Question

Estimate the force exerted by a $75W$ bulb on your finger tip (are $1c{m}^2$), when is at a distance of $8cm$ from the bulb. (Assume $100\%$ absorption)

• A. $1N$
• B. $3.1\times {10}^{-10}N$
• C. $4\times {10}^{-6}N$
• D. $2\times {10}^{-4}N$

Answer 1

The correct option is B $3.1\times {10}^{-10}N$

$\begin{array}{c}P=\frac{\overline{S}}{c}=\frac{\frac{P}{4\pi d^2}}{c}=\frac{\frac{75W}{4\pi {\left(8.0\times {10}^{-2}m\right)}^2}}{3.00\times {10}^{8}m/s}=3.1\times {10}^{-6}N/m^2.\\ Theforceispressuretimesarea,\\ weapproximatetheareaofafingertiptobe1.0c{m}^2\\ F=PA=\left(3.1\times {10}^{-6}N/m^2\right)\left(1.0\times {10}^{-4}{m}^2\right)=3.1\times {10}^{-10}N\end{array}$

Hence,

option $\left(B\right)$ is correct answer.