- #1
henpen
- 50
- 0
First, energy of a disk:
[tex] \int \frac{dm}{2}r^2 \omega^2 =\frac{\omega^2}{2}\int_0^R m\frac{2 \pi r dr}{\pi R^2}r^2 =\frac{m\omega^2}{ R^2}\int_0^R r^3 dr=\frac{m\omega^2 R^2}{4 }[/tex]
Which agrees with other sources. However, in the following lies my problem:
The equation for a circle: [itex] r^2+x^2=R^2 \Rightarrow r^2=R^2-x^2[/itex]
Energy of a sphere- integrate infinitesimal disks's rotational kinetic energy (assuming rotational energy is additive, which makes sense physically), all of which have their centre through the x-axis:
[tex] \int_0^R \frac{m\omega^2 r^2}{4 }dx=\frac{m\omega^2}{4 }\int_0^R(R^2-x^2)dx=\frac{m\omega^2}{4 }\frac{2R^3}{3}=\frac{m\omega^2 R^3}{6 }[/tex]
This differs from the result here of [tex]\frac{I \omega^2}{2}=\frac{m\omega^2 R^2}{5 }[/tex] quite substantially. Where have I gone wrong?
[tex] \int \frac{dm}{2}r^2 \omega^2 =\frac{\omega^2}{2}\int_0^R m\frac{2 \pi r dr}{\pi R^2}r^2 =\frac{m\omega^2}{ R^2}\int_0^R r^3 dr=\frac{m\omega^2 R^2}{4 }[/tex]
Which agrees with other sources. However, in the following lies my problem:
The equation for a circle: [itex] r^2+x^2=R^2 \Rightarrow r^2=R^2-x^2[/itex]
Energy of a sphere- integrate infinitesimal disks's rotational kinetic energy (assuming rotational energy is additive, which makes sense physically), all of which have their centre through the x-axis:
[tex] \int_0^R \frac{m\omega^2 r^2}{4 }dx=\frac{m\omega^2}{4 }\int_0^R(R^2-x^2)dx=\frac{m\omega^2}{4 }\frac{2R^3}{3}=\frac{m\omega^2 R^3}{6 }[/tex]
This differs from the result here of [tex]\frac{I \omega^2}{2}=\frac{m\omega^2 R^2}{5 }[/tex] quite substantially. Where have I gone wrong?