Two equally charged particles, held apart, are released from rest. The initial acceleration of the first particle is observed to be and that of the second to be . The mass of the first particle is . We have to find (a) the mass of the second particle and (b) the magnitude of the charge of each particle.

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Let the charge on each particle be . Separation between the two charged particles is . According to Coulomb’s law the force on each charged particle due to the other will be

We recall

Therefore,

The initial acceleration of the first particle is and its mass is . From Newton’s second law

We therefore have the equation

(b)

The initial acceleration of the second particle is . Its mass can be found by applying the Newton’s third law of motion