Friday, November 27, 2015

Thor's hammer

I came across this video on YouTube which I found quite fascinating. A certain guy was able to lift a 19 kg weight over his head with little effort.
 
https://www.youtube.com/watch?v=GeyDf4ooPdo

When the wheel is at rest, he has to apply a force an upward force, Fb to balance out Fa and mg; he also has to apply Fa to act counterclockwise to Fa and mg. Once the wheel starts rotating, it generates a a torque that also acts counterclockwise to the original torque and balances it out. Hence he only has to apply Fb to balance out mg; Fa is no longer needed and the wheel and shaft feel lighter.

Assume Fa is 0.4m and Fb is 0.3 m from the axis of rotation 2.
mass of apparatus = 19 kg. At rest,
Fa + mg = Fb
 Fb - Fa = 19*9.8 = 186.2 N

0.4Fa = 0.3Fb

Fa = 558.6 N   Fb = 744.8

Στ = Iα, I = mr2 = 9.8 kg * 0.15 m = 1.47 kgm, assume α = 30 rad/s2
torque = 1.47*30 = 4140 Nm

This torque generated is big enough to balance out the torque created by the weight of the wheel and make it feel lighter.













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