Torque produced by a perpendicular force F at a lever arm length l is:

• A. tau = F / l
• B. tau = l + F
• C. tau = l - F
• D. tau = l * F

Answer: (D)

The rotational effect of a force depends on how far the force is applied from the pivot and how much of the force actually tends to rotate the object. When the force is exactly perpendicular to the lever arm, the entire force contributes to turning, so the torque magnitude is the product of the lever arm length and the force: tau = l F.

If the force isn’t at a right angle, only the component perpendicular to the lever arm matters, giving tau = l F sin(theta), where theta is the angle between the force and the lever arm. That makes sense physically: pushing along the lever as you rotate it doesn’t produce rotation, so the torque goes to zero if theta is 0 degrees.

The other expressions don’t fit because they don’t have the correct units or interpretation. F divided by l has units of N/m, not N·m, and l plus or minus F mixes lengths and forces, which isn’t a meaningful way to describe rotational tendency.