two spheres of same mass m and radius r rolls down without slipping down an inclined plane . one of sphere is solid and the other is a thin spherical shell . Explain which one will reach the bottom of the plane first , if released from the same point on the plane
ANSWER –
For a rolling down a spheres , total kinetic energy at the bottom of the plane
K=KT + KR = 1/2mv2 + 1/2Iω2 = 1/2v2(m+ I/r2) [ω=v/r]
If the velocities of solid and hollow spheres at any point on the plane be Vs and Vh respectively , we have for solid sphere
K_s=\frac{1}{2}V_s^{2}(m+\frac{2}{5}\frac{mr^{2}}{r^{2}})=\frac{7}{10}mV_s^{2}
And for the hollow sphere
K_h=\frac{1}{2}V_h^{2}(m+\frac{2}{3}\frac{mr^{2}}{r^{2}})=\frac{5}{6}mV_h^{2}
but at any point on the plane the total kinetic energy , when falling from the same point will be same for both the spheres
Ks = Kh
\frac{7}{10}mV_{s}^{2}=\frac{5}{6}mV_{h}^{2}
\frac{V_s^{2}}{V_h^{2}}=\frac{50}{42}
VS > Vh