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Five suspended balls labelled 1 to 5 are tested in pairs. Pairs (1,2), (2,4) and (4,1) attract each other, whereas pairs (2,3) and (4,5) repel. What can be concluded about ball 1?

1. It is neutral
2. It is positively charged
3. It is negatively charged
4. It is made of metal

Correct answer: It is neutral

Solution

Repulsion (2,3) and (4,5) proves balls 2 and 4 are charged. Ball 1 attracts BOTH 2 and 4. If 1 were charged, it could not be opposite in sign to both 2 and 4 unless 2 and 4 carried the same sign - but a charged ball attracting two same-sign balls is impossible. The only consistent possibility is that ball 1 is neutral, attracting charged balls by induction.

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