When an α-particle of mass 'm' moving with velocity 'v'

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When an α-particle of mass 'm' moving with velocity 'v' bombards on a heavy nucleus of charge 'Ze', its distance of closest approach from the nucleus depends on m as :

At closest distance of approach, the kinetic energy of the particle will convert completely into electrostatic potential energy.
Kinetic energy

K.E. =	1	mv²
	2

Potential energy

P.E. =	KQq
	r

	1	mv² =	KQq	⇒ r ∝	1
	2		r		m

	1
	√m

	1
	m²