Question

The frequency of a radar is 780MHz. The frequency of reflected wave from an airoplane is increased by 2.6kHz. The velocity of the airoplane is

• A. 2km/s
• B. 1km/s
• C. 0.5km/s
• D. 0.25km/s

Answer 1

Answer: (C)

0.5km/s

Given : $\delta =780MHz=780\times {10}^{6}Hz$

${\delta }^{{}^{\prime }}=2.6\times 780\times {10}^{6}Hz$

$v_s=0m/s$

$v=330m/s$

By Doppler effect formula

${\delta }^{{}^{\prime }}=\delta \left(\frac{v-v_0}{v-v_3}\right)$

$2.6\times 780\times {10}^6=780\times {10}^6\left(\frac{330+v_p}{330-0}\right)$

$2.6\times 330=330+v_p$

$\Rightarrow v_p=330\left(1.6\right)$

$=500m/s$

$=0.5km/s$