Torque from tangential forces requires zero net force for "only a torque"

Steps:

• Calculate net force: vector sum of all F; must be zero for pure torque.
• Calculate torque: each tangential F contributes FR (lever arm R); total magnitude is sum considering direction (clockwise/counterclockwise).
• For magnitude FR, net torque equals FR with no translational acceleration.
• Identify arrangement where forces balance translationally but produce net rotational effect of FR.

Why B is correct:

• Two forces of F at 120° apart on the edge create zero net force (vector sum cancels) and net torque FR via unbalanced rotational components.

Why the others are wrong:

• A: Four forces produce torque 2FR (full couple) with zero net force.
• C: Three forces at 120° yield zero torque and zero net force (symmetric).
• D: Single force gives torque FR but net force F (translational motion).

Final answer: B