Question

Find the mass of an object whose rest mass is 1000g when it is travelling at 90% of the speed of light?

Answer 1

The mass of an object

$$m=\frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}=\frac{1000\:\rm g}{\sqrt{1-0.9^2}}=2294\:\rm g$$