Relative speed of two particles pulled by gravity – Rankers Physics
Topic: Gravitation
Subtopic: Gravitational Potential Energy

Relative speed of two particles pulled by gravity

The gravitational force between two particles with masses \(m\) and \(M\), initially at rest at great separation, pulls them together. When their separation becomes \(d\), then speed of either particle relative to the other will be :
\(\sqrt{G(M+m)/2d}\)
\(\sqrt{G(M+m)/d}\)
\(\sqrt{4G(M+m)/d}\)
\(\sqrt{2G(M+m)/d}\)

Solution:

Using energy conservation in the center-of-mass frame: \(\frac{1}{2} \mu v_{\text{rel}}^2 = \frac{GMm}{d}\), where \(\mu = \frac{Mm}{M+m}\) is the reduced mass. Substituting \(\mu\) yields \(v_{\text{rel}} = \sqrt{\frac{2G(M+m)}{d}}\).

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