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\(A 2.5-m\) titanium rod in a moving spacecraft is at an angle of \(45^{\circ}\) with respect to the direction of motion. The craft moves directly toward Earth at \(0.98 \mathrm{c}\). As viewed from Earth, (a) how long is the rod and (b) what angle does the rod make with the direction of motion?

Short Answer

Expert verified
(a) The rod is 0.5 m long; (b) the rod's angle is 81.9掳 with the direction of motion.

Step by step solution

01

Understanding Length Contraction

Due to the phenomenon of length contraction, the length of an object moving at relativistic speeds is shorter relative to a stationary observer. The contracted length \(L'\) can be found by the formula \(L' = L \sqrt{1 - v^2/c^2}\), where \(L\) is the proper length (length in the spacecraft's frame), \(v\) is the velocity of the spacecraft, and \(c\) is the speed of light.
02

Convert Velocity to Expressed Terms

The velocity of the spacecraft is given as \(0.98c\). Thus, \(v^2/c^2 = (0.98)^2\). Calculate this value to use in the formula for length contraction.
03

Calculate the Contracted Length

Plug the values into the formula: \( L' = 2.5 \text{ m} \times \sqrt{1 - (0.98)^2}\). Simplify to find the contracted length as observed from Earth.
04

Determine the Contracted X-component

Since the rod makes an angle of \(45^{\circ}\) with the direction of motion, find the x-component of the rod (the component along the direction of motion). This is given by \(L_x = L \cos(45^{\circ})\). Substitute \(L = 2.5 \text{ m}\).
05

Apply Length Contraction to X-component

Contract only the x-component using the length contraction formula. Find the contracted x-component: \(L_x' = L_x \sqrt{1 - (0.98)^2}\).
06

Calculate Uncontracted Y-component

The y-component (perpendicular to the motion) does not undergo length contraction. Calculate \(L_y = L \sin(45^{\circ})\).
07

Compute Observed Angle

Use the contracted x-component and uncontracted y-component to find the observed angle using \(\tan(\theta') = L_y / L_x'\). Solve for the angle \(\theta'\), the angle the rod makes with the direction of motion as seen from Earth.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Special Relativity
Special Relativity is a fundamental concept introduced by Albert Einstein in 1905. It revolutionizes the way we understand space and time, especially for objects moving at speeds close to the speed of light. Special Relativity is based on two key postulates:
  • The laws of physics are the same for all observers, regardless of their relative motion.
  • The speed of light in a vacuum is the same for all observers, regardless of their relative motion.
These principles lead to some fascinating consequences, such as time dilation and length contraction. Length contraction specifically refers to the phenomenon where the length of an object parallel to the direction of its motion appears shorter to an observer moving relative to the object. This contraction only occurs at relativistic speeds, meaning speeds approaching that of light, denoted by the symbol 鈥渃.鈥 As a result, experiments and observations need to account for these relativistic effects, especially in fields like astronomy and particle physics.
Lorentz Transformation
The Lorentz Transformation is a set of mathematical equations that describe how measurements of space and time are related between two inertial frames of reference moving at a constant velocity relative to each other.These transformations are crucial for understanding how observations change when moving from one frame to another, especially at velocities close to the speed of light. They are derived from the axioms of Special Relativity, addressing differences in time (time dilation) and space (length contraction).For length contraction, the relevant Lorentz transformation is given by:\[ L' = L \sqrt{1 - \frac{v^2}{c^2}} \]where:
  • is the proper length (length measured in the rest frame of the object),
  • is the length observed in a moving inertial reference frame,
  • is the relative velocity between the observer and the object,
  • is the speed of light.
This equation shows that as the velocity v approaches c, the factor \( \sqrt{1 - \frac{v^2}{c^2}} \) approaches zero, indicating significant length contraction.
Relativistic Effects
Relativistic Effects refer to phenomena that become significant when objects move at speeds comparable to the speed of light. These effects include length contraction, time dilation, and increases in mass, all predicted by Einstein's theory of Special Relativity. When a spacecraft travels at a velocity close to the speed of light, such as 0.98c, these effects become prominently observable. In the context of length contraction, the rod in the spacecraft appears shorter to an observer on Earth. This is because the dimensions of objects moving at high speeds along the direction of motion are compressed. Another key relativistic effect is time dilation, where time appears to pass more slowly for observers on the fast-moving spacecraft than it does for stationary observers. For observers on Earth watching the spacecraft, these effects seem counterintuitive but have been experimentally confirmed in various ways, including through the observation of cosmic rays and in experiments with high-speed particles. These effects underscore the need to account for relativistic corrections in practical scenarios, ensuring more accurate scientific and technological applications.

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Most popular questions from this chapter

Suppose the speed of light is \(35 \mathrm{mi} / \mathrm{h}\). A paper girl riding a bicycle at \(22 \mathrm{mi} / \mathrm{h}\) throws a rolled-up newspaper in the forward direction, as shown in Figure \(29-30\). If the paper is thrown with a speed of \(19 \mathrm{mi} / \mathrm{h}\) relative to the bike, what is its speed, \(v,\) with respect to the ground?

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