A car of mass $300\mathrm{kg}$ is moving on the horizontal ground. Find work done by gravity when the car has moved $30m$.

 Work done is the dot product of Force and Displacement.

= Work Done $\left(W\right)=$ Forced $\left(F\right)\times$ Displacement $\left(S\right)\times \cos \theta$ 

Here$\theta$ is the angle between force and displacement.

When a car is moving horizontally, the direction of gravity force is downwards at any given time, and the direction displacement is in the direction of motion, which is horizontal (perpendicular to the downward direction).

As a result, the angle formed by gravity force and displacement will b:$\theta ={90}^o$.

$Work done \left(W\right) = Gravity force\left(F\right)\times Displacement\left(S\right)\times \cos \theta$

$W=F\times S\times \cos 90°$

$W\times F\times S\times 0=0$