Physics · Work, Energy And Power · NEET
In circular motion the centripetal force points toward the centre, but at every instant the displacement (velocity direction) is along the tangent, which is perpendicular to the radius. So the angle between force and displacement is theta = 90 degrees, and W = F d cos 90 = 0. This is why the speed of a satellite or a stone whirled on a string stays constant: gravity or tension does no work on it.
When a coolie walks on level ground carrying a load, he lifts the load with an upward force equal to its weight, but the load moves horizontally. The upward force and the horizontal displacement are at 90 degrees, so cos 90 = 0 and the physics work on the load is zero. He feels tired because his muscles use up biological energy, but that is different from mechanical work as defined by W = F d cos(theta).
No. You apply a large force, but the wall's displacement is d = 0, so W = F d cos(theta) = 0. Work in physics needs the object to actually move in the direction of the force. Your muscles still get tired because they burn energy internally, but zero mechanical work is done on the wall.
No. The string tension always points toward the centre (along the radius), while the stone moves along the tangent. The two directions are perpendicular, so theta = 90 degrees and the tension does zero work. That is why the stone's kinetic energy and speed do not change even though a force is acting on it.
Yes, for purely horizontal motion. Gravity acts vertically downward while the displacement is horizontal, so the angle is 90 degrees and the work by gravity is zero. Gravity only does work when the object rises or falls (has vertical displacement).
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Work W = F d cos(theta) is zero when: (1) displacement d = 0 (object does not move), (2) the force is perpendicular to displacement so theta = 90 degrees (cos 90 = 0), or (3) the net force F = 0.
The centripetal force points toward the centre while the displacement is along the tangent, making them perpendicular. Since theta = 90 degrees, cos 90 = 0, so the work done is zero and the speed stays constant.
No. The normal force is vertical (perpendicular to the surface) while the sliding displacement is horizontal. The angle is 90 degrees, so the normal force does zero work in this case.
Yes. If the object does not move (d = 0) or moves perpendicular to the force (theta = 90 degrees), the work is zero no matter how large the force is. Work depends on force, displacement and the angle between them together.
Your muscles keep contracting and relaxing, using chemical energy, which makes you tired. But because the weight has no displacement (d = 0), the mechanical work done on it in physics is zero. Physical tiredness is not the same as physics work.
