A particle of mass m is projected at an angle of `60^(@)` with a velocity of `20m//s` relative to the ground from a plank of same mass m which is placed on smooth surface Initially Plank was at rest The minimum length of the plank for which the ball will fall on the plank itself is `(g=10m//s^(2))`

Options

A. `40sqrt(3)m`
B. `20sqrt(3)m`
C. `10sqrt(3)m`
D. `60sqrt(3)m`

Answer

A

Explanation

As net external horizontal force is zero and initial velocity of system is also zero
`:. M_(ball)xxx_(ball)=m_(plank)xxx_(plank)(backward)`
distance travelled by the ball is its range
` R=(u^(2)sin20)/g`
`l/2=(20)^(2)/10xxsqrt(3)/2`