Which of the following is an example where the work done by a force is zero?

Question

BlackTom AI · Accepted Answer

Consider the general idea: work done by a force depends on how much the force and the displacement align. 
Option a: Gravitational force to pull an object across the floor. The displacement is horizontal while gravity acts vertically downward. The angle between the gravitational force and the displacement is 90 degrees, so the work done by gravity is W = F · d = F d cos(90°) = 0. This matches the scenario where gravity does no work on horizontal motion. 
Option b: External force to pull an object across snow surface. Here the external force is typically aimed along or partly along the displacement of motion. Unless the force is exactly perpendicular to the displacement (which would be unusual for “pull across” motion), the component of the force along the displacement is nonzero, giving W = F·d cos(θ) with θ < 90°, so the work is not zero. 
Option c: Friction force to pull an object across a horizontal table. Friction acts opposite to the direction of motion, so the angle between friction and displacement is 180 degrees. The work is W = F_fric · d cos(180°) = -F_fric d, which is negative and not zero (unless the displacement is zero). This means friction does negative work during the motion. 
Option d: Tension force in a string to pull an object along the ground. The tension force typically acts along the string toward the object. If the object is pulled along the ground, the displacement has a component in the direction of the tension, so the work is W = T · d cos(θ) with θ between 0 and 90°, which yields nonzero work. 
In summary, only option a describes a scenario where the force (gravity) is perpendicular to the actual displacement, resulting in zero work for horizontal motion, while the other forces have a nonzero component along the displacement or oppose the motion, producing nonzero or negative work.

UCIC 202503 PHYS101 Quiz 3

Which of the following is an example where the work done by a force is zero?

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