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Ch. 36 - The Special Theory of Relativity
Giancoli Douglas - Physics for Scientists and Engineers 5th edition
Giancoli Douglas5th editionPhysics for Scientists and EngineersISBN: 9780137488179Not the one you use?Change textbook
Chapter 35, Problem 62

A spaceship moving toward Earth at 0.65c transmits radio signals at 95.0 MHz. At what frequency should Earth receivers be tuned?

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1
Identify the problem as a relativistic Doppler effect scenario, where the frequency of the signal received on Earth is affected by the relative motion of the spaceship and Earth.
Write the formula for the relativistic Doppler effect: fre=1+vc1-vcfe, where fr is the received frequency, fe is the emitted frequency, v is the relative velocity of the source (spaceship) with respect to the observer (Earth), and c is the speed of light.
Substitute the given values into the formula: fe=95.0 MHz, v=0.65c. The formula becomes: fr=1+0.651-0.6595.0.
Simplify the square root term: 1.650.35. This term represents the relativistic factor due to the motion of the spaceship.
Multiply the simplified relativistic factor by the emitted frequency 95.0 MHz to find the received frequency fr.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Doppler Effect

The Doppler Effect describes the change in frequency or wavelength of a wave in relation to an observer moving relative to the source of the wave. In this scenario, as the spaceship approaches Earth, the frequency of the transmitted radio signals will appear higher to the Earth receivers due to the relative motion, which is a key aspect of the effect.
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Relativistic Doppler Shift

The relativistic Doppler shift accounts for the effects of special relativity when dealing with objects moving at significant fractions of the speed of light. It modifies the classical Doppler effect equations to include time dilation, which is crucial for accurately calculating the observed frequency of signals from fast-moving sources like the spaceship.
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Frequency and Wavelength Relationship

The relationship between frequency and wavelength is governed by the equation c = fλ, where c is the speed of light, f is the frequency, and λ is the wavelength. Understanding this relationship is essential for converting between frequency and wavelength when analyzing the signals received on Earth, especially when considering the effects of the Doppler shift.
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Related Practice
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A certain galaxy has a Doppler shift given by ƒ₀ - ƒ = 0.1015 ƒ₀. Estimate how fast it is moving away from us.

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An atomic clock is taken to the North Pole, while another stays at the Equator. How far will they be out of synchronization after 1.5 years has elapsed? [Hint: Use the binomial expansion, Appendix A–2.]

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(III) (a) In reference frame S, a particle has momentum p=pxi\(\overrightarrow{\mathbf{p}\)}=p_{x}\(\mathbf{i}\) along the positive x axis. Show that in frame S’, which moves with speed v as in Fig. 36–12, the momentum has components

px=pxvE/c21v2/c2p_{x}^{\(\prime\)}=\(\frac{px-vE/c^2}{\sqrt{1-v^2/c^2}\)}

py=pyp_{y}^{\(\prime\)}=py

pz=pzp_{z}^{\(\prime\)}=pz

E=Epxv1v2/c2.E^{\(\prime\)}=\(\frac{E-p_{x}\)v}{\(\sqrt{1-v^2/c^2}\)}.

(These transformation equations hold, actually, for any direction of p\(\overrightarrow{\mathbf{p}\)}, as long as the motion of S' is along the x axis.) (b) Show that px, py, pz, E/c transform according to the Lorentz transformation in the same way as x, y, z, ct.

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