the bottom of the spring in less than half the time taken by an object released from rest at the same time and height.

The time total to collapse the spring is the time taken for the centre of mass of the spring to fall with acceleration "g" to the level at which all the turns touch. Because the initial centre of mass of the spring when hanging is more than half way from the top to the bottom (closer to the bottom) the time taken to collapse the spring is the time taken for a stone to fall from the level of the centre of mass before release, less than half the time taken for a stone to fall from the level of the hand.

The spring is not a rigid body: the lower coils do not move until the contraction pulse reaches them. The effect causes some confusion. Dragging the clip forwards and backwards frame by frame is interesting. Filmed at 240 fps.

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The physics teacher's trick that's been around for a century. A classic and ever fascinating demonstration of the conservation of energy, or for the more relaxed among the audience, a demonstration of controlled falling. I find the real time version a bit too fast. To compare with the slow-mo version see the comment below.

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Physics types will note potential energy lost to KE during each drop. I note the apparent "looking and hesitation" before the foot goes down on the next step. An iPhone by my daughter at 240 fps, replayed at 30 fps for an eight times time dilation. The sound effects is also slowed and I think better for it.

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