Question

A car P is moving with a uniform speed `5sqrt3 m//s` towards a carriage of mass 9 kg at rest kept on the rails at a point B as shown in figure. The height AC is 120 m. Cannon balls of 1 kg are fired from the car with an initial velocity `100m//s` at an angle `30^@` with the horizontal. The first cannon hall hits the stationary carriage after a time`t_0` and sticks to it. Determine `t_0`.

At `t_0`, the second cannon ball is fired. Assume that the resistive force between the rails and the carriage is constant and ignore the vertical motion of the carriage throughout. If the second ball also hits and sticks to the carriage, what will be the horizontal velocity of the carriage just after the second impact?

Answer 1

Correct Answer - `t_(0)=12sec,v=100sqrt3//11`
(a) `100m//s` velocity of the ball is relative to ground
[Unless and until it is mentioned in the question, the velocity is always relative to ground]

Horizontal component of velocity of cannon ball,
`u_(x)=ucos30^(@)`
or `u_(x)=(100)xx(sqrt3)/(2)=50sqrt3m//s`
and vertical component `o` its velocity
`u_(y)=usin30^(@)`
`u_(y)=100xx(1)/(2)=50m//s`
and vertical displacement of the ball when it strike the carriage is `-120m` or
`s_(y)=u_(y)t+(1)/(2)a_(y)t^(2)`
`rArr" "-120=(50t)+((1)/(2))(-10)t^(2)`
`rArr" "t^(2)-10t-24=0t=12"s or"-2s`
Ignoring the negative time, we have
(b) When it strikes the carriage, its horizontal component of velocity is still `50sqrt3m//s`. It strikes to the carriage. Let `v_(2)` be the velocity of (carriage+ball) system after collision. The applying conservation of linear momentum in horizontal direction
(mass of ball) (horizontal component of its velocity before collision)=(mass of ball+carrigage)(v_(2))`
` therefore" "(1 kg)(50sqrt3m//s)=(10 kg)(v_(2))`
`therefore" "v_(2)=5sqrt3m//s`
The second ball is fired when the first ball strikes the carrige i.e. after 12s. In these 12x the car will move forward a distance of `12v_(1)` or `60sqrt3m`.
The second ball also take 12s to travel a vertical displacment of `-120m`. This ball will strike the carriage `60sqrt3m` in these 12s. This is possible only when resistive carriage after first collision `(v_(2))=5sqrt3m//s`
Hence, at the time of second collision
Horizontal component of velocity of ball`=50sqrt3m//s` and horizontal velocity of carriage + first ball `= 5sqrt3m//s`. Let `v` be the disired velocity of carriage after second collision. Then conservation of lilnear momentum in horizontal direction gives

`11v=(1)(50sqrt3)+(10)(5sqrt3)=100sqrt3`
`therefore" "v=(100sqrt3)/(11)m//s` or `v=15.75m//s`
In this particular problem values are so adjusted that even it we that the velocity of ball with respect to car, we get the same results of both parts, although the method will be wrong.