There is a long solenoid carrying electric current. Straight conductor carrying constant current is placed along the axis of the solenoid. How will the two systems apply magnetic force on each other?

Explanation

We can see that the direction of magnetic field inside the solenoid is parallel to its axis and hence it will not apply any force on straight current placed along its axis. If the applied magnetic field is parallel to current flowing in the wire then no magnetic force will act on the wire.
We can see it in other ways also. Mometic field lines around the straight current are circular loops and hence parallel to the current flowing in the solenoid, so the current of straight conductor cannot apply force on the solenoid.