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An isolated particle of mass m moves in the horizontal x-y plane along the x-axis at some height above the ground. It suddenly explodes into two fragments of masses m/4 and 3m/4. Shortly after, the smaller fragment is located at y = +15 cm. Where is the larger fragment at the same instant?

1. y = -5 cm
2. y = +20 cm
3. y = +5 cm
4. y = -20 cm

Correct answer: y = -5 cm

Solution

Since the CM stays on the x-axis (y_cm = 0), (m/4)(15) + (3m/4)(y2) = 0, giving y2 = -5 cm for the larger fragment.

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