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# Accidents will happen

Wednesday 19 May 2021

I was at an outside party the other night and someone got out a sort of skateboard thing with a motor. One of the teachers had a go and some interesting physics took place. The motor moved the board forward resulting in an unbalanced torque causing angular acceleration around her centre of mass. She broke her wrist.

This is what she should have done.

Gravity acts forwards on her centre of mass and her feet are accelerating forwards. Einstein's principle of equivalence states that an accelerating frame of reference is equivalent to a gravitational field but in the opposite direction. If you are standing in a box accelerating upwards in space at 10 ms-2 it would feel the same as if the box was on the earth. So, the equivalent gravitational field is equal to the downwards gravitational field - the acceleration, if she leans forward at the correct angle the equivalent gravity points from her centre to her feet so she does not rotate about them. Note, the animation is not fixed, this really works. To bring some biology into it (she's a biology teacher) the rider does not have to calculate the angle, they can feel when they are aligned with the gravitational field, it's what we always do when we stand up. The trick is not to think about it. Small children are able to master this easily, they just shift their weight to stand up as normal, they do not know that the board is going to tip them up.