A cannon and a target are `5.10 km` apart and located at the same level. How soon will the shell launched with the initial velocity `240 m//s` reach the target in the absence of air drag?

Explanation

Here,`u_(0)=240 ms^(-1),R=5.10 km=5100m,g=9.8ms^(-2),alpha=?`
`R=(u_(0)^(2)sin 2alpha)/g`
`sin 2alpha=(Rg)/u_(0)^(2) rArr alpha=30^(@) or 60^(@)`
Using, `T=(2u_(0)sin alpha)/g`
When `alpha=30^(@),T_(1)=(2xx240xx0.5)/9.8=24.5s`
when `alpha=60^(@),T_(2)=(2xx240xx0.867)/9.8=42.46s`