Under which conditions is work done

Analysis of the Question

In Physics, "work" is defined differently than in everyday language. To determine if work is done, we must look for two specific criteria:

1. A force must be applied to an object.
2. The object must undergo a displacement (movement) in the direction of the applied force.

The formula for work done is:

Where:

• W = Work done
• F = Force applied
• d = Displacement
• θ = The angle between the force and the direction of motion

If there is no displacement ($d = 0$), then the work done is zero, regardless of how much force is applied.

Evaluation of Options

• A. A man supports a heavy load on his head with hands: The man is applying an upward force to hold the load, but the load is not moving. Since displacement is zero, no work is done.
• B. A woman holds a pot of water: Similar to option A, the woman is exerting force to hold the pot, but there is no displacement. No work is done.
• C. A boy climbs onto a table: The boy applies a force (his weight) to move his body against gravity, and he covers a vertical distance (the height of the table). Because there is both force and displacement, work is done.
• D. A man pushes against a stationary petrol tanker: The man is exerting a large force, but the tanker does not move. Since displacement is zero, no work is done.

Conclusion

Work is only done when the application of force results in a change in the position of the object. In option C, the boy moves from the floor to the table, satisfying the physical definition of work.

Exam Tip

Whenever you see a question about "Work Done," ask yourself: "Did the object move?" If the object stayed in the same place, the work done is zero, no matter how tired the person feels!