The quantity dE/dv, the rate of increase of energy with speed, is the amount of additional energy a moving object needs per 1 m/s increase in speed. A 25,000 kg rocket is traveling at 0.90c. How much additional energy is needed to increase its speed by 1 m/s?
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35. Special Relativity
Special Vs. Galilean Relativity
Problem 69c
Textbook Question
The sun radiates energy at the rate 3.8 x 1026 W. The source of this energy is fusion, a nuclear reaction in which mass is transformed into energy. The mass of the sun is 2.0 x 1030 kg. Fusion takes place in the core of a star, where the temperature and pressure are highest. A star like the sun can sustain fusion until it has transformed about 0.10% of its total mass into energy, then fusion ceases and the star slowly dies. Estimate the sun's lifetime, giving your answer in billions of years.

1
Step 1: Use Einstein's mass-energy equivalence formula, E = mc², to calculate the total energy that can be produced by the Sun. Here, m is the mass of the Sun that can be converted into energy (0.10% of the Sun's total mass), and c is the speed of light (approximately 3.0 × 10⁸ m/s).
Step 2: Calculate the mass that can be converted into energy by multiplying the Sun's total mass (2.0 × 10³⁰ kg) by 0.10%. This gives m = 0.001 × 2.0 × 10³⁰ kg.
Step 3: Substitute the value of m from Step 2 and c = 3.0 × 10⁸ m/s into the equation E = mc² to find the total energy that can be radiated by the Sun.
Step 4: The Sun radiates energy at a constant rate of 3.8 × 10²⁶ W (watts). To estimate the Sun's lifetime, divide the total energy calculated in Step 3 by the Sun's energy output per second (3.8 × 10²⁶ W). This gives the Sun's lifetime in seconds.
Step 5: Convert the Sun's lifetime from seconds into years by dividing by the number of seconds in a year (approximately 3.15 × 10⁷ seconds per year). Finally, express the result in billions of years.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Nuclear Fusion
Nuclear fusion is a process where two light atomic nuclei combine to form a heavier nucleus, releasing a significant amount of energy. In stars like the sun, hydrogen nuclei fuse to create helium, which powers the star and produces the energy we receive as sunlight. This process occurs under extreme temperature and pressure conditions in the star's core.
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Mass-Energy Equivalence
Mass-energy equivalence, expressed by Einstein's equation E=mc², states that mass can be converted into energy and vice versa. In the context of the sun, a small fraction of its mass is converted into energy during fusion, which sustains the sun's luminosity. Understanding this principle is crucial for estimating how long the sun can continue to produce energy before exhausting its fuel.
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Stellar Lifespan
The stellar lifespan refers to the duration a star can sustain nuclear fusion before it exhausts its nuclear fuel. For the sun, this is approximately determined by the total mass available for fusion and the rate at which it converts mass into energy. The sun can sustain fusion for about 10 billion years, and its current age is around 4.6 billion years, indicating it has several billion years left before it transitions to the next stage of its lifecycle.
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