Snigdha,

Work can be negative as well. It depends on your perspective.

For example, if we declare the work done by an elevator engine lifting the carriage "positive", then the work done by gravity on the engine is negative. It depends on how you choose to label things.

Remember that Force is Mass times Acceleration. If Acceleration is negative (such as when braking), then Force is negative. Since Work is Force times Distance, if Force and Distance are different signs, then Work is negative.

Cheers,
Stephen La Rocque.