Here,`u=72km//h=(72xx1000)/(60xx60)m//s=20m//s`
As `(1)/(2)m_(1)upsilon_(1)^(2)=(1)/(2)m_(2)upsilon_(2)^(2):.(upsilon_(1))/(upsilon_(2))=sqrt((m_(2))/(m_(1)))`
`(p_(1))/(p_(2))=(m_(1)upsilon_(1))/(m_(2)upsilon_(2))=(m_(1))/(m_(2))xxsqrt((m_(2))/(m_(1)))=sqrt((m_(1))/(m_(2)))=sqrt((1)/(4))=(1)/(2)`