Question

$5$ Examples of negative work done.

Answer 1

Negative Work Done

1. Work done is defined as the force applied to displace a body from its original state.
2. Work done is a scalar quantity.
3. Force and displacement are vector quantities.
4. We know that the dot product of two vectors gives a scalar quantity as its result.
5. Therefore work done is the dot product of force and displacement.
6. Unit of work and energy and equal and it is termed as joules or $J$.

If $\overrightarrow{F}$ is the force applied and $\overrightarrow{S}$ is the displacement of the body, then the work done $W$ can be given as,

$W=\overrightarrow{F}.\overrightarrow{S}$

$=F.S.\cos \theta$

where, $\theta$ is the angle between the direction of force vector and displacement vector.

Let $\theta =180°$

$\therefore FS\cos 180°=-FS=-W$

Hence, if the angle is obtuse, work done is always negative.

Examples

1. The work done by the buoyant force of water in the drowning of a ship.
2. The work done by gravitational force when one climbs a tree.
3. The work done by frictional force when pushing a block on a hard surface.
4. The work done by gravity on a rocket goes perpendicularly upwards.
5. The work done by us on pulling water from a well.