Gravity is keeping the cars on the ground while inertia is the tires being the action and the ground pushing the car would be the reaction. The car would not move unless in drive or reverse and that's inertia.
As the two cars race the little car goes in head because of its smaller weight.the engine pushes the smaller car faster
The bigger car ends up in head because of it's bigger engine. The bigger engine has more power to it than the little car.