The motion of one projectile as seen from another always:

The motion of one projectile as seen from another can be described as a combination of both straight line and parabolic paths. To understand this concept, let's consider two projectiles, A and B, and observe the motion of projectile A as seen from projectile B.

Straight Line Motion:
When viewed from projectile B, the motion of projectile A may appear as a straight line if both projectiles are moving with constant velocities in the same direction. In this scenario, both projectiles have parallel paths, and the relative motion between them is linear. This is similar to observing a moving car from another moving car, where the motion may seem straight.

Parabolic Motion:
On the other hand, if projectile A is launched vertically upwards and projectile B is stationary, the motion of projectile A as seen from projectile B will be parabolic. As projectile A rises in the air, it slows down due to the effect of gravity until it reaches its highest point. From the perspective of projectile B, this motion will appear as a parabolic arc, as it observes projectile A's vertical motion.

Combined Motion:
In most cases, the motion of one projectile as seen from another will involve a combination of straight line and parabolic paths. This occurs when both projectiles have different initial velocities, directions, or accelerations. The relative motion between the two projectiles will then produce a trajectory that is a combination of straight line and parabolic segments.

For example, if projectile A is launched at an angle while projectile B is stationary, the motion of projectile A as seen from projectile B will initially appear as a straight line (horizontal component of motion) until it reaches its maximum height. Then, as projectile A starts descending, the motion will appear as a parabolic arc (vertical component of motion).

In conclusion, the motion of one projectile as seen from another can exhibit both straight line and parabolic characteristics, depending on the initial conditions and relative motion between the projectiles.