In the radioactive decay of (), the centre of the emerging particle is, at a certain instant, from the centre of the residual nucleus. We have to find at that instant (a) the force on the particle, and (b) its acceleration.

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We note that the atomic number of Th is 92. That is a nucleus of Th has units of charge; and that the atomic number of He is 2 and it has units of charge. Therefore, at the instant when particle is from the particle, the magnitude of the electrostatic force on it will be

(b)

Molar mass of is 4.0026 g. Therefore, rest mass of a particle will be

Acceleration of the particle at that instant, if calculated using the Newtonian mechanics, will be

But as the particle will be relativistic, we should use its relativistic mass in calculating its acceleration.