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Positive and negative work

When the object moves in the same direction as the force, the work done is positive (i.e. work is done by the force).

When the object moves in the opposite direction of the force, the work done is considered to be negative (i.e. work is done by the object).

When a man tries to push a car, but the car moves further towards the man, the man performs negative work on the car.

When positive work is done by the force, negative work is done by the object and vice versa. This arises from Newton's third law (for every action, there is an equal and opposite reaction).

When work is said to be done by the object, it is in fact the reaction force of the object that does work. This reaction force is necessarily in the opposite direction of the applied force and hence takes on a negative sign.

The work done by the object is therefore the negative of the work done by the force.

If the motion of an object is perpendicular to the direction of the force, no work is done by the force.