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An electron and a proton, initially at rest and infinitely far apart, are released and pulled toward one another by their mutual attraction. Which statement about their kinetic energies is correct?

1. The electron's kinetic energy exceeds the proton's
2. The electron's kinetic energy is smaller than the proton's
3. Both have equal kinetic energy
4. It cannot be decided; it depends on their separation

Correct answer: The electron's kinetic energy exceeds the proton's

Solution

Internal forces are equal and opposite, so momentum is conserved; both acquire equal magnitude of momentum p. Kinetic energy = p²/(2m). Since the electron has much smaller mass, its kinetic energy (inversely proportional to mass for equal p) is far greater than the proton's.

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