Give two examples of (i) positive work, (ii) negative work, (iii) zero work.

When we talk about work in physics, it’s essential to understand how it relates to force and displacement. Work is defined as the product of the force applied to an object and the displacement of that object in the direction of the force. Let’s break down the concepts of positive work, negative work, and zero work with some clear examples.

Positive Work

Positive work occurs when the force applied to an object and the displacement of that object are in the same direction. Here are two examples:

• Example 1: Pushing a box across the floor. If you apply a force to push the box forward and it moves in that direction, you are doing positive work on the box. The force you exert and the displacement of the box are aligned.
• Example 2: Lifting a weight. When you lift a dumbbell straight up, the force you exert (upward) is in the same direction as the displacement of the dumbbell (also upward). This results in positive work being done on the weight.

Negative Work

Negative work happens when the force applied to an object and the displacement are in opposite directions. Here are two examples:

• Example 1: A friction force acting on a sliding object. If you slide a book across a table, the friction force opposes the motion. As the book moves forward, the friction force acts backward, resulting in negative work done by friction.
• Example 2: Lowering a weight. When you lower a dumbbell, the force of gravity pulls it down while you exert an upward force to control its descent. In this case, the displacement of the dumbbell is downward, while the force you apply is upward, leading to negative work.

Zero Work

Zero work occurs when either no displacement occurs, or the force applied is perpendicular to the direction of displacement. Here are two examples:

• Example 1: Carrying a bag while walking. If you hold a bag of groceries and walk forward, the force you exert (upward) is perpendicular to the direction of your movement (forward). Since there’s no displacement in the direction of the force, the work done is zero.
• Example 2: A satellite in orbit. A satellite is constantly being pulled toward the Earth by gravity, but it moves in a circular path. The gravitational force acts toward the center of the Earth, which is perpendicular to the satellite's displacement. Therefore, the work done by gravity on the satellite is zero.

Understanding these concepts helps clarify how forces interact with objects in various scenarios. By analyzing the direction of forces and displacements, we can determine whether work is positive, negative, or zero, which is fundamental in physics and engineering applications.