while travelling in a car if we suddenly apply the brakes why does our upper part get affected and our lower part of body stays in rest

When the car is running, the whole body of the passenger is in the state of motion. When the car stops suddenly, the lower part of the passengers body which is in contact with the car, comes to rest. But the passengers upper portion remains in the state of motion due to the inertia of motion. This is the reason for the passenger to lean forward when the car stops suddenly.