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A block of mass m is attached to a second block of mass M by a light spring of spring constant k. Both blocks lie on a frictionless horizontal surface. At the beginning, the system is at rest and the spring is at its natural length. A constant force F is then applied to the block of mass M to pull it. Determine the force exerted by the block of mass m.

1. MF/(m + M)
2. mF/M
3. (M + m)F/m
4. mF/(m + M)

Correct answer: mF/(m + M)

Solution

The whole system accelerates at a = F/(m+M). The only horizontal force on block m is the spring force, which must equal m*a = mF/(m+M).

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