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HW #6-1-solutions

# HW #6-1-solutions - pasha(sep635 HW 6-1 Antoniewicz(56445...

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pasha (sep635) – HW 6-1 – Antoniewicz – (56445) 1 This print-out should have 21 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 (part 1 of 3) 2.0 points An electron has mass 9 . 1 × 10 - 31 kg. The speed of light is 3 × 10 8 m / s. If the electron’s speed is 0 . 982 c (that is, v/c = 0 . 982), what is its particle energy? Correct answer: 4 . 33607 × 10 - 13 J. Explanation: The electron’s particle energy, or total en- ergy, is given by E = γ m c 2 = 1 1 - v c 2 m c 2 = 1 1 - 0 . 982 2 (9 . 1 × 10 - 31 kg) c 2 = 4 . 33607 × 10 - 13 J . 002 (part 2 of 3) 2.0 points What is its rest energy? Correct answer: 8 . 19 × 10 - 14 J. Explanation: The rest energy is given by E r = m c 2 = (9 . 1 × 10 - 31 kg)(3 × 10 8 m / s) 2 = 8 . 19 × 10 - 14 J . 003 (part 3 of 3) 1.0 points What is its kinetic energy? Correct answer: 3 . 51707 × 10 - 13 J. Explanation: The kinetic energy is just the di ff erence be- tween the particle energy and the rest energy: K = E - E r = 4 . 33607 × 10 - 13 J - 8 . 19 × 10 - 14 J = 3 . 51707 × 10 - 13 J . 004 (part 1 of 4) 2.0 points The point of this question is to compare rest energy E r and kinetic energy KE at low speeds. A baseball is moving at a speed of 11 m / s. Its mass is 150 g (0 . 15 kg). The speed of light is 3 × 10 8 m / s. What is its rest energy? Correct answer: 1 . 35 × 10 16 J. Explanation: The rest energy is given by E rest = m c 2 = (150 g)(3 × 10 8 m / s) 2 = 1 . 35 × 10 16 J . 005 (part 2 of 4) 1.0 points Is it okay to calculate its kinetic energy using the formula 1 2 m v 2 ? 1. Yes, because | v | c . correct 2. Yes, because accuracy isn’t too important here. 3. No, because | v | c . 4. No, because baseballs are not subatomic particles Explanation: It’s okay to use the approximate formula here, because the baseball’s speed is so small compared to the speed of light. 006 (part 3 of 4) 1.0 points What is its kinetic energy? Correct answer: 9 . 075 J. Explanation:

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pasha (sep635) – HW 6-1 – Antoniewicz – (56445) 2 Since we can use the approximate formula from Newtonian mechanics, the kinetic energy is KE = 1 2 m | v | 2 = 1 2 (150 g)(11 m / s) 2 = 9 . 075 J .
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