The path of one projectile as seen by an observer on another projectile is a/an:

Explanation

Let a ball `A` is thrown with speed `u_(1)` and angle of projection `theta_(1)` with horizontal. Another ball `B` is thrown will speed `u_(2)` at angle `theta_(2)` with horizontal, simultaneously.

After time `t`,
Ball `A`: `x_(1) = u_(1) cos theta_(1) t`
`y_(1) = u_(1) sin theta_(1) t - (1)/(2)g t^(2)`
Ball `B`: `x_(2) = u_(2) cos theta_(2) t`
`y_(2) = u_(2) sin theta_(2) t - (1)/(2)g t^(2)`
Position of `B` as seen from `A`

`tan alpha = (y_(2) - y_(1))/(x_(2) - x_(1)) = (u_(2) sin theta_(2) - u_(1) sin theta_(1))/(u_(2) cos theta_(2) - u_(1) cos theta_(1)) = const ant`
The path of `B` as seen by `A` will be inclined straight line `(alpha != 0^(@))`, if `u_(1) cos theta_(1) = u_(2) cos theta_(2)`, path will be vertical straight line `(alpha = 90^(@))`.
Note: 1. if a bomb is dropped from an aeroplane moving with constant velocity, the path of bomb as seen by the pilot wii be vertical straight line and for observer on the ground, path will be parabola.
2. If a particle is moving under gravity and angle between the initial velocity and acceleration due to gravity is other than `0^(@)` or `180^(@)`, the path will be parabola.