GRE Subject Test: Physics : Four Vectors

Study concepts, example questions & explanations for GRE Subject Test: Physics

varsity tutors app store varsity tutors android store

All GRE Subject Test: Physics Resources

33 Practice Tests Question of the Day Flashcards Learn by Concept

Example Questions

Example Question #1 : Four Vectors

A reference frame S' is moving at 0.6c in the z direction with respect to a stationary frame S.  An event occurs in S' with the coordinates (x', y', z', ct')=(1, 0, 2, 3).  What are the coordinates (x, y, z, ct) of the event with respect to the stationary frame S?

Possible Answers:

(1, 0, 2, 3)

(1, 0, 2.64, 3.3)

(1, 0, 3.3, 2.64)

(1, 0, 5.25, 4.75)

(1, 0, 4.75, 5.25)

Correct answer:

(1, 0, 4.75, 5.25)

Explanation:

To apply a Lorentz transformation, we need gamma and beta:

\(\displaystyle \beta=\frac{v}{c}=\frac{3}{5}\)

\(\displaystyle \gamma=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}=\frac{5}{4}\)

Then, apply the Lorentz transformation:

\(\displaystyle x=x'=1\)

\(\displaystyle y=y'=0\)

\(\displaystyle z=\gamma(z'+\beta ct')=\frac{5}{4}(2+\frac{3}{5}\cdot 3)=4.75\)

\(\displaystyle ct=\gamma(ct'+\beta z')=\frac{5}{4}(3+\frac{3}{5}\cdot 2)=5.25\)

 

Example Question #2 : Four Vectors

A container of water is carried on a space ship traveling at 60% the speed of light relative to the Earth. From the perspective of an observer on Earth, what is the speed of the light in the water?

\(\displaystyle n_{water}=1.33\)

Possible Answers:

\(\displaystyle 3 c\)

\(\displaystyle \frac{9}{29}c\)

\(\displaystyle \frac{27}{29}c\)

\(\displaystyle \frac{27}{47}c\)

\(\displaystyle \frac{9}{47}c\)

Correct answer:

\(\displaystyle \frac{27}{29}c\)

Explanation:

The speed of light through water in a stationary frame is found from the index of refraction of water, which is approximately \(\displaystyle \frac{4}{3}\)

\(\displaystyle v'=\frac{c}{n}=\frac{3}{4}c\)

To find the relative speed, use the following equation:

\(\displaystyle v_{p}=\frac{v'+v}{1+\frac{vv'}{c^2}}\)

\(\displaystyle v\approx0.6c\)

All GRE Subject Test: Physics Resources

33 Practice Tests Question of the Day Flashcards Learn by Concept
Learning Tools by Varsity Tutors