A pushing force is applied via the handle to the mowing part of the lawn mower. Only the part of the pushing force that is in the same direction as the direction of motion results in work being done. The part of the pushing force perpendicular to the direction of motion does not do any work. If we assume that the lawn mower moves with constant velocity, then the net force in the direction of motion has to be zero because of Newton's first law of inertia. The force that cancels out the applied pushing force is the force of friction which is always in the opposite direction of the motion. Since there is no motion in the vertical direction, the net force in the vertical direction has to be zero, too, because of Newton's first law. pushing force along the handle force of friction normal force direction of motion force of gravity or weight The forces acting on the lawn mower and their corresponding work can be summarized as follow: The pushing force in the direction of the motion does positive work. This is answer (a). The pushing force perpendicular to the direction of motion does no work. The force of gravity on the lawn mower points vertically downward. It is therefore perpendicular to the direction of motion and does no work. This is answer (b). The normal force from the lawn onto the lawn mower points upward, and it is equal to the force of gravity on the lawn mower plus the pushing force perpendicular to the direction of motion, and it does no work. The force of friction between the lawn mower and the lawn points in the opposite direction of the motion and it does negative work. This is answer (c). The pushing force in the direction of the motion has to be equal to the force of friction since we assume that the lawn mower moves at constant velocity. Therefore the work done by the person is the work to compensate for the energy lost by friction. Q1. Assume that a horse pulls a sleigh in a way that the sleigh moves at constant velocity. What is the work being done on the sleigh (a) by the horse, (b) by the Earth, and (c) by the force of friction? Determine whether the work is positive, zero, or negative.