Positive, Negative, and Zero Work Class 9 Science | Work, Energy, and Simple Machines | CraftExam

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Student-friendly explanation

Positive work increases or supports motion, like gravity doing work on a falling stone. Negative work opposes motion, like friction on a sliding box. Zero work occurs when force does not produce displacement in its direction, such as centripetal force in uniform circular motion at an instant.

How to write this in exams

1
Start with the exact idea
The sign of work depends on the angle between force and displacement: work is positive when both are in the same direction, negative when opposite, and zero when perpendicular or when displacement is absent.
2
Then show how to use it
Mark displacement direction, mark force direction, compare the angle, assign the sign, then calculate if values are given.
3
Add one concrete example
When a ball moves upward, gravity acts downward, so work done by gravity is negative.
4
Avoid this incomplete answer
Calling frictional work positive is wrong because friction usually acts opposite to displacement.

Definition

The sign of work depends on the angle between force and displacement: work is positive when both are in the same direction, negative when opposite, and zero when perpendicular or when displacement is absent.

Example

When a ball moves upward, gravity acts downward, so work done by gravity is negative.

Rule to remember

Using W = F·s·cosθ: θ = 0° gives positive work, θ = 180° gives negative work, and θ = 90° gives zero work.

Memory hook

Same helps, opposite resists, perpendicular does no work.

Examples and method

Worked example

A 10 N frictional force opposes a box moving 2 m. Work by friction = -10 × 2 = -20 J because force is opposite to displacement.

Method to apply

Mark displacement direction, mark force direction, compare the angle, assign the sign, then calculate if values are given.

Diagram support

Use three small cases with arrows: force parallel to displacement, force opposite to displacement, and force perpendicular to displacement.

How CBSE asks it

Exams ask for examples of positive, negative, and zero work, or ask students to identify the sign from a situation.

Avoid common mistakes

Common confusion

Students often say work is negative only when the object moves backward, but it actually depends on force direction relative to displacement.

Common wrong answer

Calling frictional work positive is wrong because friction usually acts opposite to displacement.

Exam tip

Draw force and displacement arrows. Same direction gives positive work, opposite direction gives negative work, and perpendicular gives zero work.

Quick check

Why is the work done by friction on a sliding book negative?

The work done by friction is negative because friction acts opposite to the displacement of the sliding book.

Answer writing and exam use

1-mark answer

The sign of work depends on the angle between force and displacement: work is positive when both are in the same direction, negative when opposite, and zero when perpendicular or when displacement is absent.

2-mark answer

The sign of work depends on the angle between force and displacement: work is positive when both are in the same direction, negative when opposite, and zero when perpendicular or when displacement is absent. Using W = F·s·cosθ: θ = 0° gives positive work, θ = 180° gives negative work, and θ = 90° gives zero work. When a ball moves upward, gravity acts downward, so work done by gravity is negative.

3-mark answer

Positive work increases or supports motion, like gravity doing work on a falling stone. Negative work opposes motion, like friction on a sliding box. Zero work occurs when force does not produce displacement in its direction, such as centripetal force in uniform circular motion at an instant. Using W = F·s·cosθ: θ = 0° gives positive work, θ = 180° gives negative work, and θ = 90° gives zero work. A 10 N frictional force opposes a box moving 2 m. Work by friction = -10 × 2 = -20 J because force is opposite to displacement. Exams ask for examples of positive, negative, and zero work, or ask students to identify the sign from a situation. Calling frictional work positive is wrong because friction usually acts opposite to displacement.

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Positive, Negative, and Zero Work

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Examples and method

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Answer writing and exam use

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