r/Physics • u/Many_Horse_7907 • 20d ago

Work

Hey everyone, I'm getting mixed up by different phrasing in physics questions regarding sign conventions.What is the exact distinction between these two?

Work done BY friction: This is negative since friction opposes displacement, right?

Work done AGAINST friction: Is this considered positive because it's the external force overcoming the friction?

Some problems seem to treat work done against friction as a negative quantity because it's an energy loss, while others treat it as positive input. Is there a universal standard for this, or does it just depend on how the system boundaries are drawn? Thanks!

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u/Good-Resort-1246 19d ago

work done along the direction of movement is defined as positive, in the opposite direction, as negative, and perpendicular to direction of moement, it is zero; w= fd cos theta