主题：【整理】发几个有趣的物理现象，可以用来当魔术玩 -- 铁手

铁球的运动半径在减小，但是能量不变，着就要求大的向心力。加入铁链足够坚强，那么铁联和转动轴之间的压力是增大的.假定摩擦系数是常数. f max=n*N

the work of the ball did during the man' fall: W= intergral( mv^2/r*dr) from r0 ->0, we neglect the effect of the ball's gravity.

so finally, W=e^(r0/r)*(mv^2)=1/2.*mv_final**2+Mg*H

M is mass of the man and H is the height of the man's drop.

N=mv_final**2/r=2*(W-Mg*H)/r

fmax=n* N

fmax>=Mg.

finally the force depends on the ratio between r0/r, which can be always sufficient large to provide force to balance the weight of the man and compensate the energy due to fall, when r->0.