A uniform slender bar AB of mass m and length L supported by a frictionless pivot at A is released from rest at its vertical position as shown in figure. Calculate the magnitude of the reaction at pivot when the bar just acquires the horizontal position shown dotted.
√374mg
Since the pivot is frictionless and there are no other un-conservative forces, conservation of mechanical energy can be applied.
At this position (vertical)
Ki+ui=0+mg(ℓ2)
At horizontal position
kf+uf=12Iω2+0
ki+ui=kf+uf[law of conservation of energy]
mg(l2)=12Iω2
⇒ω=√mglI=√mgl(ml2)3=√3gI[I=ml23
about [Remember this is ω at horizontal position] pivot, as rod is hinged at end]
Considering free body diagram of the rod in horizontal position.
The normal reaction due to hinge can be resolved as Nx and Ny in x and y directions
τ on the rod = mg(ℓ2)
as τ = Iα
mg(l2)=ml23α
α=3g2l
Nx=max[ax is the centripetal acceleration i.e., towards the centre]
=m.ω2(l2)
=m3gl.l2=3mg2
Ny=mg−may
=mg−m.(αl2)
[ay is the tangential acceleration = angular acceleration × 1 distance of com from axis]
=mg−m(l2)(3g2l)
=mg−3mg4=mg4
so the total normal reaction magnitude
|→N|=√N2x+N2y
=√(mg4)2+(3mg2)2=√374mg
if θ is the angle with the horizontal then
tan θ=NyNx=16⇒θ=tan−1(16)