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An iron sphere and an aluminium sphere of equal radius are released from rest at the top of a 100 m high tower (neglect air resistance). When they pass a point 40 m above the ground, which quantity is identical for both spheres?

1. acceleration
2. momentum
3. kinetic energy
4. potential energy

Correct answer: acceleration

Solution

In free fall both spheres have the same acceleration g regardless of mass, so they reach 40 m with the same speed. But momentum (mv), kinetic energy (1/2 mv²) and potential energy (mgh) all depend on mass; iron is denser than aluminium so those differ. Only acceleration is identical.

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