Question

A pulse of light of duration 100 ns is absorbed completely by a small object initially at rest. The power of the pulse is 30 mW and the speed of light is $3\times {10}^8$ m/s. The final momentum of the object is

• A. $0.3\times {10}^{-17}kgm{s}^{-1}$
• B. $1.0\times {10}^{-17}kgm{s}^{-1}$
• C. $0.3\times {10}^{-17}kgm{s}^{-1}$
• D. $9.0\times {10}^{-17}kgm{s}^{-1}$

Answer 1

The correct option is B $1.0\times {10}^{-17}kgm{s}^{-1}$

$t=100\times {10}^{-9}s,P=30\times {10}^{-3}W,C=C\times {10}^{8}m{s}^{-1}$

$Momentum=\frac{Pt}{C}=\frac{30\times {10}^{-3}\times 1000\times {10}^{-9}}{3\times {10}^8}=1.0\times {10}^{-17}kgm{s}^{-1}$