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An electron with (rest mass m₀) moves with a speed of 0.8 c. Its mass when it moves with this speed is:

1. m₀
2. m₀/6
3. 5m₀/3
4. 3m₀/5

Correct answer: 5m₀/3

Solution

The relativistic mass of an object is given by m = m₀ / √(1 - v²/c²). Substituting v = 0.8c, we get m = m₀ / √(1 - (0.8)²) = m₀ / √(1 - 0.64) = m₀ / √0.36 = m₀ / 0.6 = 5m₀/3.

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