The angular speed of a motor wheel is increased from $$1200 \mathrm{~rpm}$$ to $$3120 \mathrm{~rpm}$$ in $$16 \mathrm{~s}$$. The angular acceleration of the motor wheel is
The centre of mass of an extended body on the surface of the earth and its centre of gravity
A metallic rod breaks when strain produced is $$0.2 \%$$. The Young's modulus of the material of the $$\operatorname{rod} 7 \times 10^9 \mathrm{~N} / \mathrm{m}^2$$. The area of crosssection to support a load of $$10^4 \mathrm{~N}$$ is
A tiny spherical oil drop carrying a net charge $$q$$ is balanced in still air, with a vertical uniform electric field of strength $$\frac{81}{7} \pi \times 10^5 \mathrm{~V} / \mathrm{m}$$. When the field is switched OFF, the drop is observed to fall with terminal velocity $$2 \times 10^{-3} \mathrm{~ms}^{-1}$$. Here $$g=9.8 \mathrm{~m} / \mathrm{s}^2$$, viscosity of air is $$1.8 \times 10^{-5} \mathrm{Ns} / \mathrm{m}^2$$ and density of oil is $$900 \mathrm{~kg} \mathrm{~m}^{-3}$$. The magnitude of $$q$$ is