Unbalanced rocket thrusts cause net nonzero force

Steps:

• Weight W acts downward; pivot exerts R upward equal to W, balancing vertical forces.
• Each rocket exerts force F on the rod at perpendicular distance d from pivot.
• Rockets face opposite directions, so their forces F act in the same linear direction on the rod, producing net force 2F.
• Net force is nonzero, so firework accelerates linearly and is not in equilibrium.

Why D is correct:

• For rotational-translational equilibrium, net force must be zero (Newton's first law); here, unbalanced rocket forces give nonzero resultant force.

Why the others are wrong:

• A: Firework not in equilibrium; net force nonzero prevents it despite any torque consideration.
• B: Firework not in equilibrium, but not because net force is zero—it's nonzero.
• C: Torque may be nonzero, but lack of equilibrium stems from nonzero net force, not zero torque.

Final answer: D