Question

A man, who weighs 60 kg on earth, weighs 61 kg on a rocket, as measured by an observer on earth. What is the speed of the rocket?

(a) 2.5 X 10^8 m/s

(b) 2.5 X 10^7 m/s

(c) 5.5 X 10^7 m/s

(d) 5.5 X 10^8 m/s

I had been asked this question in an online interview.

My question is taken from Einstein’s Special Theory of Relativity topic in chapter Ultrasonics & Relativity of Engineering Physics I

Answer 1

Correct choice is (c) 5.5 X 10^7 m/s

The best I can explain: Now, as we know m = \(\frac{m_0}{\sqrt{1-\frac{V^2}{c^2}}}\)

Here, m = 61 kg and m0 = 60 kg

Therefore, 61/60 = 1/\({\sqrt{1-\frac{V^2}{c^2}}}\)

v^2/c^2 = 1 – (60/61)^2 = 121/3600

v = 11 X c/60 = 5.5 X 10^7 m/s.