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Question

Figure (10-E15) shows a small spherical ball of mass m rolling down the loop track. The ball is released on the linear portion at a vertical height H from the lowest point. The circular part shown has a radius R.
(a) Find the kinetic energy of the ball when it is at a point A where the radius makes an angle θ with the horizontal.
(b) Find the radial and the tangential accelerations of the centre when the ball is at A.
(c) Find the normal force and the frictional force acting on the if ball if H = 60 cm, R = 10 cm, θ = 0 and m = 70 g.

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Solution

(a) Let the velocity and angular velocity of the ball at point A be v and ω, respectively.
Total kinetic energy at point A =12mv2+12Iω2



Total potential energy at point A=mgR+Rsinθ
On applying the law of conservation of energy, we have:
Total energy at initial point = Total energy at A
Therefore, we get:
mgH=12mv2+12Iω2+mgR1+sinθmgH-mgR1+sinθ=12mν2+12Iω212mv2+12Iω2=mgH-R-Rsinθ ...1Total K.E. at A=mgH-R-Rsinθ

(b) Let us now find the acceleration components.
Putting I=25mR2 and ω=vR in equation (1), we get:
710mv2=mgH-R-Rsinθv2=107gH-R-Rsinθ ...2
Radial acceleration,
ar=v2R=107gH-R-RsinθR
For tangential acceleration,
Differentiating equation 2 w.r.t. 't' ,2vdvdt=-107gRcosθdθdtωRdvdt=-57 gRcosθdθdtdvdt=-57 gcosθat=-57 gcosθ

(c) At θ=0, from the free body diagram, we have:



Normal force = N=mar
N=m×107gH-R-RsinθR =701000×107×10 0.6-0.10.1 = 5 N

At θ=0, from the free body diagram, we get:
fr=mg-mat (fr = Force of friction)
fr=mg-at =m10-57×10 =0.0710-57×10 =1100 70-50=0.2 N

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