A nucleus of ${ }_{84}^{210} \mathrm{Po}$ originally at rest emits $\alpha$ particle with speed v. What will be the recoil speed of the daughter nucleus

4v/206 4v/214 v/206 v/214

4v/206

${ }_{84} \mathrm{Po}^{210} \rightarrow{ }_{82} X^{206}+{ }_2 \mathrm{He}^4$ Using conservation of linear moments $206 v'+4 v=0 \Rightarrow v'=-\frac{4 v}{(206)} \Rightarrow\left|v'\right|=\frac{4 v}{206}$