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On the conservation law of physical mechanical energy The radius of the smooth track fixed in the vertical plane is r, and a small ball with mass m moves in a circle on the track in an anti clockwise direction. If the pressure of the ball on the track is 8 mg at the lowest point a, the pressure on the track is 8 mg when the ball moves to the highest point B______ The answer is 2mg, but I don't know why
At the lowest point a, the supporting force of the track on the ball (the reaction force of the pressure of the ball on the track) - Mg = centripetal force
8mg-mg=mV^2/r
mV^2=7mgr
In the process from a to B, the mechanical energy is conserved, and the velocity at point B is u
(1/2)mV^2=mg(2r)+(1/2)mu^2
mu^2=mV^2-4mgr=7mgr-4mgr=3mgr
At point B, the pressure of the track on the ball is f
F + Mg = centripetal force Mu ^ 2 / r = 3mgr / r = 3mg
F=3mg-mg=2mg
The pressure of the ball on the orbit is the reaction force of F, which is also 2mg
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