Question

Why does a fast car slow down if its engine switched off

Answer 1

engine is basically a energy system , which provides continuous energy for working of a machine , if engine is switched off the vehicle will slow down as different forces including frictional force , air drag etc will oppose the accelerated car which after some time will stop when a mechanical equilibrium is reached . as engine is stopped no energy will be provided to car to overcome this force .

Answer 2

For a car to move constant force should be applied. We know that F=Ma M is the mass of the vehicle including the weight of the people inside it. a is the acceleration created by engine. When engine stops, a=0. That means acceleration is absent and hence force will be 0. But the car wont be stopped suddenly. Due the inertia in motion, car will move slowly forward until the resistance stops it