Two particles each of mass m are traveling toward each other with equal speeds v in reference frame O. They collide and form a new composite particle of mass M. (a) Is the mass M greater than 2m, equal to 2m, or less than 2m? (b) Observer O ′ is in motion relative to O with speed v in the same direction as one of the particles, so that particle appears to be at rest in O ′. Will O ′ agree with the answer to question (a)?

Two particles each of mass m are traveling toward each other with equal speeds v in reference frame O. They collide and form a new composite particle of mass M. (a) Is the mass M greater than 2m, equal to 2m, or less than 2m? (b) Observer O ′ is in motion relative to O with speed v in the same direction as one of the particles, so that particle appears to be at rest in O ′. Will O ′ agree with the answer to question (a)?

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